Xeon th MG Pony
11-16-2007, 05:50 AM
Seems I've dumped a ton of info in a couple of threads so I figure why not put them in a dedicated thread for easy viewing!
Total heat of Rejection:
When a hermetic compressor runs it converts electrical energy to mechanical, a fundamental law of thermal dynamics says nothing can be 100% efficient, thus we have losses, some electrical energy is converted to heat, plus some mechanical energy is converted into heat.
So with a running compressor we must remove this heat added by the 2 losses of energy, as the hermetic is surrounded in cool refrigerant vapor we naturally dump the heat there and discharge it along with all the latent heat absorbed from the evap.
The term Total Heat Rejection refers to A: Rejecting the heat taken from evap & B: The heat taken to cool the compressor its self. On average the heat added by the compressor is between 25 to 35% depending on the type.
Technically we have three heat gains, one is the evaporator, two is the Suction Line, and finally Three is the Compressor, when we add all three together we get the Total Heat of Rejection, and that is the size of the condenser you will need, in order to get good efficiency it is considered good practice to then over size it by 15 - 25% to help give reserve capacity during hot pull down or un-usually hot days.
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the term of heat rejected by the compressor means How much heat the compressor can move at a set of points for example it can move 6,000BTU/h @ 35C (95f) condensing @ 7.6C (45.7F) Evaporating. You use this to match a compressor to your application.
Things you need to select a compressor:
The evap temp
Amount of heat to be removed at evap temp
Refrigerant type
Condensing temp
The compressor must fit into the operating window of the design.
__________________
Design Data:
Standard delta for Water cooled HX: 10K
Standard entering water on recirculating loop 29.4C (85F) (EDIT: The 29.4C (85F) will be measured at HX inlet 6" back)
Standard Multiplier for hermetic compressors for heat rejection 35% ie Total heat absorbed from evap * 35% = Total heat to be rejected
Standard design temp for Ambient 35C (95F)
Standard required flow for recirculated water system 3GPM/Ton H
8 to 15K Delta is acceptable for a water cooled HX with an ideal condensing range of 29.4C to 43.3C (85F to 110F)
Max Discharge 99C to 121C (210F to 250F) your oil will carbonize on the valves and refrigerant will turn to acid (Measures 6" away from compressor outlet
Max compression ratio for Hermetics 20:1, 2 stage should be used after 10:1
Rotary compressors max compression ratio 5:1.
For a high efficiency Air cooled condenser 15 to 20K Delta
1000FPM Face velocity with around 600-1200CFM/Ton H
Ideal condensing temperature range is between 29.4C to 43.3C (85F to 110F) for air cooled
Standard efficiency Condensers are 20 to 35K Delta air flow the same as above
Calculating the amount of heat need to be removed to reach a certain temp: Qs=M*C*Dt
Qs = Sensible Heat in BTUs
M = Mass
C = The constant of specific heat
Dt = Delta T, Starting temp - ending temp
Total heat of Rejection:
When a hermetic compressor runs it converts electrical energy to mechanical, a fundamental law of thermal dynamics says nothing can be 100% efficient, thus we have losses, some electrical energy is converted to heat, plus some mechanical energy is converted into heat.
So with a running compressor we must remove this heat added by the 2 losses of energy, as the hermetic is surrounded in cool refrigerant vapor we naturally dump the heat there and discharge it along with all the latent heat absorbed from the evap.
The term Total Heat Rejection refers to A: Rejecting the heat taken from evap & B: The heat taken to cool the compressor its self. On average the heat added by the compressor is between 25 to 35% depending on the type.
Technically we have three heat gains, one is the evaporator, two is the Suction Line, and finally Three is the Compressor, when we add all three together we get the Total Heat of Rejection, and that is the size of the condenser you will need, in order to get good efficiency it is considered good practice to then over size it by 15 - 25% to help give reserve capacity during hot pull down or un-usually hot days.
__________________
the term of heat rejected by the compressor means How much heat the compressor can move at a set of points for example it can move 6,000BTU/h @ 35C (95f) condensing @ 7.6C (45.7F) Evaporating. You use this to match a compressor to your application.
Things you need to select a compressor:
The evap temp
Amount of heat to be removed at evap temp
Refrigerant type
Condensing temp
The compressor must fit into the operating window of the design.
__________________
Design Data:
Standard delta for Water cooled HX: 10K
Standard entering water on recirculating loop 29.4C (85F) (EDIT: The 29.4C (85F) will be measured at HX inlet 6" back)
Standard Multiplier for hermetic compressors for heat rejection 35% ie Total heat absorbed from evap * 35% = Total heat to be rejected
Standard design temp for Ambient 35C (95F)
Standard required flow for recirculated water system 3GPM/Ton H
8 to 15K Delta is acceptable for a water cooled HX with an ideal condensing range of 29.4C to 43.3C (85F to 110F)
Max Discharge 99C to 121C (210F to 250F) your oil will carbonize on the valves and refrigerant will turn to acid (Measures 6" away from compressor outlet
Max compression ratio for Hermetics 20:1, 2 stage should be used after 10:1
Rotary compressors max compression ratio 5:1.
For a high efficiency Air cooled condenser 15 to 20K Delta
1000FPM Face velocity with around 600-1200CFM/Ton H
Ideal condensing temperature range is between 29.4C to 43.3C (85F to 110F) for air cooled
Standard efficiency Condensers are 20 to 35K Delta air flow the same as above
Calculating the amount of heat need to be removed to reach a certain temp: Qs=M*C*Dt
Qs = Sensible Heat in BTUs
M = Mass
C = The constant of specific heat
Dt = Delta T, Starting temp - ending temp