Power required in a compressor

[Friobernal]

Hi all,

Definitely i know nothing about refrigeration!!!

Today i was reading a brochure of a manufacturer of compressors, when i realised that, for the same compressor, when the evaporating temperature becomes lower, the Refrigerant capacity becomes lower too (OK i´m agree looking a P-h diagram) but the power required also decreases... WHY??

My first thought was; ok it is a semi-hermetic compressor, let´s look open compressors, and happens the same

Please an answer!!! (I have to come back to school )

Regards

[Gary]

As the evaporating temperature rises, the pressure in the low side of the system rises. With higher pressure pushing it, more refrigerant vapor is forced into the cylinder on each downstroke of the piston, thus the compressor capacity is increased. Since the compressor is doing more work (pumping more refrigerant vapor), more energy is required.

[Prof Sporlan]

As Gary notes, the compressor must compress greater refrigerant mass at higher evaporator pressures. Not only must the compressor work harder, it also pumps more pounds of refrigerant.

It is also worth noting that the fact a compressor consumes less power at lower suction pressures does not mean it will operate more efficiently at lower suction pressures. In fact, the opposite will be the case if the compressor isn't being overloaded.

[bersaga]

Dear Friobernal,

To further get what Gary and the Prof had explained , just go to any compressor manufacturer's website e.g.

http://www.bitzer.de or
http://www.bock.de
or
grasso or copeland

Download their compressor selection software - select a compressor - and then vary the evaporating temperature say, from -30 to 0 ( depending on the operating envelope ) and at each Tevap , record down the 1. Refrigeration Capacity, Power required and the Mass Flowrate and you will see the trend. ( Fix the condensing temp and other parameters i.e subcool,superheat etc )

Also the P-h diagram appears to show that if the Tevap is lowered , the delta H of the compressor increases and this gives the false impression that the compressor power intake is higher. But then :

Power required by the compressor is proportional to the product of 'delta H of the compressor ' and ' the mass flowrate '

( delta H of compression = ideal work of compression)

And from your observation of the data you have recorded, you will find that the mass flowrate - kg/s- ( due to the higher specific volume m3/kg) gets lower as the T evap decreases.

If you actually plot this power curve, you will find that at very low and very high evaporating temperatures, the power required is low and therefore there is a peak between these extremes. In reality. manufacturers do not make compressor to cover such a wide range of loads.

Hope this helps !