The Direct Expansion (DX) and Chilled Water central air conditioning plants are both used at different places depending on the applications and size of the place to be air conditioned. Both of them have their own advantages and disadvantages. Let us see the comparison of DX and chilled water central air conditioning plants.
1) DX Central Air Conditioning Plants are more Efficient
In the DX type of central air conditioning plants the air used for cooling the room is directly chilled by the refrigerant in the cooling coil of the air handling unit. Due to this heat transfer process is more efficient, since there is no middle agency involved for the heat transfer resulting in higher cooling efficiency.
In case of the chilled water system, the cooling effect from the refrigerant is first transferred to the chilled water, which is then used to chill the air used for cooling the room. There is some loss of the cooling effect when it is being transferred from the refrigerant to the chilled water and from there to the air due to which the chilled water systems have lesser cooling efficiency. The chilled water acts as the secondary medium for cooling the room air in air handling unit.
Further, the chilled water has to flow long distance along the whole building. On its way it tends to get heated due to friction of flow and also due to surrounding heat absorption. The chilled water also has to be pumped by the pump, which adds more heat to it. Thus as the chilled water flows from the chiller to the air handling unit and again back to the chiller, apart from the heat from air, it also absorbs lots of additional heat, which leads to high increase in its temperature. The chiller has to remove this additional heat from the water due to which its efficiency decreases or rather additional power is required to remove this additional heat. In the DX type of central air conditioning plants the refrigerant travels only through the small distances and there is no pump involved so the additional heat absorption is less, which makes the DX plants even more efficient.
2) The Chilled Water Plants can be used for the Large Multi Storey Buildings
In the direct expansion types of the central air conditioning plant, the refrigerant like R22 flows through the whole air conditioning system including the air handling unit. When the refrigerant flows in the refrigeration piping there is lots of drop in its pressure. Due to this the length of the refrigeration tubing and the distance between the condenser and the air handling unit should be kept minimum possible.
If the air handling unit is kept at the height more than the condenser, the loss in pressure is pronounced since refrigerant travelling from the condenser to the air handling unit is in liquid state. As the distance between the air handling unit and the condenser increases the loss in pressure also increases. At certain point the losses may be so high that the refrigerant may not be able to reach the air handling unit, leading to complete failure of the system. At the larger height difference there is also oil return problem from the refrigerant to the compressor.
Due to these reasons, in direct expansion type of the central air conditioning plant there is limitation on the distance between the condenser and the air handling unit. The distance between the two cannot be too high. This limits the application of the direct expansion type of central air conditioning systems to the small buildings or a number of rooms on the single floor. In such cases the plant room and air handling room and the rooms to be cooled are located on the same floor. The height difference between the condenser and the air handling units has to be quite reasonable so that they can function well.
One of the solutions to increase the capacity of the DX systems can be to increase the number of air handling units on upper floors. But with this will be additional number of joints in the refrigerant tubing from which there will be higher chances of leakage of highly expensive refrigerant. This leads to too many operational and maintenance problems.
One may think that we can employ compressor of very high capacity to increase the refrigerant pressure, but this will lead to highly excessive capital and running costs of the plant. This is because we will have to install the compressor of capacity much higher than needed.
Thus the direct expansion types of the air conditioning plants can be used only for smaller buildings or various rooms on the same floor.
There are no pressure loss problems in the chilled water system. In this system chilled water is pumped by the pump at very high pressure, which is good enough to carry it to various floors of the multi storey building. The losses in the pressure of water are accommodated by the sufficient capacity of the pump, which has low capital and running cost. Further, the water doesn’t carry any oil so there are no oil return problems.
In case of the chilled water system the compressor, condenser, expansion valve and the chiller are all kept at the same level in the single plant room. There is no problem as such of the height difference between the condenser and the air handling unit since the refrigerant does not travels to the air handling unit. The flow of the refrigerant is limited to the chiller plant. The water chilled in the chilled flows to the various air handling units kept on different floors of the building. The whole arrangement and the structure of the chilled water type of central air conditioning plant makes it more suitable for cooling the large multi storey buildings and even for very long distances along the same floor level. This makes the chilled water central air conditioners more popular than the direct expansion type ones.
1) Basic Refrigeration and Air-conditioning by P N Ananthanarayanan, Tata McGraw-Hill Publishing Company Limited.