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stuman
24-06-2008, 06:36 AM
I have a consultants reports from several years ago that I just cannot get to add up.

We have a hall screw compressor model 2031 running at 50 Hz. Accoridng to the hall web site, the model 2031 has a swept volume of 1728 m3/hr @ 50 Hz

Running ammonia with a suction temp of -2C and a discharge temp of 33C, they got 1590 kW of cooling. When the suction temp lowers to -9C, they got 1180 kW.

Based on my calcs
-2C hg = 1442.2 kj/kg
33 hf = 337.7 kj/kg

delta = 1104.5 kj/kg
vg at -2C = 0.3110 m3/kg

hence with the 2031 running at 2980 rpm (1728 m3/hr) I get 1704 kW at 100% efficiency. They have included separately 297 kW for motor power.

Can someone please explain to me why there is a difference? Also based on my calcs, changing to -9C suction only lowers the cooling to 1692 kW, not 1180!

Any help in understanding this would be great.

Thanks
Stu

Rajkumar
24-06-2008, 08:38 AM
Your calulation are oK but you have not considered subcooling/ superheating.
Take the same to rectify the calculation.

Raj

GXMPLX
24-06-2008, 08:40 PM
Your calculations are all wrong! You are using saturated suction vapour condicions (apples) and saturated liquid at discharge conditions (pears). Apple and pears cannot be subtracted to calculate capacity.

You can get the correct calculations all over the net, like here:
http://en.wikipedia.org/wiki/Vapor-compression_refrigeration

Two more hints: Compressor discharge conditions ar not even near saturation and you should try to understand your assignment before asking someone else to do your homework.

US Iceman
24-06-2008, 10:48 PM
Calculating compressor capacity is a sticky issue. The first thing you have to verify are the operating conditions at the compressor. That's where the capacity is determined.

If you have subcooling for the liquid then the liquid enthalpy value is lower. The interesting issue is how are you measuring the suction conditions since they determine the gas density & resulting volume.

If the suction gas has some superheat in it then the gas density is less and the volume increases. In this case the vapor enthalpy will be higher because of the total heat content (phase change + superheat).

In an industrial system you can consider the superheat as non-useful to the refrigeration effect (to be safe).

If the suction gas is essentially saturated the gas density is higher and specific volume is slightly lower. However, the vapor enthalpy will also be less.

Any calculation can be made to look better by adjusting the assumptions used for determining compressor capacity. It happens all the time in manufacturers catalogs. Look and see how much subcooling and superheat they include. Either of these can be changed to artificially inflate the capacity. And, the important thing to remember is... the compressor will only develop that capacity at those conditions.

It's not surprising to see the difference you calculated to be less than the consultants report. As the suction pressure decreases the pressure ratio increases. This causes a reduction in the volumetric efficiency (VE) of the compressor. Therefore, swept volume X VE = actual volume flow while operating at that condition.

The swept volume is the same for a fixed speed, but the VE changes (+/-) as the overall pressure ratio changes also.

The bottom line answer is: You can't compare or calculate compressor capacity unless you know the exact operating conditions on which the performance values are based off of.

GXMPLX
25-06-2008, 01:10 AM
Compressor capacity can be sticky only in field measurement because you have no easy accurate way to estimate mass flow but it's very easy in the lab.

It is also easy in the field if you can install a flowmeter or have any device that can act as a calibrated orifice (like a BPHE or something you have enough accurate information of flow vs pressure drop).

It seems to me in this case he needs to know where in the cycle the compressor capacity is measured! Then he will begin to understand what you are talking about.

Peter_1
25-06-2008, 07:24 AM
Your compressor is not running at 100% in real world conditions.
Pressure ratio is also important like US Iceman told you.

GXMPLX
25-06-2008, 03:49 PM
Your compressor is not running at 100% in real world conditions.
Pressure ratio is also important like US Iceman told you.

That's exactly why you should use a flow meter!

US Iceman
25-06-2008, 04:40 PM
Flow meters and orifice plates are nice for lab work but present more issues with trying to use these in the field. Placement and interpretation of data is more complex than the subject at hand.

The easiest way to do this is use the manufacturers swept volume and the refrigerant properties. You can derive the VE, but it's only meaningful if you use the correct operating conditions and properties in the first place.

Or as you say, you can do an energy balance across the evaporator if you can get the data. This might be more difficult than just making some good assumptions and using good judgements though.

GXMPLX
25-06-2008, 06:22 PM
Flow meters and orifice plates are nice for lab work but present more issues with trying to use these in the field. Placement and interpretation of data is more complex than the subject at hand. .
That's why in the field you can use anything that works like an orifice. It could even be a piece of pipe if it has enough pressure drop for your instruments to measure and you have information on pressure drop vs mass flow.


The easiest way to do this is use the manufacturers swept volume and the refrigerant properties. You can derive the VE, but it's only meaningful if you use the correct operating conditions and properties in the first place. .
This is what stuman is trying to do but will get inaccurate results not knowing the compressorīs volumetric efficiency.
His main mistake was not using the correct points in the cycle. And by correct I mean they have to be the catalog's conditions for stating it's capacity, only doing this he can compare figures. But will get differences because of using swept volume.

Or as you say, you can do an energy balance across the evaporator if you can get the data. This might be more difficult than just making some good assumptions and using good judgements though.
Glad we agree in something. But stuman's original problem was comparing catalog values to calculated values (based on catalog data), it is you who brought "measuring and assumptions" into this problem ... I just got carried away!

US Iceman
25-06-2008, 06:59 PM
And by correct I mean they have to be the catalog's conditions for stating it's capacity, only doing this he can compare figures. But will get differences because of using swept volume.


That is very true. If the exact conditions that are used in the catalog are not used to calculate the data the other numbers are meaningless. However, to do this correctly you have to know if the superheat that may have been used as the suction rating is useful or non-useful to the net refrigeration effect as it will increase or decrease the performance values. Assuming the superheat is useful or not is what I meant by the assumptions that can be used.

Sometimes the manufacturer will not state this in the catalog. It's possible to artificially inflate the ratings by using different data, so whoever is doing this needs to know what the assumptions are and what they are based on.

Assumptions also comes into play with the measuring of data as I'm sure you will agree. That's why I said what I did.;)

We both probably agree on more than what we have discussed so far. You have to admit it's difficult to guess at what someone means if they don't write it out plainly (and that's not entirely directed at you:D).

GXMPLX
25-06-2008, 08:24 PM
... is useful or non-useful to the net refrigeration effect as it will increase or decrease the performance values.
For the way I've seen these measures done (generally not under any standard) the evaporator is too close to the compressor to be able to differentiate between useful superheat or not.
I'd say this is a system distinction, not an issue if you want to measure the compressor capacity.
What is useful or not is application and installation and maintenance dependant.


Sometimes the manufacturer will not state this in the catalog. It's possible to artificially inflate the ratings by using different data, so whoever is doing this needs to know what the assumptions are and what they are based on.

If I published a catalog Iīd use it all, yes you are inflating it but not in a unethycal way.
Not stating subcooling would be wrong! Many standards now a days ask for no subcooling.

I live on assumptions! Yes we agree ... except that I don't like to smell that stuff!

GXMPLX
25-06-2008, 08:48 PM
... it's difficult to guess at what someone means if they don't write it out plainly (and that's not entirely directed at you:D).

I forgot to tell you Iīm sorry for this, but (as my wife says) I tend to talk too much too!