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Selection of suitable natural refrigerants pairs for two stage Cascade Refrigeration
#1

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Submitted by:
Arjun Lal

Under the guidance of
Dr. Jahar Sarkar
(Asst. professor)

Department of Mechanical Engineering
Institute of Technology
Banaras Hindu University
Varanasi 221005



Abstract

Conventional refrigerants are being phased out worldwide to avoid ozone layer depletion and global warming and natural refrigerants are fast gaining favour nowadays. Single stage and multistage refrigeration system fail to widen the gap between heat source and heat sink temperatures required in many industrial applications and cascade system appears to be best alternative. Modest work has been done on the cascade system based on the natural refrigerants. In this study, a thermodynamic analysis of 8 natural fluids as ultra low temperature refrigerants in cascade system has been done to get the suitable refrigerants pair in a wide range of temperatures based on best system performance (maximum COP) as well as on operating characteristics. The natural refrigerants which have been considered here are:
i. Ammonia
ii. Carbon dioxide
ii. Propane
iv. Propylene
v. n-Butane
vi. Isobutane
vii. Ethane
vii. Ethylene

Out of these 8 natural refrigerants, there is possibility of using each refrigerant as HT fluid or LT fluid, thus giving rise to 8P2=56 possible combinations of HT fluid and LT fluid. However, there are some required characteristics of refrigerants which make them suitable for certain temperature range only. These important characteristics are normal boiling point and critical point of both HT and LT fluid. The operating temperatures of a refrigerant should be in-between its critical point and normal boiling point. Considering these characteristics, a thermodynamic analysis has been done to get suitable refrigerants pairs for a wide range of condensing temperature of HT circuit and evaporative temperature of LT circuit. Furthermore comparison has been also done in terms of refrigeration capacity per unit volume of suction vapor and mass flow ratio. A chart has been prepared which describes which refrigerant pair should be used for a range of condensing temperature of HT circuit and evaporative temperature of LT circuit.


Introduction

Many industrial applications require very low temperature cooling which can t be achieved effectively by single stage or multistage systems due to individual limitation of refrigerant and a cascade system is the best alternative in these situations. A cascade refrigeration system is suitable for the applications where very low evaporating temperature in the range -30 C to -90 C is required.
Cascade refrigeration system: The use of single refrigerant in simple vapor compression cycle for the production of low temperatures is limited by the following reasons:
1. Solidification temperature of the refrigerant.
2. Extremely low pressures in the evaporator (problem of leakage in the system) and large suction volume if a high boiling refrigerant is selected.
3. Extremely high pressures in the condenser if a low boiling refrigerant is selected.
4. Very high pressure ratio for compressor, which leads to a decreased compressor efficiency and hence lower COP.
5. Difficulties encountered in the operation of any mechanical equipment at very low temperatures.
The multistage compression is employed when low temperatures are required and pressure ratio is high. If the vapor compression system is to be used for the production of low temperatures, the common alternative to stage compression is cascade system in which a series of refrigerants, with progressively lower boiling points are used in a series of single stage units. An additional advantage of the cascade system is that oil from one compressor cannot wander to the other compressors as it generally happens in the multistage systems. The cascade refrigeration system combines two or more vapor compression units as shown in Fig. 1.
The high temperature circuit produces refrigeration at a certain low temperature TCASE (which is also known as IT). The low temperature circuit produces refrigeration at a still low temperature TE, using the refrigerating effects of HT circuit at temperature TCASE for rejecting heat in its condenser at temperature TCASC, which in the limit is equal to TCASE. In practice, however, there is a certain overlap between these temperatures, i.e. the temperature TCASC is about 5-10 C is higher than temperature TCASE. The difference between these two temperatures is called overlapping temperature (OT)

It is also to be noted that each circuit works on a separate refrigerant. Each refrigerant can be chosen in such a way that it operates best within the required comparatively narrow temperature limits.

The high temperature circuit uses high boiling refrigerants such as ammonia, R22, propane, propylene and ethanol etc., whereas the low temperature cascade uses a low boiling refrigerant such as CO2, ethane, ethylene, methane, N2O etc. depending on the requirements. The use of low boiling and therefore, a high pressure refrigerant ensures a smaller compressor displacement in the low temperature circuit and a higher COP. Here we have considered 8 natural refrigerants which are
i. Ammonia
ii. Carbon dioxide
ii. Propane
iv. Propylene
v. n-Butane
vi. Isobutane
vii. Ethane
vii. Ethylene
Out of these 8 natural refrigerants, there is possibility of using each refrigerant as HT fluid or LT fluid, thus giving rise to 8P2=56 possible combinations of HT fluid and LT fluid. However, there are some required characteristics of refrigerants which make them suitable for certain temperature range only. These important characteristics are normal boiling point and critical point of both HT and LT fluid. The operating temperatures of a refrigerant should be in-between its critical point and normal boiling point. Based on these characteristics we must have normal boiling point temperature of refrigerants should be less than Evaporating temperature of corresponding circuit, i.e.
TE NBPLT
IT NBPHT
This ensures that no vacuum is created inside the evaporator and hence eliminates the possibility of entry of air and moisture into the system, also operating at temperature well above NBP would result in higher vapor density at compressor inlet, so that sufficient mass flow rate is maintained.The critical point temperature of refrigerants should be much greater than condensing temperature corresponding to that circuit, i.e.
TCASC CTLT
TC CTHT
This will ensure that a reasonable refrigerating effect is achieved which becomes very small if the state of refrigerant before expansion is near the critical point. Considering these characteristics, a thermodynamic analysis has been done to get suitable refrigerants pairs for a wide range of condensing temperature of HT circuit and evaporative temperature of LT circuit. Furthermore comparison has been also done in terms of refrigeration capacity per unit volume of suction vapor and mass flow ratio. A chart has been prepared which describes which refrigerant pair should be used for a range of condensing temperature of HT circuit and evaporative temperature of LT circuit. The general data of the refrigerants used in this study are given below in Table 1.


Literature review

A suitable selection of refrigerants used in HT and LT cycles can provide the required low temperature while attaining good system efficiency. Synthetic refrigerants are being phased out worldwide to combat the twin menace of the ozone layer depletion and global warming and several natural refrigerants are regaining their importance and are on a revival path. For cascade systems, lower temperature limit of the HT side is termed as intermediate temperature (IT), the system performance depends upon IT and hence they have to be optimized.
High temperature circuit can be fed with ammonia, propane, propylene and ethanol etc. out of these, ammonia offers better system performance. Although its toxicity remains main concerned, it can be used in industries away from residential areas. For LT side we can use CO2, ethane, ethylene, propane, nitrous oxide etc. Getu & Bansal [1] presented a thermodynamic analysis of ammonia- carbon dioxide cascade refrigeration system to optimize the design and operating parameters for the system operating between evaporating temperature of -50 C and condensing temperature of 40 C. Dopazo [2] performed a theoretical analysis of a CO2 NH3 cascade refrigeration system for cooling applications at low temperatures and determine the optimum CO2 condensing temperature and also showed the influence of compressor isentropic efficiency on system performance. Bingming [3] performed Experimental investigation on the performance of NH3/CO2 cascade refrigeration system with twin-screw compressor, and compared the cascade system with NH3/CO2 with two stage NH3 system and single stage NH3 system with & without economizer and showed that cascade system is very competitive in low temperature applications.
Lee TS, Liu CH, Chen TW. 2006 [4] presented a thermodynamic analysis of optimal condensing temperature of cascade-condenser in CO2/NH3 cascade refrigeration systems to determine optimum condensing temperature of CO2 in lower temperature circuit. A parametric study and optimization of CO2 C3H8 cascade system was reported by Bhattacharyya [5] for simultaneous refrigeration at -40 C and heating at 80 C and relevant expressions for design parameters were developed for optimum system performance.
Kruse & Russmann [6] studied the use of nitrous oxide as an option to substitute for low temperature synthetic refrigerants. Kruse and Russmann (2006) reported a theoretical investigation on natural refrigerant based cascade refrigeration systems and compared with an R23 R134a LT HT cycle, and showed that the transcritical carbon dioxide HT cycle with nitrous oxide LT cycle cascade system exhibited lower performance than the R134a/R23 cascade system. Bhattacharyya [7] studied the effects of design and operating parameters on system performance of a novel N2O-CO2 cascade system for refrigeration and heating and also showed that use of internal heat exchanger has marginal effect on system performance. DiNicola et al. (2007)[8] experimentally determined COP of carbon dioxide and nitrous oxide binary mixture in low temperature cycle with R404a in high temperature cycle and results were compared with R23 in low temperature cycle and R404a in high temperature cycle. DiNicola [9] presented Blends of carbon dioxide and HFCs as working fluids for the low-temperature circuit in cascade refrigerating system. A. Kilicarslan & M. Hosoz [10] studied energy and irreversibility analysis of a cascade refrigeration system for various refrigerant couples, namely R152a R23, R290 R23, R507 R23, R234a R23, R717 R23 and R404a R23, and showed that Refrigerant couple R717 R23 is the best couple for the vapour compression cascade refrigeration systems among all considered couples and if there are some limitations for the use of the natural refrigerants, the couple R152a R23 is the solution. However in all the above studies, either analysis of one or two natural refrigerants pair has been done or comparison of natural refrigerants with CFCs and HCFCs has been done. All the above mentioned work lacks the comparison between various natural refrigerants. I this study a thermodynamic analysis have been done for cascade system with 8 natural refrigerants, and a chart has been prepared which is helpful in selecting best refrigerants pairs for a wide range of evaporator and condenser temperatures.
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