hvac/r terminology

Stewing about terminology used in the havc/r industry that isn't consistent or precise. Perhaps the Prof is thinking too much, and needs to find a beer... :)

Consider the term "compression ratio". It is a volumetric ratio defined as the swept volume to the clearance volume, and most often associated with internal combusion engines. And as most car enthusiasts know, a compression ratio of 8 to 10 or so is common for gasoline engines, and approx double that for diesel engines.

But it seems many (if not most) in the hvac/r industry use this term as a pressure ratio to describe the ratio of discharge pressure to suction pressure across a compressor, using absolute pressure units. For example, many in the industry will say that an R-22 compressor operating at a 240 psia discharge pressure and a 60 psia suction will be operating at a "compression ratio" of 240 / 60 or 4 : 1. Of course, this cannot be the case, since swept volume to clearance volume in a compressor is fixed, and has nothing to due with pressures. The correct term to use here is simply "pressure ratio".

So why are there so many (incorrect) references to "compression ratio" in the hvac/r industry. Prof diatribe #107.

The Prof also has a problem with the term "thermostatic expansion valve", which suggests a valve that controls constant temperature... :)

"Suction" pressure is another misnomer used in the refrigeration industry. This term implies system low side pressures are below atmospheric pressure, which was typically the case with the R-12 freezers manufactured many years ago, but is hardly the case now. In fact, good system design practices require low side pressures remain above atmospheric pressure whenever possible. "Evaporator" pressure is the better term to use here….

I'll see you on that beer and raise you one, or raise one in salute.
I also tend to agree with your thinking on the terminology subject, although I admit to being guilty using just what you are talking about. The industry has accepted and passed down these
misnomers. Not pointing fingers but Copeland bulletin 17-1268
refers to the "compression ratio as it affects compressor reliability". Stoecker however refers to it correctly calling it "pressure ratio". Maybe not what you read but who you read is important in learning correctly. As far as TXV goes read 11.13 in the handbook. And yes "suction" is another one that maybe just comes from the fact that the compressor sucks! More appropriate term would be compressor entering pressure. That would cover all the varying systems instead of just 1 or 2 groups. Another that I found to be interesting is that of figuring "net oil pressure".
Most if not all mechanics read net oil pressure as compressor entering pressure - oil pump outlet pressure. The correct way is to take compressor crankcase pressure - oil pump outlet pressure.
This way you get almost true pressure entering the bearings and lose the possible pressure drop from compressor inlet to crankcase pressure. Learned that from a post somewhere and credit is hereby given to whomever. Credit is also given to "Industrial Refrigeration Handbook" by Wilbert F. Stoecker for those who don't know. If you do fridge work and don't have it, get it and read it repeatedly. Now for that beer...
Mike Hopkins

Good points, both of you. I also highly recommend Stoeckers book, although it is pricey. It is more than a good read, but a wonderful reference manual.

When I first started trying to learn fridge I was confused as heck by the free interchange of different descriptions of valves and other components. In one manual I found 7 different ways of describing a winter condensing pressure control valve.. or is it holdback valve.. or is it discharge pressure regulator... or is it a flooding valve... or is it a receiver pressure regulating valve....or a head pressure regulator... I cannot remember them all.

Also subcooling is often misused when what is being talked about is mostly only liquid cooling. It is sort of nonesensical to say that subcooling permits you to lower discharge pressures, for example.

I don't like "heat of rejection" when we are really describing heat rejection. I am not too fond of inverted traps, for that matter.

The Prof has had the pleasure of meeting Prof Wilbert F. Stoecker, attended one of his refrigeration seminars at the University of Illinois--Champaign many years ago, and owns a copy of his "Industrial Refrigeration" book, a book anyone involved with industrial refrigeration should own. Prof Stoecker is a true refrigeration Prof!

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And yes "suction" is another one that maybe just comes from the fact that the compressor sucks!

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LOL!!!!! If the Prof were responsible for compressor application and service literature, all references to "suction" pressure would be removed, and at least refer to it as "evaporator" pressure.

Unfortunately, any industry that has been around as long as ours has grown some barnacles. Consider the ubiquitous unit of power we use for cooling capacity: tons of refrigeration. By definition, it is the cooling provided by melting one ton of ice in one day, with the latent heat of fusion for water defined as 144 Btu/lb_m. Therefore, one ton of refrigeration equals: 144 Btu/lb_m * 2000 lb_m/ton / 24 hr/day = 12,000 Btu/hr. But we have a slight problem here.... the latent heat of fusion of water is really closer to 143.4 Btu/lb_m (based on International Table Btu), which gives us only 11,950 Btu/hr per ton refrigeration. We have been overestimating our cooling capacities from the very beginning... :)

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Originally posted by Fridgetech
I think the guys in the USA need to learn the difference between dT and TD !!!

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That's how I seperate the A/C guys from the Refrigeration men!!!!

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That's how I seperate the A/C guys from the Refrigeration men!!!!

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And the real smart refrigeration men can explain what LMTD means... :)

:(

Got me there. I understand the concept.

But I wasn't implying that I am anywhere near the brightest bulb here.

I was left hangin' the last time the issue arose.

oh....

Marc, why do I find myself having to read everything you post, fourteen times? :) LOL

Nonetheless, I do inquire as to one's understanding of dT vs. TD as a barometer of their knowledge.

I'm gonna' get that Stoeker (sp?) book.

But my point anyway was, typically when I ask "What is TD", the victim answers, "The difference of the temp of the air entering (evap) and the air exiting (evap)". I then am thrust into the explanations of dT and TD!

Quickly reviewing the compressor chapter in ASHRAE's 2000 HVAC System and Equipment Handbook, the Prof finds some inconsistency relating "pressure ratio" and "compression ratio" terms. For example, the following equation is used several times in the chapter to equate the compressor's volume ratio to its internal pressure ratio:

p_i = V_i^k

where:
p_i = internal pressure ratio
V_i = compressor volume ratio
k = specific heat ratio of the refrigerant

The purpose of this equation is to allow the compressor designer to determine the necessary compression ratio or compressor volume ratio to obtain the desired internal pressure ratio. For efficient compressor operation, the internal pressure ratio should closely match the system's pressure ratio, i.e., the ratio of the absolute pressures across the compressor.

In the rotary vane compressor section, however, p_i is referred to as "internal compression ratio", and V_i is referred to as "compression volume ratio", instead of simple calling it "compression ratio". The problem is obvious here... you can't have both variables being called "compression ratio"

But in the twin screw compressor section, p_i is referred to as "internal pressure ratio", and V_i is referred to as "compressor volume ratio". This would have met with the Prof's approval, but then the section defines "compression ratio" as the ratio of the absolute pressures across the compressor.

But then the chapter later shows an efficiency vs pressure ratio curve for a scroll compressor.

Argghhhh!!!!!

surely the evaporating pressure does not equal the compressor inlet pressure as the suction line can be subject to increased thermal load and therefore increased superheat and pressure?

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I wouldn't call the internal volume ratio a compression volume ratio, I would call it "internal volume ratio (V_i)"

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Agreed. The term "compression volume ratio" is an apparent aberration used in the rotary vane section to distinguish it from the "internal compression ratio" term.

The Prof was happy with the terminology used in the twin screw section, except for its use of compression ratio. "Compressor volume ratio" is clearly the same as "internal volume ratio", and "internal pressure ratio" is in line with his thinking.

Perhaps the Prof is thinking too much like an automotive engineer. These guys will, of course, tell you that "compression ratio" is clearly a volume ratio, defined as the swept volume to clearance volume in an engine, and it is prominently noted in engine specifications. But it is interesting that the hvac/r industry has redefined it, even though the automotive definition is applicable to a compressor. Those in the hvac/r industry who recognize this inconsistency use the more generic "pressure ratio" term to describe the ratio of high and low side pressures. The Prof believes terminology should be consistent, though the English language makes it difficult at times… :)

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Perhaps we should agree on what is technically most correct and then stick to it and watch and see how many years it takes this to permeate the industry globally

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This BBS will need many more members to make it happen in our lifetimes…:)

Frank, I agree with the beginning of your post:

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Originally posted by frank
surely the evaporating pressure does not equal the compressor inlet pressure as the suction line can be subject to increased thermal load and therefore increased superheat and pressure?

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But not necessarily your conclusion. There is a definite difference in the Evap pressure and the compressor suction pressure. If there were not a drop in pressure between these points there would be no flow of the refrigerant fluid and without that flow, now refrigeration would be possible. This is why I agree with
Fridgetech. (did I say that ;) )

How about renaming the thermostatic expansion valve (TEV) to superheatstatic expansion valve (SEV)??? ..... :) on the basis that it controls at a constant superheat, and not temperature. Maybe isosuperheat expansion valve???

The automatic expansion valve (AEV) could be given a clever name: isobaric expansion valve...

Consider the possibilities. The Prof might just have to find another beer... :)

How about "air conditioning"? Medium and low temp systems also condition air. As do heating systems.

There is definetly going to be a difference in evaporating pressure
versus the compressor entering pressure. Most likely due to pressure drop from the line design and possible devices placed in the line. Also dependent on the system. A close coupled reach in freezer is not going to have the same pressure drop as a built up screw skid running -10*F at 500 tons. I have heard from the old timers a term called "true suction" which would be compressor entering pressure. I think (going out on a limb) what the prof is trying to say is many refer to evap pressure as suction pressure and maybe not consider the pressure drop. They figure if I put my gauge on the compressor inlet then that's my evap pressure. Manufacturers haven't made it easy for us either, their goal being reducing the number of openings on a system, reducing the chance of leaks. So they don't put an access on the evap. outlet where we can hook a gauge to. This other thing about the DT TD isssue, how many refrig. mechanics know what approach on a chiller is ? Or as some call it LTD and when do you take it and why, what's it supposed to be? Discharge superheat ? Screw , centrifugal, recip. what's it supposed to be and why, at what load. What happens when we're loaded , unloaded, undercharged, overcharged, flows aren't right? From my little outlook on the world of the fridge game DT is a measure of performance and TD is a measure of design. Not to say we can't tweak a little to change either. Both are an insight to the system but it's up to the design people to get it right before it's put in service. Once it's in service then we have to deal with it. We know that checking the pressure ratio on a compressor tells us much about whether the compressor can handle the duty it's subjected to. Another measure of design. If all the design engineers out there were perfect and right on the
money our jobs would be much easier. And if the owners\operators didn't change things after the fact we would be like the maytag repairman. Never a perfect world and so we face these problems and work to solve them. Now for one of my
other pet peeves, don't call us technicians, as we are mechanics, at least I am. The word "tech" bothers me immensely. JMO :)
No, Gary that's not a stab at the "tech method" and no offense intended. I think what you have there is an excellent program for the initial diagnosis.
Good day eh!
Mike Hopkins

No offense taken, Mike, and thank you for the kind words. :)

I have a similar pet peeve. Being called an engineer. I don't calculate and predict; I measure and analyze.

They got the senior part right, though. I'm older than dirt... LOL

Don't tell Mom I said that. She'll make the connection and smack me.

Mike, I think what we call ourselves is what we perceive ourselves. I would be put out if I was called a mechanic, indeed I can remember being offended when someone from Cimco called me a mechanic in the course of a conversation.
I spent the extra two years day release, losing out on pay just to study my technicians exams, so I feel I have earned the term "Technician"
Anyway that was then, now I have to aply what I learned everyday, that is where the real benifit from the technicians exams become useful.
Regards. Andy.

I remember when I first came to Florida, my title was "Region Branch Engineer." (My education was English and Journalism and Technical Writing.)

Meeting with a degreed engineer for a supermarket chain he asked me if I am a "real" engineer. I told him no, if he meant a degreed engineer. I then asked him what his degree was in. "Civil Engineering" he replied smugly. I said, "Then would you mind acting more civil?"