How to select orifice for expansion valve

  EXPANSION VALVES FOR REFRIGERATION SYSTEMS

CONSTRUCTION

AS shown in diagram sensing bulb is connected to the expansion valve with 1.5 meter of capillary tubing. Bulb pressure is transmitted to the expansion valve top. At the outlet of evaporator sensing bulb pressure functions of the temperature. It’s an angled configuration body, made from forged brass. Pressure is transferred to the plug inside the orifice with the help of steel rod inside the expansion valve body. When the temperature at evaporation outlet increases the thermostatic charge pressure also increases, due to that diaphragm get deflected downward and transfers this motion to the plug. Seat gets lifted and allows liquid to pass through orifice. Superheat at the evaporation outlet can be adjusted by side spindle which is connected to the spring tension. Static superheat can be increased and decreased by turning the spindle clockwise and anticlockwise respectively. Element (Thermostatic) is connected to forged brass body by brazing to avoid leakage. It comes with both internal and external equalizer. It comes in flare connection or with solder connection. If require nuts for flare type connection and inlet type brazing adapter need to ordered separately.

OPERATION

TXV (Thermostatic expansion valve) is a throttling device of refrigeration system between the high pressure and low-pressure sides. The main function of TXV is to match the refrigerant flow to the rate of evaporation of liquid refrigerant into evaporator. It feeds more liquid refrigerant in the evaporator if actual superheat is higher than the set point and decreases the flow of liquid refrigerant if the actual superheat is lower than the set point. It ensures that evaporator is utilized at full capacity and no liquid enters the compressor.

SUPERHEAT

It is a measure control parameter of TXV. It is measured at outlet of the evaporator. Superheat is the difference between evaporating temperature and actual bulb temperature. , evaporator temperature is calculated from evaporator pressure(You can use chart or Danfoss application). Superheat must be maintained to prevent liquid entering the compressor.

SUBCOOLING

Subcooling means deference between the condensing temperature. It is calculated from condensing pressure (Danfoss application can be referred) and expansion inlet valve temperature. Subcooling has measure role in increasing the refrigeration capacity. It is considered while selecting the expansion valve. Consideration of subcooling is necessary to prevent flash gas from forming in the liquid line. Flash gas in liquid line reduces he expansion valve capacity drastically.

ORIFICE FOR REFRIGERATION SYSTEMS

ORIFICE ASSEMBLY

Orifice is interchangeable and comes in various range of capacity from 0.5 to 15.5 kW. Following are the elements. Spring and strainer, plug, seat, housing. Design is rigid wot withstand types any type of operation. Design can withstand with liquid hammering, sudden variation of pressure, cavitation and temperature contaminants.

 ORIFICE SELECTION

At all selection of orifice make sure at the start of refrigeration system load on the expansion is around 120-130% more than 100% to avoid the liquid flood back to the compressor, Now it depends on the type of compressor, its capacity & operating range, at the end of the evaporating attempting temperature this load can be around 20-25%

Eg- If the load on the evaporator side is 1kW, Make sure orifice is selected for 120-130% load which means load on the expansion device is high and initial super heat of 10K is maintained taking care of no liquid flood back to the compressor.

Now as per the superheat on the exit of evaporator, expansion valve will adjust the liquid flow in the evaporator till the end of evaporator require temperature,this flow will keeps on reducing depending on superheat at the outlet of evaporator.

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