What's an expansion valve in an electromechanical system?

Expansion Valve is an important part of AC Unit (DX type) which controls the flow of refrigerant as well as control the evaporation by sudden change of pressure which cause cooling effect of cooling coil since liquid when enters from high pressure to low pressure it starts evaporation 

Hope this is clear

 the expansion device like a  traffic light  designed to regulate the rate of  liquid refrigerant flows into the evaporator.

The thermostatic expansion valve (TXV) is a precision device, which is designed to regulate the rate at which liquid refrigerant flows into the evaporator. This controlled flow is necessary to maximize the efficiency of the evaporator while preventing excess liquid refrigerant from returning to the compressor (floodback) .

Expansion Valve is a vital part at any cooling circuit HVAC. it controls the pressure of refrigerant (Ammonia, Freon). it takes high pressure liquid and transform it into low very low pressure  liquid which leads to huge decrease in temperature. without exp. valve there would be no cooling.

Electronically controlled electric valves have been accepted for years in larger applications, just as chillers have been, and have proven to add efficiency to the systems. The key to the increase in efficiency is due to their ability to control superheat to a low, stable setting. Since many chillers use almost infinitely variable screw compressors, an expansion valve that has the ability to follow these radical changes in capacity is required. Traditionally, mechanical thermostatic expansion valves (TXVs) are normally rated from about50 to110 percent of nominal rating. This rating is based on relatively constant head pressure and liquid quality conditions. These valves may not allow efficient control of the chiller during low load or head pressure conditions.

The electric expansion valve (EXV) has the ability to follow load, in most cases, from about5 to115 percent of nominal load.

In addition, flash gas in the liquid line is less damaging to the operation of the system because the relatively large port and large stroke of the EXV allows them to open wide, purge the flash gas, and then re-establish the desired superheat. The advantages of the EXV are clear, but since the valve is an electric component, it requires some form of electric or electronic control.

An expansion valve in an ES will be in the cooling part of this system or depending on the use of the system in the system itself.

I think most of us think deeply and forget the basic. First is the TXV (Thermostatic Expansion Valve) is a principal part of the refrigeration cycle. Not in the HVAC (Heat Ventilation and Air Conditioning). There are three types of expansion valves, known as Externally equalized, Internally equalized and electronics expansion valves. Externally equalized valves has a external equalising tube and it is not in the internally equalized valves. In this both types has a sensing bulb to operate the valve according to the fluctuation of the temperature of leaving gas of evaporator. But electronic expansion valves do not have a sensing bulb. It is operating according to the electric signal generated by the temperature sensor. However, all of these components are refrigerant metering devices. Further, after the expansion valve there should not be100% liquid. If TXV supplied liquid, the evaporator gets flood. It cause due to over opening the valve. Next is arcing the evaporator. The TXV is providing very low amount of refrigerant to the evaporator. Next is hunting. The amount of refrigerant provide by the TXV is fluctuating. Once it release over capacity and then very low amount. These all conditions are because of improper tuning. After the TXV there should be a mixture of vapor and liquid refrigerant. These total amount should be fully evaporate before leaving from evaporator.    

I agree with the expert answers.

The expansion device is the fourth major component in air conditioner units. It’s also known as meter devices.

Air conditioner expansion valve is the divided point between the low side and the high side of the air conditioner units. Another dividing point is air conditioner compressors.

The meter device is located indoor (air handler) units with the evaporator coils. It’s small and hard to see, unless you open the evaporator compartment.