Does a same coil (heat exchanger) show different performance when used as evaporator and when used as condenser ? I would like to know this in both theoretical and practical case ?
I expect the coil to behave the same whether operated as an evaporator or as a condenser, theoretically. I would like to know more about this.
Thanks in advance.
Since you say 'coil' I am taking it as the finned 'coil' of air conditioners. In all theoretical respects, they are identical. As for practical aspects, water vapor in the conditioned air condenses on the evaporator and then forms frost on the fin surfaces, causing a blockage of airflow in the fin spaces. There are ways to minimize its effect but this problem has not yet been solved entirely satisfactorily by the air conditioning industry.
A second point of difference is that in a condenser, the entering medium is dry vapor with a quality of 1. Hence the problems of maldistribution are less severe. In the case of the evaporator, the vapor entering the coil after the expander is wet. Maldistribution of two-phase flow is another kind of animal. Evaporator maldistribution too is an area of intense research.
As for refrigerant-side heat transfer coefficients, they are somewhat higher in condensation than in evaporation.
Since you say 'coil' I am taking it as the finned 'coil' of air conditioners. In all theoretical respects, they are identical. As for practical aspects, water vapor in the conditioned air condenses on the evaporator and then forms frost on the fin surfaces, causing a blockage of airflow in the fin spaces. There are ways to minimize its effect but this problem has not yet been solved entirely satisfactorily by the air conditioning industry.
A second point of difference is that in a condenser, the entering medium is dry vapor with a quality of 1. Hence the problems of maldistribution are less severe. In the case of the evaporator, the vapor entering the coil after the expander is wet. Maldistribution of two-phase flow is another kind of animal. Evaporator maldistribution too is an area of intense research.
As for refrigerant-side heat transfer coefficients, they are somewhat higher in condensation than in evaporation.
Condensers and evaporators are sensible heat exchangers in which the refrigerant changes phase. The capacitance rate during a phase change is effectively infinite since the fluid energy changes without a corresponding change in temperature. However, the refrigerant is often superheated or subcooled, and the overall capacitance rate ratio is not zero. For evaporators, treating the exchanger as one with a capacity rate ratio of zero is usually sufficiently accurate. In a condenser, there is usually significant desuperheating of the vapor prior to condensation. Dividing the exchanger into two sections, one with a finite capacitance ratio and the other zero, improves the estimation of performance.
In the evaporation and condensation of refrigerants additional mechanisms over those in single phase heat transfer are present. The process of changing phase from liquid to vapor in evaporators and vapor to liquid in condensers affects the flow patterns and, consequently, the heat transfer and pressure drop for the flow. In evaporation on the outside of tubes the phase change takes place at the surface and the rising bubbles create secondary flow patterns in the fluid. For evaporation inside tubes, bubbles forming at the wall surface move to the center of the flow, also causing mixing and secondary current. Condensation on the outside of tubes creates a downward flow of condensate over the surfaces that promotes mixing. For condensation inside tubes there is a layer of condensing liquid along the wall surface and a flow of vapor down the center of the tube, creating a complex flow pattern. The heat transfer coefficient and pressure drop correlations for evaporation and condensation are empirical and bring in the important parameters. They can be used as guides to estimate the heat transfer and pressure drop for these heat transfer situations.
My longish answer of yesterday as well as Tinku Mittal's answer are missing!
Tunku, your answer is essentially OK, but there is an error, perhaps inadvertent. Exchangers transferring sensible heat are no-phase-change exchangers. Just a minor point, mainly for other young minds who might be reading our posts.
Dear rajesh,
Condensers handle high pressure vapour and requires addtional fan to enhance the heat transfer. In the case of evaporator it handles low pressur liqiud (does not require external aid always).
Hence if a condenser coil is used as evaporator coil or vice versa, the system balance will be affteded. Desired output cooling or heating may not occur due to unbalanced system.
You can check with simple LMTD or NTUmethod for the same input /output temperatures for condenser or evaporator.
Best of luch
With regards,
P.Thirumal
Dear Rajesh
In practical aspects the condenser and evaporator are geometrically different as they are designed to handle different phases of the refrigerant. So you have to take care of the velocities with those dimensions which in turn will effect the heat transfer/performance. It is often adviced to select the dimensions such that the the following velocities ranges are not crossed in the working range:
velocity of gas in the evaporator = 4.5 to 20 m/s
velocity of gas at the inlet of the condenser = 10 to 18 m/s
max velocity of liquid in the condenser = 1.5 m/s
therefore, you may find that the condenser tube diameter is generally less than the evaporator tube dia for a given system. Though there are many practical systems in which same coils serves both the purpose when the cycle is reversed during heating or cooling modes.
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