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Trevor Matthews, Eric Kaiser, Don Gillis, and Jim Bergmann join Bryan for an educational live stream about compression ratio, efficiency, and capacity. They also touch on sensible heat ratio (SHR) and the conditions that affect SHR and compression ratio.
Compression ratio is the ratio of head pressure to suction pressure in an HVAC/R unit. Generally, compression ratios approach 2.5:1 on high-efficiency, 16+ SEER A/C systems. The highest compression ratio in A/C compressors is typically 11:1. Refrigeration compression ratios tend to be quite a bit higher (up to 26:1).
To calculate a system's compression ratio, you take your suction and discharge pressure right by the compressor. Then, you turn them into absolute readings. To do that, you add atmospheric pressure (14.7 PSI) to your gauge pressure reading. You divide your head pressure (discharge pressure) by your suction pressure to yield a ratio.
When you pump down a scroll compressor, the floating seal prevents a system from running with too high of a compression ratio. High head pressure AND low suction pressure both contribute to high compression ratios.
Lately, air conditioning manufacturers have attempted to drop compression ratios by increasing the sizes of their condenser coils. Increasing the sizes of condenser coils helps expand the surface area; when that happens, you can get the refrigerant temperature closer to the outdoor ambient temperature. (As you could probably assume, outdoor ambient also affects compression ratio.) While those reduce head pressure, they don't do anything to address issues about low suction pressures.
Mass flow also plays a role in compression ratio. It correlates with the suction and discharge pressures. Higher discharge pressure indicates lower mass flow, and higher suction pressure indicates higher mass flow.
Compressor ratio and efficiency go hand-in-hand. Scroll compressors tend to have lower compression ratios than reciprocating compressors and are more efficient. Reciprocating compressors retain some gas in their pistons; the retained gas expands after most of the discharge gas leaves, leading to inefficiency.
Sizing is also a vital component of the compression ratio. Oversizing, in general, is NOT a best practice in installation. (You can see the benefits of large evaporator coils in dehumidification and large condenser coils in reducing compression ratio, but they are not perfect fixes and can lead to other complications.) Suction filter/driers are often improperly sized and end up being too restrictive. Improperly sized suction filter/driers have the same effect as a kinked suction line, which horribly impacts the compression ratio.
Sensible heat ratio (SHR) indicates the amount of humidity removed from an airstream across the evaporator coil compared to the sensible heat. Essentially, it compares latent heat to sensible heat and is expressed as a percentage of sensible heat. It typically does NOT impact the compression ratio, but conditions that affect the SHR may also affect the compression ratio.
When it comes to capacity, we often encounter limiting factors in each system. There may be difficulties rejecting or absorbing heat. Additionally, not all latent capacity can be converted to sensible. So, increasing airflow won't directly increase your capacity because of that inconvertible latent heat.
We also answer viewers' questions and talk about:
Vapor injection
Theoretical (and real) ways to reduce compression ratios with water
Dirty evaporator effects on volumetric efficiency
Dew point and its impacts on SHR and suction pressure
Manual J and latent heat load
MeasureQuick testing and functioning
Noisy compressors
Read all the tech tips, take the quizzes, and find our handy calculators at

I've got trevor matthews here to begin with, i invited a few others, but trevor's always uh great about making time for us, and this topic, especially the topic of compression ratio and efficiency and capacity, actually applies a lot to the refrigeration side, which is sort of your Cup of tea trevor, so uh, so yeah thanks for doing this, oh thanks for having me, i appreciate it and you're going to notice some of our um. Some of our students are probably going to be trickling in here soon as they get off from work. So, on the actual zoom meeting um, this is this: is both a live stream to the interwebs and also a class as part of the lake technical apprenticeship program. So there's a few students who still need a little bit of uh a little bit of hours through the summer and so we're i'm going to continue doing these classes so we'll also benefit them virtually.

So anyway, that's what's going on. That's the story! That's my story and i'm sticking to it. I love it all right. So, let's start with um just the topic of compression ratio in general um.

We you know trevor you and i have talked about this - a lot already on the podcast um. But, let's just start by when you hear compression ratio, what do you think of? Where do you start really on when i'm teaching it or talking about it? I i really want to make sure people understand how to calculate it and what it is like that's step. One you need to make sure what you're looking for and uh really how to properly calculate it. Uh that's step one right so go ahead and go ahead and start there yeah.

So you you need to get your pressures, so put your gauges on your system on your compressor and get your suction and discharge pressure and then turn those into absolute. That's that's step one and so absolute means. You add in atmospheric pressure to those numbers which at sea level, which is what we didn't just kind of use, that as a default that is 14.7 psia. So you just take your your suction right at the in and really both of these numbers for them to be ideally compression ratio.

They need to be right before and after the compressor. So if you're doing it based on suction pressure and liquid line pressure, that's not really accurate, because there's going to be some pressure drop, you wouldn't want to do it say at the evaporator coil or on the inlet side of a suction dryer. You'd want to do it right before it hits the compressor right after it leaves the compressor. You take those two numbers you add 14.7 to both, and then you look at the ratio in between those two.

So it's just as simple as taking your absolute head and dividing it by your absolute section and then that number is your number yeah exactly and i usually tell a lot of people. You know 14.7, you can get on a calculator if you use 15, it's okay! Yeah, you know what i mean: it's 0.3. It might change it a little bit but for uh good, easy calculations, i just say add 15 to that number: yeah, yeah and um. But if it's on a test, 14.7 yeah, um and another thing uh to factor in there, because a lot of people will say well, if you're adding it to both sides or just another thing to get your head around.

I guess i should say if you're adding it to both sides, why does it matter right, but it does matter significantly because in proportion to one another, your suction pressure obviously is much lower in proportion to your head. So when you're adding 14.7 to that that drives your compression ratio down, so if you don't add those in, then it's going to make your compression ratio look higher than it actually is, and that's a really common mistake that people make on tests or in a lot Of different areas, i see that on nate and other places where people fail to add those to both sides, and then they come up with the wrong number yeah and i've seen it in tests that i've given out to technicians yeah exactly so okay, so first you Got ta know how to calculate it. It's really simple, but then um, because as soon as people hear something like compression ratio, they immediately want to jump to. What should the number be? You know it's sort of like you know, suction or head pressure or sub cooling or super heat, or anything people always want to jump to what what should the number be and um? That's, not the one.

I got ta. Let somebody else in here: that's not how we do this, because it really depends on the application, so in kind of your modern, r410a high efficiency, air conditioner sort of 16 plus year, which is a lot of what we're installing nowadays we're going to see compression ratios That you know are approaching 2.5, so they're very low. That would be a very low number, but in air conditioning. That's that's something that you're going to see a lot, whereas in refrigeration - and you know, grocery store applications that sort of thing you're going to see much higher uh compression ratios and what are some common compressed compression ratios that you would see in those areas.

So for sure we we do have uh what we talked about uh in our scroll compressors. For example, we have what we call floating seal and the design of the compressor from a refrigeration compress scroll to a air conditioning scroll is different and, as you mentioned in the refrigeration, you got a lot lower suction pressure. You got a higher uh compression ratio so on our air conditioning compressors. You want to always keep it uh below 11, 11 to 1., so that floating steel stay engaged, has a high compression ratio.

You just said 2.5, but it's designed to handle about 11 to 1.. Anything above that that floating seal is going to unbalance and separate it where now, our design of our refrigeration scroll are about 26 to 1. yeah, and so just in terms of kind of your maximum design. One thing that immediately comes to mind that i think is worth mentioning here is because when you hear 1101 on air conditioning you think well, we would never run 11 to 1 compression ratio.

I mean, if you do the math on that - that's really high, but what you forget about compression ratio is that, when your suction pressure drops that compression ratio goes up really fast, so in fact suction pressure changes are going to impact your compression ratio even more than Head pressure changes because it's that smaller number and so where we see this in air conditioning where you're going to see this, this floating seal, come unseated and and cause equalization, is when you're trying to do a pump down. So if you're trying to pump down a scroll compressor, we've all seen this happen, where you're pumping it down that suction pressure's dropping and then all of a sudden, it starts going back up again you're like what the heck's going on here and that's what that is. It's got that built-in protection in order to prevent it from running too high of a compression ratio, and it's important that we remember that compression ratio isn't just a matter of running high head pressure, so high head pressure that will cause high compression ratio, but so will Low suction yeah, and why we design it that way? It's it's! So we don't uh that floating seal, so we never run that scroll into a vacuum. So we want that to separate before that happens, yeah run it scroll into a vacuum.

You can actually create shorting, but then also you can create overheating, which damages the oil damages, the compressor all that as well so yeah a lot of good stuff there kind of on the basics. So now i want to talk a little bit about um. Well, first off i want to recognize all of the students who have made it into the uh into the zoom room. For those of you didn't hear, uh live on youtube.

This is both a live, a youtube, live stream and also a class for our students. So we have josh clemente, damien, fox, jessica egan is out there and then chad, maneer chad, is actually a grocery tech. He's a grocery refrigeration check so uh for those of you, students here in zoom. If you have any questions or anything that you want to add into the conversation, feel free to unmute yourself and and ask away as we go um, i don't have any slides today, we're not going to bore you with any slides.

This is just a kind of a free-flowing conversation and, as and as those of you on youtube, also have questions. Um feel free to ask those or if you have any any comments you want to make feel free to ask those in the chat, because i am monitoring the chat as well so um. This next topic is one of my kind of my favorite topics. This is a little i don't.

Actually i was going to say it's more of an air conditioning thing, but it's really not because compression ratio and efficiency are huge in the refrigeration space as well so i'll. Let you kind of comment on that, but one of the things that we've seen manufacturers do in the air conditioning space is attempt to drop compression ratios and the biggest way that they've done that or the primary way that they've done. That in design is by increasing the size of their condenser coils, so you have a bigger condenser coil. What happens? You have more surface area? You can get that liquid inside the condenser closer to your outdoor temperature.

Some manufacturers call that approach the temperature difference between your liquid coming out of the condenser and your outdoor temperature, the lower you get your approach, the more efficient um you're running in terms of compression ratio. I actually kind of rushed ahead there when i say compression ratio. The the lower i, the lower the compression ratio number, and so that was always kind of a trick way to get your compression ratios lower, was by um was by decreasing your head pressure, and i want you to talk about that. A little bit in terms of the grocery space, because i know in terms of um or the refrigeration space, because i know there's been a lot about controlling compression ratio in that world as it relates to like floating head pressure and that's a term that those of Us in air conditioning probably never heard, but talk a little bit about that and what that means yeah.

So that's really nothing new to refrigeration. Even when i was on the tools years and years ago, and industrial refrigeration, it's it's just that's the norm, uh, but what we call about low, condensing our floating head, we're trying to reduce that head pressure. So, instead of running at 225, we'll say we want to run it down at 150., but all like there's a lot of considerations in there. The best thing to do for anyone out there is to get a compressor performance chart and look at it.

Maybe i can pull one up after, but when you look at it when a condenser is running at say 120 or 130, you don't run them usually that high in refrigeration and say 105 and you all of a sudden drop that down to 80 or 70 you're Going to see that compression ratio drop off you're going to see power go down and capacity go up, which is very interesting, yeah and that's, and so you mentioned there needing to be some other things that you do some design things um and in the refrigeration side You know there's kind of these two battling. I don't want to say they're schools of thought, but two battling ways of doing this. One is to say, let's keep everything fixed. Let's keep our head pressure at a certain number and that way our suction pressure stays at a certain number and our superheat stay all fixed.

Our valves don't have to do that. Much fluctuation, you know, keep everything really fixed and in terms of just super super stable. So that there isn't a lot of variability, that makes sense, but the problem is: is that you're allowing your compression ratio to rise as you're doing that so um rather than right, i should say you're preventing your compression ratios from falling um as ambient conditions or whatever Allow for it and so that results in lower efficiency. So i want to talk about that a little bit we have eric kaiser.

Here i was. I was glad, i'm glad that you made it uh with us eric so uh. So we know you can we? Can you can hear us hey brian how's, it going hey great to see you again yeah you too yeah good company here, awesome, yeah excellent company, so i'm gon na throw you just straight out of the frying pan into the fire eric oh wow, yeah. Just right right off the bat um on on the topic on the topic of compression ratio, i want to kind of talk a little bit about that mass flow rate through the compressor, because i think that once you understand that, then it starts to make sense.

Why keeping your compression ratio low can result in better efficiency and better capacity, so uh? Do you feel you feel prepared to just comment on that quickly, not at all, not at all, especially when i got trevor over here sitting next to me, i'm like i'm, like you got the refrigeration god here next to me, i pretend to know a little bit All right: here's! What i'll do here's, what i'll do i'll bungle it and then you can correct me how's that we'll try it that way. So when we think about because there's there's several different factors here - and this is actually something that my thinking has evolved on very recently - so i would say even in the last couple months - i've started to get a little bit more clarity on this, and that is That you know, compression ratio is a huge factor, but i like to start with thinking about suction gas density and an easy way to think of that is just you know. If, if you have a fixed refrigerant inside the system - which, of course you do - and so that variable is, is, is fixed, then the higher the suction pressure, the heavier or more dense the gas is that's entering. The compressor is that is that a fair statement would both of you agree with that, so yeah, higher suction pressure equals heavier more dense gas, and the reason i like thinking of it this way is when you think about things like trevor, and i just talked about Um in a recent podcast, i think it was you you and i trevor we were talking about um cpr, uh, uh, crankcase pressure, radiation yeah and this idea of um high load operation for a compressor like when a system comes out of defrost or something like that.

Because when we think of i, i know i'm kind of jumping all the place here, but i'm going somewhere with this because you can actually have suction gas. That is so heavy. That's so dense that the compressor actually overloads because you're giving it more stuff to move. So when we think of higher suction pressure, a way that i think about it is you've got more stuff in your suction line and therefore your compressor is moving more stuff.

Is that anything to add there that works? Okay? So then, then you have this kind of separate question which is now: how much does this compressor have to push against in the discharge line now, depending on the compressor technology, a scroll versus a reciprocating compressor and a reciprocating compressor? You actually have some re-expansion in the uh in the cylinder and so that's an inefficiency with with scroll compressors, it's continuous compression. So so that's not really a factor but you're still pushing against this higher pressure. And so now that's a factor in how much your compressor can move. So if you kind of think of those two things separately, higher head pressure equals less mass flow because you're pushing against more force lower head pressure, equals more mass flow because you're pushing against less force go to suction side.

Heavier suction gas, denser suction gas, means higher mass flow. Why? Because it's more mass like there's more there to move right, lighter suction gas or lower pressure, means lower mass flow. So to me, when i think of those separately that just helps my brain work around that i'll, let you add whatever you like, i'm going to pull up actually a performance chart of a compressor, i'm just gon na. Look it up because it'll it'll make it'll help.

People understand a little bit more. If you just give me a second here, yeah yeah, that's that's! Bang on like because when you look at the performance chart, as you start to do, that, lower condensing or floating head you'll see that mass flow start to change, which makes a big difference i'll. Just randomly pick a compressor here, i'm going to sit over here and nod my head along. Are you going to nod your mustache a little bit? Yeah, that's good! Another thing i want to mention quickly is: how awesome it is that both of you guys showed up with literally no notice.

Okay, i sent an email and just said: hey, i'm doing a live stream this evening. You want to join me, here's a topic and that's it's. It's awesome to have you guys here and by the way, also, if any of you need to leave, feel free, you know as we go, you don't have to stay the whole time um. This is going to be just sort of a flowing conversation on the topic, but there's a lot to cover here, so i appreciate being invited.

I mean that was some pretty high company on that email yeah. I know what so what's funny is jim joined 15 minutes before and then he left and didn't join again so uh, you know who knows oh but look, look who look who just joined now. I don't know if you guys can see this. Oh we have.

We have the don gillis just joined us as well. Exactly oh boy, the american version trevor's getting worried. Okay, this is great, so uh everybody on youtube. If you can just let make sure that uh that you can see the screen that trevor's sharing just let me know once that comes around that you can see it um, but you can go ahead and and uh and just kind of cover.

What we're looking at here trevor, so this is a performance share that probably should zoom in a bit more, so every compressor, every cobot compressor has one of these. You see this is i'm talking about a zp104 here. So this is a 410a here's, the voltage, but every one of copeland compressor has one of these. If you go on coplo mobile, you can see this and pull it up in the performance chart maker, but across the top here, here's your evaporator temperature, your saturated uh dew point pressure along the top.

This is the numbers here and then along the bottom, because we're talking about the condensing lowering the condensing here's, the condensing temperature condensing pressure here, so we were talking about earlier. Here's the right here is uh m is for mass flow. I should scroll down a little bit here: capacity, power and mass flow, and if you look right here for an example, the mass flow for for this 120 fahrenheit or 418 psi is at 1440. But if we all of a sudden drop that down to say 100, which is 318, we're at 1216., see how that mass flow starts increase.

Just like you talked about brian yeah, exactly yeah. So and again it's just a so we say: compression ratio and compression ratio is just a more simple number that takes into account all of this, but it isn't even just as simple as compression ratio as this chart shows, because if it were that simple, then this Chart would just say you would just track a compression ratio, it's actually you're, actually looking at your um, your low side and high side pressures and that's where you're plotting, um your mass flow and your compressor performance. Yes, um yeah good point there right and you can find all of this in the um in the copeland mobile app as well right, yeah yeah, it's just a really great tool. I'm just gon na stop.

Sharing. Now, okay, great, i have to say hi to don. You there dawn - i am here hello, hi trevor, hi, brian, hey buddy, thanks for joining us, you bet. Do you have anything to add to uh to that conversation a little bit that you've already heard in terms of using the uh your kind of your design conditions and looking up your compressor specifications in order to uh plot performance? Well, i actually just did that today.

Up in south bend trevor did a good job there. So, basically, what he said is real: it's it's real uh. It really brings it to life and i would show the students today up in south bend that you know, as trevor said, i missed part of it but uh. If you change that compression ratio, especially on the suction side, when i think of compression ratio and very layman's terms, i think you know the compressor is getting hot right anytime.

We get those numbers apart. That ratio gets farther apart and you're you're. I don't know what i missed, but you know your lower your lower refrigerants, your lower temperatures, your minus 30s, and what have you are going to have a lot harder, more robust, compressors, uh anytime, that compressor gets hot and i apologize. We already talked about this.

We're talking about uh, the compression ratio goes high, the the compressor gets hot, we start to lose lubricity, we start to lose lubricity, the viscosity gets less, which is thinner and the way the viscosity is tested is basically just a cup with a with a hole in It you put the oil in it, you you know, stop watching and you know and that's how they dial it in. But when we start to lose our lubricity, we start to get a lot of friction, build up on whatever you're working with whether it's a scroll or a piston or a cylinder, and in pistons for example, and same as a scroll. That's the thinnest film of oil. In the whole compressor, and ironically or coincidentally, by design the hottest part of the compressor is right next to that thinnest, film of oil.

So it's a it's a big deal and i don't recommend people gauging up. But if you gauge up it's pretty darn easy to figure out and you can tell a lot like i'm selling students today. If you know your discharge temperature leaving the compressor - and you know your compression ratio and it's mostly used in refrigeration. I get that.

But you've told yourself a lot about the inside of that compressor without being able to see it, you know without it being a plexiglass. If you will, you know it tells you a lot about how that thing is running yeah for sure, and i want to hit on a couple things that you covered there, that we didn't uh necessarily cover yet, and one is for those of you who do a Lot of air conditioning to kind of go along with this idea of heavy gas versus light gas when you're working on a system that has to go down to really low temperature uh evaporator. So you have a really cold box in those cases when you're running a really cold box, just like what we see in air conditioning when you run the house real cold, what happens? Your suction pressure drops? Your air passing over the evaporator coil is a lower temperature, and so that means that your suction pressure drops remember what we said: lower suction pressure equals lighter suction gas, lower density suction gas, and that means that that compressor isn't doing as much meaning it doesn't have. As much to move, but also if you've got lighter suction gas going through, that compressor, keep in mind that it's the suction gas that cools the compressor - and this is actually kind of a tricky business, because a lot of people imagine that, just because the gas returning To the compressor is cold, meaning a low temperature that it's going to do a good job of cooling, the compressor.

But this is it's not that it's not that straightforward, because you could have cold suction gas returning. That is a very low suction pressure or a lower suction pressure than that compressor is designed for it could be, i mean a heck, it could even be freezing coming back to that to that compressor, but if it is too light, meaning it is too low. Suction pressure, you don't have the mass flow that compressor is still going to overheat, and that's because that compressor is refrigerant cooled. It requires that mass of refrigerant and again it depends on the design like trevor was saying.

If you've got a, you know, you've got a low, temp compressor. It can be designed for that, but just keep in mind that if you've got suction gas, that's lighter lower pressure than the design of that compressor. That compressor is going to run hot, and it's going to cause. Problems like like don is saying with lubrication over time, which is you know, one of the primary causes of damage is loss of lubrication.

So i want to boil all that down for you right there. A little too little uh too low of pressure, suction pressure. You know too light a gas not good too much too high, not good, there's a sweet spot in the middle between the two nailed. It nailed it, and so when we talk about like pressure limiting valves, uh like trevor and i talked about in a recent podcast or cprs in both of those cases, those are designed to keep it from getting too heavy.

So the compressor doesn't overload right. If you run a compressor say you run a medium temp compressor in a low temp application. Well, now you're going to have um suction gas returning that's too light and now that it's going to be bad for the compressor. And so it's interesting because that's why i wanted to start with this idea of oversized condenser coils as what we see in air conditioning, and so we tend to think of.

I mean at least i i did for years and if i thought about compression ratio being too high, i would always think in terms of head pressure being too high. I didn't, i didn't automatically think about suction pressure being too low, and so the manufacturers have worked really hard initially, especially when they first started coming out with high efficiency equipment to use larger condenser coils, which resulted in lower head pressure, which got our compression ratios down. A little bit which meant that that compressor didn't have to work as hard, it didn't have as much pressure to push against, but it didn't initially, we didn't do a lot with. You know the the gas coming back, but nowadays there's a lot.

That's been done in order to get our suction pressure up, but that has some that has some consequences of getting our suction pressure up, um i'll. Let you uh i'll! Let you talk about that a little bit if you're willing eric kind of what the strategies are in terms of like design, what we've seen! That's that's driving suction pressure up and why manufacturers have done that and what some of the consequences are well, as you said, they're trying to lower the compression ratio because the lower the compression ratio, the less energy we're going to use because we're not having to push You know push that refrigerant as much we're not having to compress it nearly as much the downside, of course to that is, there's a limit on the upper side to how much we can bring that down, and i think some of the higher efficiency stuff now is Down around probably 15 degree, condenser split, yeah um, that's about as low as they're gon na go and man. If they go much lower, it becomes harder to transfer that heat that the units are just going to get bigger and bigger, and i'm a tall guy and some of them, i think, are almost as tall as me now. You know we're pushing five six foot tall units at residences here.

This is getting a little a little big and then, when you add, like 15 degree, glide refrigerants in there makes it so much better. Oh yeah that that obviously works great because you know then then you've got all kinds of other fun. You know, and on the indoor side, of course, in cooling, we have to get that indoor coil cold enough, so that we can remove humidity because we need to remove some of the humidity. We have to have that sensible heat ratio.

So there's a balance and realistically um, even the high efficiency stuff, we're looking at about a 30 degree, coil td, evaporator td, and that's really about as far as they can go to narrow that compression ratio, because if we go any higher with our evaporator coil temp We're not going to be removing the moisture that we need to in most cases. Now, if we get into a dry climate, that's a different story. I mean personally i'm in kind of a wet climate brian's in a really wet climate down there yeah we have to take that humidity out. Now we go into the dry climates.

Maybe there i don't know. Maybe maybe trevor can you tell us? Are manufacturers looking at making different units for dry climates where they don't have to remove the humidity and they can just stuff more air flow at it? I guess i don't know to be honest with you. I don't know that answer a lot of the ones that i deal with they're more on the refrigeration side, but it's uh whatever the customer is looking for now. What's a lot of the oems that i work with they'll build custom equipment if they need to yeah but uh like you're, saying you're spot on with that that for the wet climates - and you know, you're gon na remove that humidity.

Even here, where i'm at in ontario, you know in the winter it gets super dry and in the summer, depending on when it is what time of the year it is, it could be super humid and hot, so you're, right, depending on where it's at in the Country in north america, i guess it'll depend on the type equipment they sell. So yeah - i don't. I don't know if making a special unit for dry climates would be a good idea because, like if we look at say last summer in l.a, they saw unprecedented humidity in southern california. They had no idea what to do with it out there.

You know i was talking to a couple contractors and they're like how do we manage this? What do we do because they they don't know they don't have those problems, yeah, um, so derek. Has the question um? It's a good question, something i want to address. He says: won't the ambient temperature affect the compression ratio. Anybody want to take that one yeah outside outdoor ambient is absolutely going to affect the compression ratio, because that drives your condenser, your condensing pressure or your saturation pressure and your condenser higher.

We still have to keep that same essentially condensing temperature on our evaporator. So, as those two pressures get farther and farther apart, our compression ratio goes up, which means more electricity usage yeah, especially in cases um like refrigeration, where your evaporator temperature needs to stay pretty darn fixed. I mean like you're, not you're, not going to float that too much. I mean there are some strategies where you can float it a little bit, but but it's going to stay pretty fixed and so, in those cases, changes in your outdoor ambient conditions are definitely going to change your compression ratio and in refrigeration.

The trick is to allow it to float down where you can, when ambient conditions allow it to flow down so meaning if it gets cooler outside that's sort of the. I mean it's been happening a long time, but you can save energy. If you can let your head pressure drop so that way, your systems are operating a little bit more efficient on the air conditioning side. Yeah, absolutely i mean if it's, if it's cooler outside you're going to have lower compression ratios and your system's going to perform better.

If you look at a performance chart on any unit, um you're gon na see that if you keep your indoor temperature the same, you keep your. You know your evaporator pressure the same and you drop your outdoor uh ambient. Then your your compression ratio is gon na drop, which in turn is going to result in better overall capacity, better overall system, performance, efficiency and capacity uh. Because now your compressor can move more refrigerant because it has less head pressure to push against the what it's pushing against is lower, which goes back to kind of what i was talking about at the beginning.

You know think of your suction in your head. Think of them separately a little bit and if you can imagine head pressure, drop compressor perform better uh suction pressure. Go up, compressor perform better, but there are still these limits, especially um. I mean it's their design limits, but if we think about air conditioning specifically like eric said, we can only do so much right, i mean we can only get that liquid temperature so close to the outdoor temperature, otherwise it just becomes diminishing returns.

You just keep making this larger and larger condenser coil, but if you're rejecting your heat to the outdoor air you're, not you can't get your liquid line. Temperature below your outdoor temperature and i've actually said that a lot, and then i had somebody challenge me because they're, like you know what about mechanical sub cooling, it's like yes, of course, mechanical subcooling is a totally different strategy and not something that we really do in Uh in air conditioning but yeah absolutely uh is is a strategy, but for mostly what we're working on in air conditioning our liquid line temperature can't drop any lower than that. So that's our limit and like eric was saying on the inside. We really can't.

I mean how warm can we get our evaporator coil, because higher suction pressure means higher boiling temperature or higher evaporating temperature of that of that refrigerant? In the evaporator coil - and i mean you know - 45 degrees, 50 degrees at design conditions, if it's 75 degrees inside and you're, only running a 25 degree, coil td. That would mean that, in a lot of cases, your evaporator coil is not going to be below. Dew point, which means it isn't going to pull any moisture to to the point that eric was making. So there's this like limitations that we have, but it doesn't change the fact that reducing compression ratio, increasing suction pressure, dropping head pressure does result in better overall compressor performance in terms of uh capacity and efficiency yeah.

That was a great question, because maybe they they don't know, i don't remember the person's name. But when you design a system, you have to design for worst case scenario. So here in ontario, like we'll get up to you, know 90 degrees or 100 degrees in the summertime. You have to design for those worst case scenarios.

So even though most of the year, you won't be running that high. Your system has to be designed to handle that or you will not be able to satisfy the need of the system yeah exactly and that's what you have to think about, and this goes down to when you're designing air conditioning you're. Looking at your, you know, your akka suite your aca manual j and then you're designing it. You know basically you're choosing a piece of equipment based on manual s and all that, and so it's not necessarily all worst case scenario, but when you're doing refrigeration.

Absolutely i mean you have to look at what is your design, evaporator temperature and then what is your worst case scenario: outdoor uh kind of condensing temperature, outdoor, aiming conditions, yeah, the air air conditioning air conditioning from my understanding, the design is for, for lack of better Words is a happy medium like where i'm at you're going to see some low 90 degree days like i pulled hundreds and thousands of customers through my lifetime. It's 90 some degrees out day and they're, calling hey my air conditioning running all day we're doing design. We didn't do a manual j for that to be in 95 degree weather, because if you design something to be 95 degree weather all the time - and you don't see 95 even once in the summer twice in the summer - we're not in florida you're going to walk Into a cold clammy home, you have other issues, moisture issues you got to dehumidify, but so there's got to be a happy medium there when they design the half the air conditioning unit and that's again, like brian said, that's, hence the reason we do manual, js, yeah And manual s and everything else beyond that too, because that that goes back to equipment sizing as well yeah oversizing equipment just to be safe, is not a not a good idea on the air conditioning side and refrigeration. You know you may have an efficiency hit, but it's not going to be as big of a deal in air conditioning.

It's a significant problem: um because you're talking about air distribution, you're talking about duct design, um, there's so many different factors that go into that and if you oversize equipment um, it causes all kinds of issues. Besides. The fact - and this is a complete aside - has nothing to do with what we're talking about, but one of our favorite things to do is to downsize equipment. If you can, because now, if you're doing a change out you're going to go back in and you that that original duct work was probably significantly undersized and you go in with a smaller piece of equipment and now everything runs better.

Because now you no longer have uh undersized tuck work, so uh. So there's a lot of advantages in air conditioning to not to not oversizing um, because if you size for worst case scenario, then you're going to be oversized for 99 of the year um. And there is sort of that happy medium, also considering that in most cases, even in hot markets, you're going to get a chance to recover um. As soon as the sun goes down.

So you're going to have all these hours that you can catch up a little bit as long as people aren't messing with the thermostat too much um. So that's that's another kind of side to this uh side of this conversation, a lot of short cycling right right, yeah, that's exactly what we want. A lot of you know slamming on and off joe shearer says. This is his idea.

I guess he says what about evaporating the waste water condensate with the liquid line as a form of a sub cooler? That's interesting, interesting yeah. I know um uh, the guy i'm dealing with in uh south africa on co2 units and co2 systems. You use adiabatic uh cooling, so you i've seen it in super mario, supermarket refrigeration, where well a lot of technicians that do supermarket in the middle of summer, when it's 35 out they'll just get the sprinkler on the roof and just start just cooler down. You know that those those condensers that were undersized on there, so they i've done it.

I've done hundreds of them pulling them up and just spraying them until for a couple days. Sometimes it's all summer, you know it's a huge waste, but if it's under size it's not going to run properly so you can have misting systems or some sort of um water to reduce that. But if you're in a wet climate, it's not regal gon na work. Right, a high humid climate but yeah in the co2 side, they're developing that exact thing where taking that waste water from certain cases, not like the meat cases and the produce cases, but certain walk-in blocks and then we're using that to on those misting systems.

Not really the sub cooling, i believe that they're talking about but yeah and it's happening out there - that's actually uh something that i've suggested to customers. This is this is kind of on the crazy side. But while we're talking crazy here, um i've had cases uh where i've had customers who had condensers that were from the 80s one specific customer. I'm thinking of uh older lady, fixed budget really couldn't afford to do a change out right then, but it was just the issue that it was 98 degrees outside 99 degrees outside and her house wasn't getting below 82..

I said you know, look i actually set it up, for you put a little sprinkler, just kind of unmissed, you know so it was just barely and just let it hit the condenser coil. I mean over time. That's going to build up, you know hard water and probably destroy it. But if you're talking about trying to get somebody through a couple really bad summer days, um it's not a bad strategy and - and if you think about that really what you're doing.

I should write an article about this because this is probably a good way to even think about compression ratio. What you're doing is you're dropping your compression ratio. That's that's what you're doing so you're dropping your head pressure, which in turn is dropping your compression ratio, which in turn is making your system uh. You know increasing the capacity of the system um.

I should do it with like measure quick and actually to show like before and after a really old. You know tiny condenser coil, that on a real hot summer day, because you can definitely you can definitely increase your capacity. That way. Joe's idea is kind of interesting to take some of that cold condensate water and cool the liquid down.

I just wonder how much of a difference that would actually make yeah some math is necessary. There there's definitely some math and experimentation necessary there. It's an interesting idea, but i'm not sure i'm not sure it's going to work too. Well, hey it's worth trying, though joe is actually kind of like the inventor of the bunch um he's he's got a lot of good ideas and he suggested also.

One of his other comments was that window units do it with the slinger fan blade um. So another good example of what we're talking about here in the case of a window unit, because your condenser coil and your evaporator are so close coupled um that you know you usually have a two shaft fan motor that goes out both sides and so there's a Slinger on that condenser fan that picks up that condensate and throws it into that condenser coil and that does increase the efficiency of the of the system, because what it's doing is rejecting heat, it's actually doing it adiabatically, which is a fancy way of saying evaporatively. It's evaporating that water off when that water evaporates it takes heat with it, so that drops that condensing temperature and in turn, obviously drops the head pressure, which makes the compression ratio drop we're covering a lot of good ground. Here.

I like this stuff. I suppose that if we took the condensate water and essentially pumped it back out to the um onto the conden directly onto the condenser coil and dripped it down, it would probably it might do some good yeah, maybe maybe on the last part of it where it Does the sub cooling yeah? I think that always worries me anytime, you're, putting water on something like that. Um is what's gon na happen with deposits. You know that that's the part that always worries me well, if you're using the condensate water, though, since it's already been evaporated out of the air, it shouldn't have a lot in it.

Have you ever seen our condensate drain lines in florida? Yeah, you guys just get slime down there. It's not it's not like you're talking about some hard water deposits or anything you're, not building up rocks on it. Yeah we'll just sling a poop snake right onto the condenser. That's exactly what we want! We have a good question from regan murphy and this is definitely for dawn and trevor um, because you guys know a lot more about this.

I actually really don't know that much about this uh reagan says: can you guys talk about how vapor injection works and then he caveats, like the new carrier, green speeds? But i think this is the same sort of thing we see in the grocery space as well. So either of you care to talk about vapor injection, so vapor injection uh, it is going into air conditioning uh designed originally for your lower temperature. So all vapor injection or liquid injection is doing is uh is is cooling the compressor down at very you know when it starts to get hot uh we're injecting refrigerant in there and vapor injection is is vapor as it says, but it's going through it's hard without A slide, but it's basically going through so you're teeing off as you leave the condenser at the liquid. You have another metering device you're coming through that metering device, so you're you're turning its saturation you're going into a brace plate, heat exchanger, okay and you're.

Transferring that heat, okay, so you're, removing sub coiling on one side of that heat, exchanger, okay and you're, removing heat from that liquid, so you're gaining on the the original metering device going into the evaporator. So if you were going in what would have been 105 degrees now you might be removing 45 degrees and going into 60.. So that's a win there you're picking up more sub cooling, so you're absorbing more heat in the original evaporator and as you're leaving on the back side of it. Okay, let's, let's put this one side that will say it's like an evaporator, the evaporator for you, we're absorbing that heat from the liquid sub cooling, picking it up on the vapor side, anytime, we're raising that pressure on the vapor side.

That's a good thing back to that: raising suction pressure, we're injecting that, in the medium part of this, let's say we're using scroll the medium section of that scroll set. So if you look at a pt chart - and you go in at 50 degrees, as opposed to let's say minus 25 degrees, if it's a low temperature application, something like that, you can see how big of a difference so you've taken a lot of work off. That compressor, that would have had to start from the bottom on up. Okay, you've already taken a lot of labor, a lot of energy off that scroll, compressor and you've injected that gas, with doing nothing more than just passing in the night.

If you will, in that bracelet heat, exchanger or whatever type of heat exchanger, you have, but that's the whole concept liquid injections very similar to it um. They call it liquid injection. I don't know the term kind of freaks people out because it's you know a lot of people. Old-Timers say it's a vapor pump.

It's not pumps are really liquid, but it's it's it's for vapor, but what it does is again there's a metering device before it enters and there's all kinds of shapes and sizes and designs that we sell but the bottom line. Is it vaporizes it? If you look at a like a spray bottle coming through the media device, very little particles, okay, so it's evaporating from the heat of compression, so it's not really putting liquid in there. But it's the same concept. You see that a lot on the low temperature stuff, but like our x lines, which look like mini splits for condensers, they have vapor injection in them they have the brace plate, heat exchanger and they got all kinds of bells and whistles we just added digital to Them also so uh go ahead and trevor whatever i missed there.

I know that's exactly right, like the vapor one, i say you know capacity, efficiency, liquid injections for cooling and then the vapor one does cooling as well, but that's how i differentiate them. But now, with these new refrigerants, like uh, four four, eight four, four nine four seven, these light refrigerant run really hot. Sometimes you need both vapor and liquid injection. So it's interesting one thing to add here on the topic of air conditioning, which is kind of what reagan was originally asking.

The reason why we're seeing this start to come to air conditioning is because we're running these heat pumps and when we're trying to produce a lot of heat because our other option, you know basically we're doing things with heat pumps that we did never thought we would Do in air conditioning because our other option for most heat pumps is electric, so we can do a lot of exotic things at low temperature um conditions that we would normally not think we would do, because our other option is so terrible in terms of efficiency. So we spin these compressors really fast, basically, and so when spinning them fast. That does what well it drives up our head pressure and it drives down our suction pressure, and in doing that, we can, you know, move a lot of refrigerant and then moving a lot of refrigerant like that. We're both we're increasing our compression ratios.

What it comes down to that's, what we're doing and because we're driving down our suction pressure driving up our head, and so when we do that. We're kind of starting to run that compressor outside of its normal operating envelope, so we're making it hotter than we normally. Would, and so we use some of these strategies in order to keep it cool during those times that you're kind of over over spinning, you know think of it like a computer, it's like you're overclocking the thing uh in order to get a little more performance out Of it, but when you do that, you got to do something in order to help cool it and - and that's my understanding of why we're starting to see this on some of these kind of fancy, high efficiency residential systems. I could be wrong, but that's my understanding.

No, that that's exactly right as trevor said, and that's what we talk about in class trevor and i and each one of us is you know just just remember like this: keep it real simple when we talk about enhanced vapor, injection or or vapor injection at all, We're doing two things at the end of the day. Both of them are cooling, the compressor down the vapor injection, all it's it's cooling, the compressor down, but it's also raising your suction pressure and taking that work off of your compression ratio. When you enter that that scroll set yep and that's all and what i love about this conversation - is we're really talking about a ton of different applications that are all about. You know protecting the compressor.

Yes, we've got to keep our compressor uh operating within its design. Envelope, that's something trevor talks about all the time. Keep it inside its design. Look at the specifications uh if you're dealing with with copeland - and you know, let's face it - we should all be just dealing with copeland.

Let's just put copeland compressors and everything scrap all those other guys, i'm just kidding, but you know it's so easy to look this stuff up, but then in cases where manufacturers start to push it, where we're where we're starting to you know, maybe operate them in some Conditions that maybe at least intermittently are outside of that. That's where all right, let's give that compressor a little bit of cooling, let's increase um that suction gas weight, let's put a little more density to that suction gas. I wanted to address tim tangwei asked he said: have you covered the effects of a dirty evaporator and he uses some fancy words here, but you know tim tim is canadian trevor, so you can relate here. You guys, like your fancy words and and your maple syrup uh and your moose as well, so there's lots of good things in canada.

It says: have you covered the effects of a dirty evaporator on specific density of vapor and how it affects volumetric efficiency? So how would you cover that dirty, evaporator on specific density of vapor and how that affects volumetric efficiency? Well, when you start talking about um the volume metric efficiency, do you want to explain what volumetric efficiency is to everyone? First right: oh you want me to do that. I was gon na. I was gone so volumetric efficiency, i'm gon na go ahead and take a stab at it because you know trevor put me on the spot. If i don't at least try, then people will think that i'm a failure and i am a failure.

My teenage children remind me of that all the time, but i'm gon na give it i'm gon na give it a shot either way so volumetric efficiency. My understanding is how much of that compressor cylinder is actually usable for the compression of gas or, basically, the ratio of how much refrigerant is making it into the cylinder versus how much is making it back out again. That's a way of thinking of that, and especially with reciprocating compressors, because you have this re-expansion where, as that as that piston hits the top of that cylinder, there's still some of that high pressure gas that has to be re-expanded. That's where you start to see kind of inefficient compression, but with scrolls we approach um, we approach 100 volumetric efficiency, although you know, and realistically that's not always the case um.

How was that yeah that was perfect so now now, when you think about that. So when you're running just say we'll say at 20 and 140 you'll have uh, maybe on a say, a semi-hermetic there's going to be more volumetric efficiency losses. So there's going to be more re-expansion of gases. So when you can drop down that just say six to one compression ratio and go down to three or four to one compression ratio, you actually don't have to compress as much gas.

It doesn't take as much work to open up that suction valve port because that's really what's happened. Like you talk about that volumetric efficiency, it takes more work to pull down to get that port open, because there's gas already in there and you'll see that volumetric efficiency change a little bit because they're, depending on the amount of volume. So you reduce that head pressure. You'll get more volume available and it'll be a bit more efficient.

That makes sense. I know i'm a little technical there, but yeah makes sense to me uh don eric anything to add there. Well, i i think you touched on it all i mean that's why we have in our scrolls, like you know, there's different styles. I was working down in missouri last week at the copeland air conditioning plant for on the assembly line, and that's why we have check valves and that's why we have check valves and discharge lines.

A lot of people don't know that we have check valves in different spots, there's a usb and the 410as and the bigger those shelves get uh on the outside. You have more capacity more gas inside the scroll that has the opportunity to come back into the scroll set and, as you said, brian with gas, we've already paid to compress already we're starting at a higher pressure. You know and and that's going to throw our compression ratio, it comes back in that scroll set and that re-expansion gas changes that volumetric uh efficiency. So back to his question, where he says yeah so back to these questions, if he says you have a uh plugged up evaporator, is that correct, yeah dirty evaporator? So you have a dirty evaporator, which you're not boiling off any refrigerant which you're getting droplets of liquid, going back to the compressor um causing fat.

I mean not necessarily so if you have a, if you have a electronic expansion valve or tx valve um, it could be throttling, and so the the primary result across the board, regardless of your metering device type, is you're going to have lower suction pressure. So that's always going to be the case. You could have flood back, but let's assume you don't have flood back just most modern metering devices yeah. So, let's think about this so now that that pressure's starting to drop, what's it doing to your compression ratio, increasing it, it starts increasing so inside the head of that we're talk, i'm talking semi-hermetic now.

Does it take more work to open that suction port? Now so, as you start to drop down that pressure now inside that cylinder, is it going to take more work to go from top dead center to bottom center of that yeah? So if, if it does then you're, it's going to cause a little bit more work, you're going to have less volumetric efficiency, you're going to lose a few points of a point yep.

16 thoughts on “Compression ratio – capacity, efficiency & shr class”
  1. Avataaar/Circle Created with python_avatars Scott kasper says:

    Don, turn your camera on fir chrissake

  2. Avataaar/Circle Created with python_avatars John Bond says:

    All joking aside: The reason compression ratio rises faster as the suction pressure drops, is the suction pressure is a lot closer to absolute zero than the discharge pressure in a well running system. Since everything is relative to a vacuum, the ratio (quotient) increases faster than the difference. As you mentioned, using pressure figures relative to gauge pressure would yield substantial errors due to the atmospheric pressure offset in both the dividend and divisor. A "vacuum" is usually considered any absolute pressure less than about 200 microns. Below that, very little air or other foreign gases should constitute negligible partial pressures, so down in the 50 to 200 micron range, it can be considered a "vacuum". When pulling a vacuum, the ratio of refrigerant to foreign gases increases dramatically, too! It's all about relativity.

  3. Avataaar/Circle Created with python_avatars John Bond says:

    When working on beer chillers, I usually measure pressure in bars.

  4. Avataaar/Circle Created with python_avatars Jamison Ausburn says:

    I’m sure I’m missing something but what comes to mind when I think of pressure regulation/consistency is a headmaster. Now obviously it would have to be engineered to the suction side. Could an accumulator contribute to balance? An electronically modulated “suction master “ like balancer with discharge pressure action to suction pressure effect/reversed headmaster/accumulator combo component?

  5. Avataaar/Circle Created with python_avatars Eli says:

    If you can relate this message to Jim: Jim I love you for all you do, but (there is always a but lol). Can you pleeeeeaaase change that when you hit the phones back button…… do I have to explain or I got many of you with me on this already 🤣. So many times I go into checking something then I want to go back to the previous page, I hit the phones back button (on Android) and it asks me if I wanna exit. Omg how many times did I hit exit by mistake, Jim the blood boils up at that point lol (feel my pain I'm in the hot attic or who knows where lol). Please for the sake of you know the thing, I beg you 🙏🏻. 🤣

  6. Avataaar/Circle Created with python_avatars gambitrc1 says:

    Great info, TY! Regarding the vapor saturation target, typically the rule of thumb has been 30-35 below return dry bulb. If humidity does play a role in that target, would wet bulb be more accurate to use? What would the rule of thumb be for wet bulb, or even better is there a calculation?

  7. Avataaar/Circle Created with python_avatars Mike Cianfrocco says:

    This was great!!!! Service area Barrhaven??

  8. Avataaar/Circle Created with python_avatars coolbreeze says:

    Awesome ! … A lot of talent in one place. Thanks to all for sharing – time well spent gentlemen.

  9. Avataaar/Circle Created with python_avatars David Rivas says:

    Awesome video ; it felt like a great seminar Are you in Nepean ?

  10. Avataaar/Circle Created with python_avatars Brian Mcdermott says:

    Great ,Great infor Bryan and guest.

  11. Avataaar/Circle Created with python_avatars more jello please says:

    Question: I missed your stream but was going to ask you why my 3 year old 5T upflow bryant/carrier is dripping water off the coil and down onto the 4" honeywell air filter, there's good flow, low static pressure, i change filters every 6mo, the drain goes into the house drain, not clogged, it's just raining off of the coils straight down onto the filter, change from Merv11 to Merv8? clean coils? if so what solution can i snag from depot or amazon? i'll also blast the outside coils and monitor before i call someone. and anyone is open to chime in, thanks!

  12. Avataaar/Circle Created with python_avatars TruTech Tools, LTD says:

    MechPic very cool idea!

  13. Avataaar/Circle Created with python_avatars RJ_Make says:

    Standing on the shoulders of giant's

  14. Avataaar/Circle Created with python_avatars MOHAMMAD AL SHAIKH HASAN says:

    Thanks Brayan

    About higher suction gas, which means denser gas so more stuff compressor to move.

    Isn't that also will increase compressor power consumption so will take the efficiency down?

  15. Avataaar/Circle Created with python_avatars Dayanand K M says:

    Watching now.. from India

  16. Avataaar/Circle Created with python_avatars Bob Y says:

    Emerson Electric, Hussmann Refrigeration, True Refrigeration and Parker Sporlan great companies in our St Louis metropolitan area. I guess you could say we have refrigerant running through our veins throughout the St Louis Metro Community. It is going to be a 102 here tomorrow with over 60% humidity ( a good day to check charges and compression ratio, not so good for the technician ). I almost forgot, go Cardinals. Service area Ottawa??

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