Printable View
I am getting confused about the meaning of compressor capacity. Sometimes I see it equated to evap capacity, is that for an ideal system? Can this value be used to calculate mass flow by using the enthalpy states into and out of the evaporator?
I know that Qin (evap load) + compressor Win (comp power) = Qout (cond heat of rejection); but where does compressor capacity fit in the picture?
Thanks for the help, Kate
Hi Kate,
That is a fun topic you started.
Compressor capacity is the Tons (or kW if you are not in the US) that the compressor will produce based on the operating conditions at the compressor flanges (suction and discharge) for a specific refrigerant.
Compressors don't really produce Tons (or kW for that matter). They only pump vapor. The use of the term Tons is a convenient way to express the relative compressor capacity for a refrigerant at various operating conditions.
In the manufacturers literature you will see the compressor pumping capacity listed as "swept volume" or "displacement". In dimensional terms this could be:
The actual useful volume of vapor pumped is lower than the above given values. This occurs due to the compressor losses in pumping, i.e., volumetric efficiency (VE).
- cubic feet per minute (CFM)
- cubic feet per hour (CFH)
- cubic meters per hour
- etc.
- Note: for a specific rotational speed
The VE will be greater for operating conditions that have a lower pressure ratio (sometimes called compression ratio, which is Discharge pressure divided by Suction pressure, in absolute terms, psia). At higher pressure ratios, the VE decreases due to increased losses within the compressor.
The greater potential pressure differential at higher pressure ratios causes the inherent gas leaks within the compressor to increase. As well as re-expanding vapor that is trapped in the cylinders at top-dead-center. Some vapor does not leave the compressor, but re-expands when the piston moves back down.
All of these impact the actual volume of vapor that can be pumped. So in Imperial terms, the swept volume is usually called CFM. The gas that is pumped is called ACFM (Actual CFM). ACFM divided by CFM is the overall VE.
The ACFM is the amount of gas the compressor will pump at a specific set of operating conditions.
From this point it's all thermodynamics.
CFM * vapor density (lbm/ft^3) = mass flow (lbm/minute)
The reciprocal of density is volume.
If the suction gas is saturated, it's density will be higher, which will be a lower specific volume. This produces a higher mass flow.
If the suction gas is superheated, it's density will be lower since the specific volume of the superheated gas is greater.
Now if we know the mass flow and the enthalpy difference due to the nature of the system we can find the capacity in Btu per minute.
The vapor enthalpy is a little tricky and depends on whether the suction superheat is useful to the net refrigeration effect or non-useful.
The liquid enthalpy is dependent on the actual liquid temperature feeding the expansion device.
One important thing to watch for, or to verify is: Some manufacturers rate their compressors on evaporating and condensing temperatures. This is not exactly correct as these temperatures occur at those respective devices.
The compressor is operating at discharge pressure and suction pressure, which are different from the evaporating pressure and condensing pressure. You have to account for pressure losses, which the rating tables do not.
On high temperature systems this does not pose too much of a capacity loss. On low temperature application however, you can be as much as 50% low if the pressure losses are not accounted for.
In short, Evaporator Tons is not the same as Compressor Tons is definitely not the same as condenser Tons. In my opinion the term "condenser Tons" is absolutely useless and can lead to a misunderstanding of how to select components.
If an evaporator will produce an equivalent capacity of 50 Tons at -40F, the compressor has to be able to produce a similar capacity. The difference is, to get the -40F vapor back to the compressor you will have some pressure loss in the suction line.
Therefore, the compressor MUST be able to produce the 50 Tons of capacity at a suction pressure equal to the evaporating pressure minus the line losses.
Time for a break...;)
that was the most thorough explaination imaginable,cooljesus himself(myself) couldn't have said it any better,,,,,,,and i've never seen "condensor tons" maybe she is refering to the appropriate cond. capacity based on size of system????must be in purchasing,office,etc. no tech
iceman have you ever seen this exact terminology used ever if so where?????i'm curious
also other than in the actual process of system design why would one even venture into comp. capacity??????what practical determinations could be made if any?
HI cooljesus, (now that's an interesting screen name)
In some of the HVAC catalogs you will see heat rejection listed as Tons which is not correct at all. If you do some digging and some number crunching they mean a condenser ton, or heat rejection ton is equal to about 15,000 BTUH.
This means the evaporator provided about 12,000 BTUH and the compressor work added about 3,000 BTUH. The numbers vary between 14,400 to about 15,000 BTUH depending on who you believe!!!
That only applies to one narrow set of operating conditions, specifically HVAC. Use of this in refrigeration applications will leave you having long discussions with unhappy customers and frustrating long service calls.
Don't blame Kate for this. I used the term especially to attract attention to something I would warn people of. BTW, she is an engineering manager, so she may make you do some math.:D Be nice now.
Because it helps you to better understand how the system works and what impact might contribute to system deficiencies.Quote:
also other than in the actual process of system design why would one even venture into comp. capacity??????what practical determinations could be made if any?
Say you have high discharge temperatures. It could be high suction superheat, normal evaporator superheat with high heat gain in the suction lines, a broken discharge valve, or any other combination of things.
Compressors just pump gas. It is the condition of the gas entering the compressor that determines how much capacity (read this as higher or lower mass flow) it will ultimately have.
The more you know, the better job you can do as a serviceman/tech/engineer.
Another example... take a supermarket refrigeration system. Long suction and liquid lines....
If the TXV's are set for too high of an evaporator superheat, the gas continues to warm up until it enters the compressor. The compressor has to run longer, since the gas is superheated (less mass flow/less capacity).
If the evaporator superheat is set to maintain a lower actual suction temperature at the compressor, the compressor will run less. (more mass flow/ more capacity = less run time) But, you still have to measure the suction superheat at the compressor to ensure no liquid is returning, OK?
If you measure the discharge temperature before and after these changes, you will see a noticeable reduction.
Now, if we can get the liquid colder so each pound of refrigerant will absorb more heat in the evaporator, the compressor will run even less. since it's capacity has increased with the subcooling of the liquid.
That's how I look at things. Warped as it is...;)
Thanks for the wealth of information! I actually didn't intend to introduce the term "condenser tons" and technically speaking, I didn't. I did, however, say "condenser heat of rejection". I think that I made that term up, either that or I picked it up somewhere along the way. Anyway, what I meant was the heat transfer rate at the condenser and I was thinking in terms of Btu/min for an ideal cycle.
What started this whole issue with me is that I need to determine the mass flow rate through a compressor. I know only the "compressor capacity", the compressor power, the SST and the SCT. I am reviewing a spreadsheet that was created by someone else and they have divided the compressor capacity by the change in enthalpy of saturated liquid at the SCT and saturated vapor at SST to determine the mass flow rate. This is a single stage system. The only way that this equation makes sense to me is if this is being treated as an ideal system with no evaporator superheat. Then if compressor capacity somehow "equals" evaporator capacity the enthalpies used would make sense.
This whole approach does not seem right to me - way over-simplified. I will need to digest what you've written and give this some thought. I'm sure that more questions will follow. Thanks for all your help.
Kate (Engineering Manager and PE.........but hey, who's counting?) :)
I was the culprit who started the term "condenser tons". I did so for a reason, as a means to point out some of the strange terminology that can be used or "developed by someone, who does not know better".
Exactly... That's why I mentioned the compressor manufacturers rating methods. You have to breakdown the rating methods to find out if the suction superheat is useful or non-useful. Once you uncover that, you can select the appropriate vapor enthalpy to use for the enthalpy difference.Quote:
Originally Posted by engrkate
The terminology of evaporator Tons equals compressors Tons is not exactly correct. The compressor does have to have a capacity equal to, or slightly greater than the evaporator capability.
However, you must provide an offset for the suction pressure losses to make it meaningful (or correct for that matter).
Look at like this. Think in terms of saturated temperatures. If the evaporating temperature is +20F (-6.6C), then the compressor would be selected on the same capacity as the evaporators at a lower temperature, say +18F (-7.7C). The two degree loss (20F-18F) is the equivalent temperature loss due to the suction line pressure loss.
Some created this spreadsheet in an over-simplified manner. If the pressure losses are not accounted for, that is the same thing as saying the system is not running. After all, that's the only way you will have no pressure losses in the system.:o
Hello Iceman,
Thank you for your information. I would like to continue this discussion a bit more. Some compressor manufactures only provide the cooling capacity at rating condition. In most of the time, we do not use compressors at rating condition. Have you ever tried to calculate compressor performances in other conditions?
The key point is to get the mass flow. Theoretically, if we know the displacement, VE (or the dead volume), and the motor curve (which is needed to determine the motor speed), we could calculate the mass flow and get the capacity. Is it correct? Do you have any experience on this? Thanks
Hi Vincent,
You can calculate the mass for only for the given condition. If you do not know the VE for a range of conditions it is hard to determine.
Having only one data point means you have to extrapolate the data, and then at that point everything is a guess.
Most of the compressor manufacturers will at least provide a range of conditions that do not exceed the operating envelope. If you have this, then yes you can determine the mass flow at various conditions.
However, the mass flow you calculate from a single operating point is only good for that operating condition. Mass flow changes as the suction gas density varies.
Yes, I do it all the time when I encounter a new refrigerant that the manufacturer does not have performance data for. It's an estimate, which is not the same as calorimeter data. But it does allow me to get a reasonably good answer.Quote:
Have you ever tried to calculate compressor performances in other conditions?
The condenser capacity should basically mean the Total Heat Rejection of the system. There are vatious systems to assess the THR based on the Evapotaive temp. , Refrigerant used etc, but the best would be to use a good compressor manufacturers software. In that you get the Compressors rated capacity at the Evaporation Temp., Condensing Temp., and the Refrigerant being used, you also get the mass flow, Evaporator Cooling Capacity and the Condenser Capacity ( Total Heat Rejection ).
:) YOU A HAVE VERY SIMPLE QUESTION!:p
And don't forget that you can plot all this information on the Mollier chart.
More use should be made of this excellent tool.
Maybe we should have a section for Mollier chart info/teaching/questions :)
Oh yes indeed I would enjoy that :)
I Got a seat in BCIT but the colledge has a 2 year wait list currently *Blink* So more I can learn here the better chances on geting in as an aprentice :D
and I've been wanting to learn how to use such a thing but doing all this me self is hard with no nudges in the correct direction ie: reinventing the weel as it where.
I posted a direct link to copelands AE4-1273 but I'm not allow to post links (less thn 15 post) anyhow, the bulletin has very good info on rated capacity and actual capacity as well as all the other factors.
Quote:
Originally Posted by engrkate
Nice thread you have here.
Nice post from US Iceman.
I am again reading my old college books (25+years old) and trying to have a deeper understanding of refrigeration system learned decades ago.
Currently, we are not taking a look on details like this as we have a lot of software from suppliers: compressor, thermal products, controls, tevs etc. to help us in our design.
Hope to read more post and keep this thread going. :)