Insulation Practices for
Gas and Liquid Refrigerant Pipes
of Split Type Air Conditioners
By TK NG
Corrigendum
Typos found in the earlier version of this article have been corrected in dark green.
Do you know the gas and liquid refrigerant pipes between the indoor and outdoor units of your split type air conditioner at home are separately wrapped in insulation (usually of the elastomeric type) or placed together within a common insulation covering?
The answer is that both practices are being adopted by installers of split type air conditioners (see Fig 1). You may wonder if there would be any advantages and/or drawbacks associated with each kind of practice.
For gas and liquid refrigerant pipes separately wrapped in insulation, our observations are:
- More insulation materials have to be used;
- Bigger wall/window opening is needed for the insulated pipes to go through;
- Gas refrigerant (cold) pipe can be properly wrapped to avoid air space between pipe and insulation and hence minimizing condensation in case of broken vapour seal.
The exact opposites apply to refrigerant pipes placed together in a common insulation covering. Besides, there is an issue that we need to take heed of. When gas and liquid refrigerant pipes are put together side by side, there will be heat flow between them due to surface temperature difference. A question may then spring up in your mind: would this affect the performance of the air conditioner?
To answer the question, we must look at what happen to the refrigerant in the refrigeration cycle when the hot liquid and cold gas pipes are put together in a common insulation covering.
Fig 2 is a typical pressure/enthalpy (P-h) chart of refrigerant showing what happen at different stages of a refrigeration cycle. The refrigeration cycle “1-2-3-4” depicted in red solid line is for split type air conditioner with the gas and liquid refrigerant pipes between the indoor and outdoor units separately insulated. You can find another cycle “a-b-c-d” in blue dotted line which is for gas and liquid refrigerant pipes being wrapped together.
When the gas and liquid refrigerant pipes are put together side by side, the refrigerant gas emerging from the cooling coil of the indoor unit is superheated (from point “1” to “a” in Fig 2) and the liquid refrigerant from the condenser of the outdoor unit is subcooled in turn (from point “3” to “c”) before throttling through the expansion valve due to heat transfer between the two pipes. The extent of superheating/subcooling will depend on the lengths of the refrigerant pipes being wrapped together. The energy for compressing the superheated gas refrigerant (hb – ha) is slightly more than that without superheating (h2 – h1) **. However, the heat removed through the cooling coil of the indoor unit in one case (ha – hd)(h1 – hd) is more than the other (h1 – h4). The resulting Coefficient of Performance (COP) is therefore higher for the case with superheating/subcooling [COP = (ha – hd)(h1 – hd)/(hb – ha)], as compared with that without [COP = (h1 – h4)/(h2 – h1)].
The above broad-brush analysis does not tell us the extent of COP improvement, which will depend on a number of different factors and may just be marginal. However, it does assure us that wrapping the gas and liquid refrigerant pipes together in a common insulation covering will not jeopardize the system cooling performance. Despite the advantages of using less insulation materials and requiring smaller wall/window opening for accommodating the insulated pipes, the practice is not without drawback. Air spaces likely remain somewhere between the refrigerant pipes and insulation if the interior profile of the insulation is not designed for accommodating two pipes. Condensation may be formed there if vapour seal for the insulation is broken.
To conclude, both ways of insulating the gas and liquid refrigerant pipes between the indoor and outdoor units of a split type air conditioner discussed above are acceptable. In any case, proper vapour sealing for the joints of insulating materials should be effected to prevent condensation along the cold refrigerant gas pipe.
**
The refrigerant gas compression process can be considered as an adiabatic one and hence follows an isentropic line on the P-h chart. 