Case-8 Basic Performance of Screw Compressor...From the compressor performance table of Figure 8-5, model with economizer: WB-II-316E at condensing pressure of 217.9 Psig and suction

Case – 8 Basic Performance of Screw Compressor

Copy Right By: Thomas T.S. Wan （温到祥)

June 24, 2010 All Rights Reserved

Case Background: The basic rating and performance given by a compressor manufacturer is usually the bare rating of the compressor without any discharge and suction penalties, not even include the pressure drops of the standard option of the discharge valve, discharge check valve, suction valve suction strainer and etc. In order to understand the basic performance of the screw compressor, it is best to know the performance table published by the compressor manufacturer and make the compressor selection by using these performance table and curves. The followings are three different inquiry examples; each requires using the performance table to make screw compressor selection, by compare the selection results, it shall provide a better understanding of the basic performance of the screw compressor given by the manufacturer. Inquiry No. 1: Refrigeration capacity 252 TR, saturated condensing temperature is 105ºF,

evaporative temperature is -15ºF, R-717, 380-3-50 power supply.

Water cooled condenser with pre-lube oil pump. Water cooled oil cooling.

10ºF liquid subcooling in condenser. No economizing is allowed. 10ºF suction superheat.

Use the information layout in the section of Rated Technical Data and Engineering Information for the Case; make a compressor selection to meet the operating conditions specified. What is (1) Compressor Size Selected, (2) Power Consumption in BHP and (3) BHP/TR.

Inquiry No. 2: Refrigeration capacity 252 TR, saturated condensing temperature is 105ºF,


Water cooled condenser with pre-lube oil pump.

Water cooled oil cooling. No liquid subcooling in condenser. Flash type economizing is used. 10ºF suction superheat.


Inquiry No. 3: Refrigeration capacity 252 TR, saturated condensing temperature is 105ºF,


Water cooled condenser with pre-lube oil pump. Water cooled oil cooling.

No liquid subcooling in condenser. Flash type economizing is used. 20ºF suction superheat. Discharge external piping pressure drop is 3.7 Psi. External suction piping pressure drop is 1.3 Psi.


Compare the compressor sizes, power consumptions and BHP/TR for the above three inquiries of 1.0, 2.0 and 3.0:

Inquiry No. 1 Inquiry No. 2 Inquiry No. 3 No Economizing No ext. pres. drop

With economizing No ext. pres. drop

With economizing With ext. pres. drops

Compressor Size Selected

Power Consumption BHP

BHP/TR

Related Technical Data and Engineering Information for the Case:

Figure 8-1 Performance Table for WB-II-222E Compressor R-717 (Model with Economizer)

Figure 8-2 Performance Table for WB-II-270 Compressor R-717 (Model without Economizer)


Figure 8-4 Performance Table for WB-II-316 Compressor R-717 (Model without Economizer)


Figure 8-6 Performance Table for WB-II-399 Compressor R-717 (Model without economizer)

Figure 8-7 Typical Part Load Power Input

Constant Condensing Temperature for High Stage Compressor

Figure 8-8 Typical Part Load Power Input with

Falling Condensing Temperature for High Stage

Figure 8-9 Standard Pre-Lube System Limit for High Stage

Clarifications for Performance Ratings of WB-II Compressor Units

1.0 Standard Conditions for the High Stage Ratings:

2.0 Selection Procedure for High Stage Compressors:

3.0 Condenser Liquid Subcooling Correction Factors for High Stage

Compressors:

4.0 Suction Superheat Correction Factors for High Stage Compressors:

Cogitation: 1.0 Inquiry No. 1: Refrigeration capacity: 252 TR Saturated condensing temperature: 105°F (CT) Saturated suction temperature: -15°F (Same as ET) Refrigerant: R-717 Power 380-3-50. compressor speed: 2,950 RPM Pre-Lube oil pump. Water cooled oil cooling. 10°F Liquid subcooling in condenser. 10°F Suction superheat. No economizing. This shall be a standard rating for high stage compressor except that the compressor

is driven by a 50 Hz motor at 2,950 RPM compressor speed. (See No. 1 and No. 2 of the Clarifications for Performance Ratings of WB Compressors):

Capacity and power consumption correction factor is 0.833. Converting the capacity

to 60 Hz, 3,550 RPM compressor speed rating, it shall be 252/0.833 = 302.5 TR. From the compressor performance table Figure 8-6, compressor without economizer: WB-II-399 at CT 105°F and ET of -15°F at 60 Hz: Capacity = 359.6 TR Power consumption = 831.7 BHP Correction Factor Capacity and power consumption is 0.833 for 50 Hz of 2,950

RPM compressor speed: Corrected Capacity = 0.833 x 359.6 = 299.7 TR Corrected Power consumption = 0.833 x 831.7 = 690.3 BHP At the rated capacity of 252 TR as specified, the compressor is actually at partial

load; refer to Figure 8-7, the partial load power input with constant condensing temperature at evaporative temperature of –15°F, the power consumption is calculated as the following:

Percent of capacity = (252/299.7) = 0.841 The power consumption correction factor = 0.88 Therefore, power consumption at 252 TR is calculated:

= 690.3 x 0.88 = 607.5 BHP Conclusion: Compressor selected: WB-II-399 Capacity at rated conditions: 252 TR Power consumption: 608 BHP 2.0 Inquiry No. 2. Refrigeration capacity: 252 TR Condensing temperature: 105°F Evaporative temperature: -15°F Refrigerant: R-717 Power 380-3-50. compressor speed: 2,950 RPM Pre-Lube oil pump. Water cooled oil cooling. No liquid subcooling in condenser. 10°F Suction superheat. Flash type economizing is used. Capacity Correction Factor for 0°F liquid subcooling for condenser: (See No. 3 of the Clarifications for Performance Ratings of WB Compressors): S.C.C.F. = 1 + (S.C. - 10°F) x 0.0025 = 1 + (0 – 10) x 0.0025 = 0.975 Correction Factors for 50 Hz of 2,950 RPM compressor speed: Capacity Correction Factor: 0.833 Power Consumption Correction Factor: 0.833 The capacity of the compressor at 60 Hz 252 = -------------------------- = 310.3 TR 0.975 x 0.833 From the Figure 8-5, the standard ratings for WB-II-316E compressor, the

compressor model with economizer at condensing temperature of 105°F and evaporative temperature of -15°F at 60 Hz:, the capacity and power consumption for the compressor are as the following:

Capacity = 340.6 TR Power consumption = 713.3 BHP Correct the capacity and power consumption for 50 Hz operation: Corrected Capacity = 340.6 x 0.975 x 0.833 = 275.6 TR Corrected Power consumption = 713.3 x 0.833 = 594.2 BHP At the rated capacity of 252 TR as specified, the compressor is actually at partial

load; refer to Figure 8-7, the partial load power input with constant condensing temperature at evaporative temperature of –15°F, the power consumption is calculated as the following:

Percent of capacity = (252/275.6) = 0.914 The power consumption correction factor = 0.93 Therefore, power consumption at 252 TR is calculated: = 594.2 x 0.93 = 552.6 BHP Therefore: Compressor selected: WB-II-316E Capacity at rated conditions: 252 TR Power consumption: 553 BHP 3.0 Inquiry No. 3 Refrigeration capacity: 245 TR Condensing temperature: 105°F Evaporative temperature: -15°F Refrigerant: R-717 Power 380-3-50, compressor speed: 2,950 RPM Pre-Lube oil pump. Water cooled oil cooling. No liquid subcooling in condenser. 20°F Suction superheat. Flash type economizing is used.

Discharge external piping pressure drop is 3.7 Psi. External suction piping pressure drop is 1.3 Psi.

The condensing pressure at 105°F is 214.2 Psig, discharge external piping pressure drop is 3.7 Psi.

Compressor discharge pressure: = 214.2 + 3.7 PSI = 217.9 PSIG The saturated evaporative pressure at 6.2°F is 6.2 Psig, the external piping pressure

drop is 1.3 Psi Compressor suction pressure: = 6.2 + 1.3 PSI = 4.9 PSIG From the compressor performance table of Figure 8-5, model with economizer: WB-II-316E at condensing pressure of 217.9 Psig and suction pressure of 4.9 Psig: Capacity = 319.7 TR Power consumption = 706.6 BHP Correction for 50 Hz of 2,950 RPM compressor speed: Capacity Correction Factor: 0.833 Power Consumption Correction Factor: 0.833 Capacity Correction Factor for 0°F liquid subcooling for condenser: (See Selection Guide, Rule No. 3: S.C.C.F. = 1 + (S.C. - 10°F) x 0.0025 = 1 + (0 – 10) x 0.0025 = 0.975 From No. 4 of the Clarifications for Performance Ratings of WB Compressors,

Suction Superheat Correction Factors for for 20°F suction superheat: 1 SHCF = ----------------------------------------- 1 + (S.H. – 10°F) x (0.0027) 1 = -------------------------------------- 1 + (20 – 10°F) x (0.0027) = 0.9737 Corrected capacity = 319.7 x 0.833 x 0.975 x 0.9737 = 252.8 TR

Corrected Power consumption = 0.833 x 706.6 = 588.8 BHP Therefore: Compressor selected: WB-II-316E Capacity at rated conditions: 252 TR Power consumption: 587 BHP Comparisons: Compare the compressor sizes, power consumption and BHP/TR for the above Inquiries of 1.0, 2.0 and 3.0:

Question No. 1 Question No. 2 Question No. 3 No economizing No ext. pres. drop

With economizing No ext. pres. drop

With economizing With ext. pres. drop

Compressor Size Selected

WB-II-399

WB-II-316E

WB-II-316E

Power Consumption BHP

608

553

587

BHP/TR

2.41

2.19

2.33

Conclusion; (A) Inquiry No. 1: The compressor is without economizer. The compressor is

larger and having higher power consumption, even no external piping pressure drops are included.

(B) Inquiry No. 2 is the compressor with economizer, but, without external

piping pressure drops being considered. The selection results in smaller size compressor and lower power consumption than the Inquiry No. 1.

(C) The compressor size of Inquiry No. 3 has higher power consumption than

the Inquiry No. 2, because external piping drops and superheat are included in the operating conditions; yet the compressor size and the power consumption are smaller than the compressor for Inquiry No. 1.

(D) The operating conditions for Inquiry No. 1 and No. 2 are idea cases; it

does not reflect the actual working condition. In an actual system design, discharge, suction external piping pressure drop and suction superheat must be included.

Documents

Case-8 Basic Performance of Screw Compressor...From the compressor performance table of Figure 8-5, model with economizer: WB-II-316E at condensing pressure of 217.9 Psig and suction