Multistage Compression Systems |
Multistage or booster compressors/ systems are used for low temp or extra low temp applications for the following reasons
-Decrease compressor ratio
-To obtain ultra low temp for cryogenic applications
*Note: As a general staging is required when compressor ratios on reciprocating compressors exceed 10 to 1 or approx -40 F of SST. Thus avoiding high discharge temps and excessive compressor ratios
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Types of Multi Stage Or Booster Compression Systems |
-Direct staging
-Cascade Staging
Direct Staging System
1. Requires refrigerants with boiling point low enough for the application yet will condense at a normal air or water temperatures
2. Direct staging practical limit ranges from -125 F for R-12, R-134A, R-502 or R-404A
3. Inter Coolers are required to cool the discharge gas from the first stage compression before entering the second stage
Cascade Staging
1. Requires use of various refrigerants
2. Normally R-13 B1 or R-502 (R-404A or R-507 is used for low stage)
3. Low stage refrigerant is condensed at rather low temps by the use of R-12, R-134A, R-22, or R-502 (R-404A or R-507)
4. Receivers are not generally used in cascade systems.
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Direct Staging systems |
Intercoolers
-Purpose: With direct staging cooling of refrigerant vapor between the various stages of compression is necessary in order to avoid overheating of the higher stage compressor (excessive discharge temperature)
Types:
-Open flash type
-Closed shell + coil type
-Dry expansion
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Multi Stage compression Systems |
Types of intercoolers: Flash type (open)
-Liquid from condenser is expanded in the intercooler and its temp reduced by flashing down to an intermediate temp. In turn the discharge gas is desuperheated by causing it to bubble up through liquid in the subcooler
-Advantages:
-Simple design
-Low $$$ cost
-Good liquid temp reduction
-One type of refrigerant
-booster bypass used for capacity control
-The LLS connected to the suction line will be liquid injection to keep the suction line temp down
Types of intercoolers: Shell + Coil Type
-Only a portion of liquid from the condenser is expanded in the intercooler while the rest passes through the intercooler by way of a coil submerged in the liquid. Thus the pressure is not reduced to saturation but rather subcooled instantly.
-Advantages:
-Elimination of flash gas at 10 to 20 F subcooling
-Higher liquid line pressure than open type
Dry Expansion Type
- The liquid is subcooled as it passes through the intercooler. Desuperheating occurs by overfeeding so that a small amount of liquid is injected into the discharge line of the first stage compression where it vaporizes and cools the hot gas
-Advantages:
-Good liquid subcooling
-Good desuperheating ability
-Simple in operation
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