04TRK Sistem Multistage

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1Kuliah 4.Sistem Refrigerasi Kompresi Uap Multistage[Sistem Tekanan Jamak ]POKOK BAHASAN :Multi-compression systemsMulti-evaporator systemsCascade systemsM. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika12

M. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika3

M. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika4Flash tankUntuk memisahkan gas cetus [ flash gas ] pada tekanan intermedit

flash gas tidak ada kontribusi nya teradap Efek Pendinginan meningkatkan kerugian tekan pada evaporator. harus dikompressi ke tekanan kondenser meningkatkan kerja kompressor.M. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika5Intercooling dalam kompressi multi-stageKerja input kompressi , reversibel, politropik uap refrigeran dapat dinyatakan sbb:

Intercooling dalam kompressi dua tingkat


Intercooling menggunakan refrigeran cair dalam flash tankIntercooling menggunakan air pendingin dalam Heat Exchanger

Intercooling dalam kompressi multi-stageM. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika71. Multi-stage system with flash gas removal and intercooling

A. Multi-compression systems


Balans massa dan energi flash tank:

Lanjutan...M. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika9The above system offers several advantages:

a) Quality of refrigerant entering the evaporator reduces thus giving rise to higher refrigerating effect, lower pressure drop and better heat transfer in the evaporatorb) Throttling losses are reduced as vapour generated during throttling from Pc to Pi is separated in the flash tank and recompressed by Compressor-II.c) Volumetric efficiency of compressors will be high due to reduced pressure ratiosd) Compressor discharge temperature is reduced considerably.M. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika10

Refrigeration system with liquid subcoolerOne disadvantage of the above system is that since refrigerant liquid in the flash tank is saturated, there is a possibility of liquid flashing ahead of the expansion valve due to pressure drop or heat transfer in the pipelines connecting the flash tank to the expansion device. Sometimes this problem is tackled by using a system with a liquid subcooler.M. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika112. Use of flash tank for flash gas removal only

M. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika123. Use of flash tank for intercooling only


1. Multi-evaporator system with A single stage system and Individual expansion valves

B. MULTI-EVAPORATOR SYSTEMSM. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika142. Multi-evaporator system with single compressor and individual expansion valves


3. Multi-evaporator system with single compressor and multiple expansion valves

h1M. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika16

4. Multi-evaporator system with multiple compressors and a flash tank for flash gas removal and intercooling

M. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika175. Multi-evaporator system with individual compressors and multiple expansion valves

M. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika18Since only one refrigerant is used throughout the system, the refrigerant used should have high critical temperature and low freezing point.

b) The operating pressures with a single refrigerant may become too high or too low. Generally only R12, R22 and NH3 systems have been used in multi-stage systems as other conventional temperatures. Operation in vacuum leads to leakages into the system and large working fluids may operate in vacuum at very low evaporator compressor displacement due to high specific volume.

c) Possibility of migration of lubricating oil from one compressor to other leading to compressor break-down.

The above limitations can be overcome by using cascade systems.Limitations of multi-stage systemsM. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika19CASCADE SYSTEMSApplications of cascade systems:Liquefaction of petroleum vapoursLiquefaction of industrial gasesManufacturing of dry iceDeep freezing, etc.

Advantages of cascade systems:Since each cascade uses a different refrigerant, it is possible to select a refrigerant that is best suited for that particular temperature range. Very high or very low pressures can be avoidedMigration of lubricating oil from one compressor to the other is preventedM. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika20

Cascade refrigeration systems Very low temperatures can be achieved by operating two or more vapor-compression systems in series, called cascading. The COP of a refrigeration system also increases as a result of cascading. M. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika211. Two-stage cascade refrigeration system

In a cascade system a series of refrigerants with progressively lower boiling points are used in a series of single stage units. The condenser of lower stage system is coupled to the evaporator of the next higher stage system and so on. The component where heat of condensation of lower stage refrigerant is supplied for vaporization of next level refrigerant is called as cascade condenser.

For a two-stage cascade system working on Carnot cycle, the optimum cascade temperature at which the COP will be maximum, Tcc,opt is given by:M. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika22For cascade systems employing vapour compression refrigeration cycle, the optimum cascade temperature assuming equal pressure ratios between the stages is given by:

Where, b1 and b2 are the constants in Clausius-Clayperon equation:ln P = a b/T , for low and high temperature refrigerants, respectively.M. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika232. Auto-cascade system two-stage

An auto-cascade system may be considered as a variation of cascade system, in which a single compressor is used (Ruhemann in 1946). In a two-stage auto-cascade system two different working fluids; a low boiling point (low temperature) refrigerant and a high boiling point (high temperature) refrigerant are used. In practice, more than two stages with more than two refrigerants can be used to achieve very high temperature lifts.These systems are widely used in the liquefaction of natural gas.

Fig.Schematic of a two-stage auto-cascade systemFig.Schematic illustrating principle of two stage auto-cascade system on Dhring plotM. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika24

CONTOH SOALA cascade refrigeration system shown in the figure given below uses CO2 as refrigerant for the low-stage and NH3 as the refrigerant for the high-stage. The system has to provide a refrigeration capacity of 10 TR and maintain the refrigerated space at 36oC, when theambient temperature (heat sink) is at 43oC. A temperature difference of 7 K is required for heat transfer in the evaporator, condenser and the cascade condenser. Assume the temperature lift (Tcond-Tevap) to be same for both CO2 and NH3 cycles and find a) Total power input to the system; b) Power input if the cascade system is replaced with a single stage NH3 system operating between same refrigerated space and heat sink. The actual COP of the vapour compression system (COPact) can be estimated using the equation:

M. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika25Ans.: Since a temperature difference of & K is required for heat transfer, the CO2evaporator and NH3 condenser temperatures are given by:

In the cascade condenser

Since the temperature lifts of CO2 and NH3 cycles are same,

From the above 4 equations, we obtain:

Substituting the values of temperatures in the expression for actual COP, we obtain:

(Tc,CO2 Te,CO2) = (Tc,NH3 Te,NH3)M. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika26The power input to CO2 compressor is given by,

Since the heat rejected by the condenser of CO2 system is the refrigeration load for the evaporator of NH3 system, the required refrigeration capacity of NH3 system is given by:

Hence power input to NH3 compressor is given by:

Therefore, the total power input to the system is given by:

M. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika27b) If instead of a cascade system, a single stage NH3 is used then, the actual COP of the system is:

Power input to single stage ammonia system is given by:

Comments:1) Using a cascade system the power consumption could be reduced by about 9.5 %.2) More importantly, in actual systems, the compared to the single stage system, the compressors of cascade systems will be operating at much smaller pressure ratios, yielding high volumetric and isentropic efficiencies and lower discharge temperatures. Thus cascade systems are obviously beneficial compared to single stage systems for large temperature lift applications.3. The performance of the cascade system can be improved by reducing the temperature difference for heat transfer in the evaporator, condenser and cascadecondenser, compared to larger compressors.M. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika28

M. ZAHRI KADIRTeknik Mesin FT UNSRITeknik Refrigerasi dan Kriogenika

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04TRK Sistem Multistage