What is the direct expansion valveand how does it work in the system what are the different between the direct expansion valve and the indirect expansion valve?

Answer 1

A direct expansion valve, sometimes called a DX valve, modulates the amount of refrigerant fluid, entering as a liquid, allowed into a heat exchanger. Past the DX valve the pressure is much lower and a warmer fluid on the other side of the piping in a heat exchanger boils the refrigerant fluid into its gas, absorbing heat and cooling the warmer fluid.

Most commonly one sees this on a household air conditioning system where the warmer fluid being cooled is air from the house, which will then be sent back into the house to cool it.

The DX valve modulates the amount of liquid refrigerant let in to cool the air and assure that the refrigerant is all boiled off by the time it leaves the cooling coil. If it were not all boiled into a gas, liquid could reach the refrigerant compressor, the next step in the refrigerant gasses circulation loop. Liquid is very difficult to compress and it would (and sometimes does) break the compressor when either the DX valve has not worked correctly or there has been some other technical malfunction.

There is no "indirect expansion valve" to contrast to the direct one. Instead the "direct" adjective distinguishes the direct cooling of the air by the expanding, boiling refrigerant from the more typical secondary fluid, usually water, used in larger systems to cool the air indirectly. "Indirectly" because water is not the primary source of cooling, the boiling refrigerant is; it cooled the water first.