Theoretical Analysis of an Indirect Air Conditioning System Using Liquid Carbon Dioxide as Secondary Fluid
Ajay Kumar Yadav1, M. Ram Gopal2,a and Souvik Bhattacharyya2,b
1Department of Mechanical Engineering, National Institute of Technology, Karnataka, Surathkal, Srinivas nagar, Mangalore 575 025, India.
2302.ajay@gmail.com
2Department of Mechanical Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721 302, India.
aramg@mech.iitkgp.ernet.in
bsouvik@mech.iitkgp.ernet.in
ABSTRACT
Due to environmental problems caused by synthetic refrigerants, there has been an upswing in the use of natural working fluids in various refrigeration and air conditioning systems. However, most of the natural refrigerants are either flammable (e.g. hydrocarbons) and/or toxic (e.g. ammonia). In such cases, confining the refrigerants to the plant room and using a secondary loop to transfer heat between refrigerated space and refrigeration plant may reduce the risk of using flammable or toxic fluids. It also results in a drastic reduction in the amount of refrigerant employed in the system, particularly in case of large system installations. Carbon dioxide, being non toxic and non flammable, offers safety operation; moreover being of food quality grade, even if it gets in contact with food, the products are not contaminated and remain suitable for consumption.
In the present study, a novel air conditioning system that uses liquid carbon dioxide as secondary fluid in a forced circulation secondary loop is analyzed. Propane (R290) is employed as refrigerant in the vapour compression refrigeration system. From the steady state analysis, a comparative study is carried out between the direct system without the secondary loop and the indirect system employing a carbon dioxide based secondary loop. The comparison is made in terms of the required heat transfer areas and net COP considering power input to the refrigerant compressor and power input to the pump used for circulating carbon dioxide in the secondary loop. Results show that due to favorable thermophysical and transport properties of carbon dioxide and near critical point operation, carbon dioxide based indirect system, even after introducing the secondary loop, offers almost the same level of performance as that of the direct system.
Keywords: Air conditioning system, Secondary loop, Carbon dioxide, Natural refrigerant, Propane, Heat transfer.
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