Does EPR rises Compressor capacity???

[georgedvf]

Hello everybody!

I'm in a dilemma... I'v being designing systems for Commercial Refrigeration for 2 years, but now talking to an Engineer in a Convention I assited a week ago, we discussed the way to select compressors in a parallel system where different evaporation temperatures are present (Different rooms, different emps). We both use EPR valves so the compressors will work at the same inlet pressure, but the dilemma comes when he tells me he uses a "factor" to rise the calculated capacity of the compressors proportional to the differential of the pressure before and after the EPR in the circuits where temperature are higher, in other words: he says that if a EPR is used in parallel systems, compressors should be calculated using a "factor" that rises their capacity...

Have you hear something like this before??????

Thanks in advanced!

[georgedvf]

Ok guys, those who didn't understand the question please say I.... jejeje......

Seriously, nobody has heard about this at all????

[US Iceman]

The use of EPR's determines what the lowest suction pressure is for the compressor. Whatever the lowest evaporating temperature is the compressor has to meet this + all other loads. One thing that is sometimes forgotten is the compressor has to meet all loads at the lowest pressure because the other loads are operating at a higher temperature/pressure and open to the lowest pressure in the suction line.

What it sounds like he is doing is adding a correction factor to say: when the lowest temperature load is off the rest of the evaporators are operating at higher suciton pressures so you might as well raise the suction pressure to match the loads. Doing this might increase the heat rejection to the condenser since the compressor capacity is now higher.

[mad fridgie]

If total load is required, then compressor must match this loead st the lowest SST. If the loads are split and happen at different times, then you need to calculate using 2 different compressor configurations.
If the system is a combination, then you can use a floating suction set point. ( for efficiency)
For example if the LT system is controlled by a liquid solenoid valve, and comp(s) load/unloading is by suction set point, then the set point can be raised when LT is not in use allowing the compressor to unload and reducing power draw, increasing efficiency

[mbc]

To calculate load of comp you should go for lowest suction pressure and if your using EPR's do not forget two thing 1- put check valve on suction pipe of rooms has a high temperature than lowest room temperature and also check rang of comp in suction pressure it can work if it can not work , you have to put CPR ( Crankcase Regulator pressure ) for them to protect your comp <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p>
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pressure to match the loads. Doing this might increase the heat rejection to the condenser since the compressor capacity is now higher

[Sandro Baptista]

Hi Guys,

My opinion is the following...see the file. This justify the increasing of capacity. R404A as example and no superheat has been considered just for simplicity.

Sandro

[HallsEngineer]

Simple answer. Efficiency. Buy a screw on a vsd with a different machine for each system then have them properly commissioned with a full cop result and proof of capacity test done at the end. Bigger install cost but it will pay for it's self in a few years.

[georgedvf]

Thanks to all.

Sandro:
could you explain me a little more about your answer, I really don't understand the increase of capacity you show on the diagram.

Thanks!!!

[Sandro Baptista]

Thanks to all.

Sandro:
could you explain me a little more about your answer, I really don't understand the increase of capacity you show on the diagram.

Thanks!!!

Geogedvf,

The difference of enthaply between +45ºC and -5ºC will be bigger than +45ºC and -30ºC. If you have refrigerated stores at +2ºC DT=7ºC, for example, the compressor (at -30ºC) will give more capacity due to the increased superheat. Of course the specif volume will increase but for R404A/R507 the final effect is still a little increase of capacity.


Sandro

[georgedvf]

Geogedvf,

The difference of enthaply between +45ºC and -5ºC will be bigger than +45ºC and -30ºC. If you have refrigerated stores at +2ºC DT=7ºC, for example, the compressor (at -30ºC) will give more capacity due to the increased superheat. Of course the specif volume will increase but for R404A/R507 the final effect is still a little increase of capacity.


Sandro

Sandro,

I have the idea that the difference of enthalpy shown in the diagram means the evaporating capacity of the system, but not for the compressor capacity.

Why do you say that the difference of enthalpy is the compressor capacity?

Also, if you go to the manufacturer's capacity tables for compresors you will see that at +45ºC and -5ºC a compressor has more capacity than at +45ºC and -30ºC.

If I'm wrong, please correct me :confused:

[D.D.KORANNE]

thumb rule : LOAD ON THE LOWEST TEMPERATURE EVAPORATOR SHOULD NOT BE LESS THAN 60% PREFERABLY MORE OF THE TOTAL LOAD . AND LOW TEMP EVAPORATOR SHOULD HAVE NRV IN SUCTION LINES ( AT THE OUTLET OF THE EVAP)

[Sandro Baptista]

Sandro,

I have the idea that the difference of enthalpy shown in the diagram means the evaporating capacity of the system, but not for the compressor capacity.

Why do you say that the difference of enthalpy is the compressor capacity?

Also, if you go to the manufacturer's capacity tables for compresors you will see that at +45ºC and -5ºC a compressor has more capacity than at +45ºC and -30ºC.

If I'm wrong, please correct me :confused:


georgedvf,

The difference of enthalpy that is hatched means the increase of capacity that the compressor will have due the fact the system can take chance of the extra superheat at the suction compressor.

Sorry, but on the previous emails I never saida that a compressor at -5ºC gives less capacity than at -30ºC. Sorry if I didn't explain too well.

[Sandro Baptista]

thumb rule : LOAD ON THE LOWEST TEMPERATURE EVAPORATOR SHOULD NOT BE LESS THAN 60% PREFERABLY MORE OF THE TOTAL LOAD . AND LOW TEMP EVAPORATOR SHOULD HAVE NRV IN SUCTION LINES ( AT THE OUTLET OF THE EVAP)

Koranne,

My opinion is if the low temperature aircoolers demand a great part of cooling capacity on the compressor and / or if the compressor can pull down well the suction pressure to low temperature regime (to temperatures below the cold stores temperatures, is enough) then no need of check valves (they represent a source of suction head loss).