Performance analysis of 404a/508b Cascade Refrigeration cycle for low temperature

Performance analysis of 404a/508b

Cascade Refrigeration cycle for low

temperature

DEVANSHU PYASI 1*

Student, IV Semester M.E (Heat Power Engineering)

R.C. GUPTA2

Associate Professor

1*, 2 Mechanical Engineering Department, Jabalpur Engineering College Jabalpur. 482001 (India) 1* Corresponding author

Abstract:

This paper presents analytical results of analyzing blends of Hfc refrigerants such as 404a and 508b in Cascade Refrigeration System. Refrigerant blend 508b is a low boiling refrigerant and advantageous in low stage of cascade refrigeration system whereas 404a is used in high stage of cascade refrigeration system because of its high boiling point which is suitable for high temperature circuit. The analysis includes three basic parameters as :Evaporator temperature(Te),Condenser temperature (Tc), and temperature difference in cascade condenser (Dt) .These parameters are varied one by one up to a limited range keeping other parameters constant and the effect of these parameters on system COP , exergetic efficiency, mass flow ratio etc is analyzed. Analysis results also give the optimum values of the evaporator, condenser and cascade condenser temperature.

Key words: Cascade refrigeration cycle, theoretical analysis, COP, Exergy, 404a, 508b etc.

Introduction

Cascade refrigeration system is a low temperature refrigeration system and is used for very low temperature range about (-40C to -130C).At such low temperature simple Vapor Compression Refrigeration Cycle (VCRS) is not efficient due to very high compression ratio that further leads to high discharge problem and low volumetric efficiencies Whereas, cascade refrigeration is much efficient for such conditions. Cascade refrigeration cycle is nothing but simply a combination of two VCRS cycles named as low and high temperature circuit that are combined together by a cascade condenser. This cascade condenser unit act as evaporator for low temperature circuit and condenser for high temperature circuit, the low temperature circuit uses low boiling refrigerants such as R23, R744 etc and high temperature uses high boiling point refrigerants such as R717, R290, R404A, R1270, R507A etc. Various refrigerants have been analyzed in cascade refrigeration system among which few are CO2/Ammonia, Propane/CO2, 507A/R23, 404A/CO2 etc. In the present study theoretical

analysis is done on cascade refrigeration system by using blends of Hfc refrigerants 404a/508b and analysis results the effect of design and operating parameters on system performance.

Cascade Refrigeration (System Description):

circuit. The temperature difference in cascade condenser is an important design parameter that decides the COP of the entire refrigeration system.

7

6

Throttling valve

W

8

5

3

2

Throttling valve

W

4

1

Figure (1) shows systematic cascade refrigeration cycle with evaporator, condenser and cascade condenser.

Theoretical Analysis:

In present work a parametric study with fixed mass flow rate in low temperature circuit and varying different parameters such as evaporator temperature, condenser temperature and temperature difference in cascade condenser have conducted to determine effects of these parameters on system performance. The analysis is done by making general assumptions so as to simplify the analytical procedure these are as follows:

a) Negligible change in kinetic and potential energy. b) Isenthalpic expansion of refrigerant in expansion valve.

c) Negligible pressure and heat loss/gain in pipe or other components d) Compressor process is irreversible and adiabatic.

(Fig:2) represents the (T-S) and (P-h) diagram showing decrease in compressor work and increase in refrigeration capacity for cascade refrigeration system.

The range of parameters used for system analysis are the evaporating temperature (Te) varies from (90°C to -70°C), Condenser temperature (Tc) varied from (30°C to 40°C) cascade condenser temperature (TCAC) varies

from (-45°C to -20°C), temperature difference in cascade condenser (Dt) varies from (2°C to 10°C).Also the

CONDENSER (TC)

EVAPORATOR (TE)

TCAE

Mass balance

∑

= ∑

(1)

Energy balance

Q – W +

∑

ℎ

-

∑

ℎ

=

0 (2)

Heat transfer rate to the cycle in the evaporator:

Q

=

m

(h1

- h4

) (3)

Total Work input to both compressor:

W

+ W

= m

( h

- h

) + m

( h

- h

) (4)

Rate of heat transfer in cascade condenser:

Q

= m

( h

- h

) = m

( h

- h

) (5)

Heat rejected by condenser of high temperature circuit:

Q

m

( h

– h

)

(6)

Mass flow ratio is given by equation:

=

(7)

Overall COP of the system:

COP =

(8)

COP Of LTC and HTC:

(COP)

=

(9)

(COP)

=

(10)

Exergetic Efficiency of the whole system is given

η

=

(11)

Flow Chart for Computational Analysis:

The analysis program is based on mathematical equations. Input parameter are taken Te= (-85°C), Tc= (35°C), and DT= (5°C) mr1= (0.2kg/min). At the initial stage enthalpies at each point are calculated which depend on operating temperatures after that the entropies and specific heat are calculated. Using these enthalpies, specific heat and entropies temperatures and enthalpies at point 2 and 6 are found out, at next step the various values which are found are used for calculating mr2,Qe,Wc1,Wc2,Qcac,Qcond, mr2/mr1 etc. Hence at last final

outcomes comes in form of COP overall , COPL ,COPH, COP Carnot , ηII.

Result and Discussion : To demonstrate the influence of operating parameters on system COP and Exergy efficiency different range of variables as discussed above are varied and there effect are studied.

Fig.1 depicts, while varying the low circuit evaporator temperature (TE) and keeping other parameters constant

overall COP of the system increases from (0.88 to 1.19) as there is decrease in pressure ratio. Hence refrigerating effect is increased with decrease in compressor work. Also Exergy efficiency increases with very small amount from (0.602 to 0.626) with increase in evaporator temperature.

Fig.2 shows the overall COP and Exergy efficiency (N II) both decreases when high temperature circuit condenser temperature (TC) is varied keeping other parameters constant. The COP deteriorates from (1.066 to

0.612) where as exegetic efficiency deteriorates from (0.652 to 0.439).If condenser temperature is varied increasingly it increases work done on compressors and decreases system COP.

Input parameter Te, Tc, Tcac, Tcae, mr1

Calculate enthalpy of points H1, H3 ,H4 ,H5 ,H7 ,H8

Calculate entropy and sp. heat S1, S2’, Cp2, S5 ,S6’,Cp6

Calculate outlet temperature of compressors and enthalpies T2 , T6 , H2 , H6

Calculate

mr2,Qe,Wc1,Wc2,Qcac,Qcond,mr2/mr1

Calculate

COP overall ,COPL ,COPH, COPCarnot, ηII

Figure:1 Effect of low temperature evaporator(TE) COP Figure:2 effect of high stage condenser (Tc) on COP and on

and exergetic efficiency Exergy efficiency

Fig.3 displays the effect of low stage circuit condenser temperature (TCAC) on COP and Exergy efficiency. As

the condenser temperature is varied both COP and Exergy efficiency increases up to a certain temperature and then stared decreasing. Therefore an optimal value of low stage condenser temperature for maximum COP and Exergy efficiency exists. The maximum COP exists at -25 C and Exergy is at (-30 C).

Fig.4 demonstrates the effect of temperature difference (DT) of cascade condenser on system COP and exergetic efficiency (N II). As the difference in cascade condenser increases the system cop reduces slightly with also decrease in exergetic efficiency. Increase in temperature difference (DT) causes increase of compressor work.

Figure 3 shows effect of varying temperature difference between Figure 4 shows effect of low temperature of cascade condenser on COP and exergy cascade condenser on COP and Exergy and Exergy

. Fig.5 depicts the effect of low stage condenser temperature (TCAC) on mass flow ratio (mh/ml). Low stage

condenser temperature is varied keeping other parameters as constant and result shows that mass flow ratio diminishes from (1.846 to 1.343) as condenser temperature is varied.

0 0.2 0.4 0.6 0.8 1 1.2 1.4

-90 -85 -80 -75 -70

COP

T E (°C)

Tc=35°c DT=5°c Tcac=-39°c COP ηII 0 0.2 0.4 0.6 0.8 1 1.2

30 35 40 45 50

COP

Tc(°C)

TE=-85°c

DT=5°c T_CAC=-39°c

COP

η II

0 0.2 0.4 0.6 0.8 1 1.2

2 4 6 8 10

COP DT (°c) Te=-85°c TC=35°c Tcac=-39°c COP

η II

0 0.2 0.4 0.6 0.8 1 1.2

-45 -40 -35 -30 -25 -20

COP

Tcac °c

Te=-85°c DT=5°c Tc=35°c

COP

Fig: 5 effect of low stage circuit condenser on mass flow ratio Conclusion

This work analyzed the overall (COP), Exergy efficiency and mass flow ratio of 404a-508b cascade refrigeration cycle. Refrigerant selection in cascade system is difficult task as it requires suitable refrigerant in each circuit thus new combination of Hfc blends is analyzed. From above study following conclusions can be made:

1) System overall performance (COP) and Exergetic efficiency gets increased by (0.88 to 1.19) and (0.60to 0.626) as the evaporator temperature is varied from (-90C to-70C) while other parameters are kept constant. 2) The system COP decreases by (1.06 to 0.61) and exergetic efficiency deteriorates by (0.65 to0.43) when condenser temperature of high stage is varied from (30C to 50C) keeping other parameter constant.

3) Both COP and Exergy efficiency first increases up to certain temperature and then decreases when temperature of lower stage condenser is varied. Thus an optimum values are determined.

4) System COP deteriorates by (0.95 to 0.84) and Exergy by (0.60 to 0.53) when temperature difference of cascade condenser is varied from (2 to 10).

5) Mass flow ratio is decreased from (1.84 to1.34) when low stage condenser temperature is varied from (-45 to -20) keeping various other parameter constant.

Nomenclature

COP Coefficient of Performance.

η II Exergetic Efficiency.

TCAC Temperature of low stage Condenser (cascade condenser).

DT Temperature difference in Cascade Condenser. Mr1 Mass flow rate in high temperature circuit.

Mr2 Mass flow rate in low temperature circuit. References:

[1] Arora, C.P. (2002), “Refrigeration and Air conditioning”, 2nd _{edition, Tata McGraw Hill, New Delhi}

[2] Bansal, P.K., Jain, S., 2007. Cascade systems: past, present.

[3] Bhattacharyya, S., Mukhopadhyay.et al, 2005. Optimization of a CO2–C3H8 cascade system for refrigeration and heating

[4] Dopazo J. Alberto .et.al Experimental evaluation of a cascade refrigeration system prototype with CO2 and NH3 for freezing process applications

0 0.5 1 1.5 2

-45 -40 -35 -30 -25 -20

mh/ml

Tcac TE=-85°C

[8] Parekh A. D. and P. R. Tailor Thermodynamic Analysis of R507A-R23 Cascade Refrigeration System. International Journal of Aerospace and Mechanical Engineering.