How to select the correct Electronic Expansion Valve?

The electronic expansion valve need to function correctly under different load conditions, therefore it is necessary to consider the following points when the valve has to be dimensioned:

  1. Showcase/Evaporator capacity 

  2. Pressure drop across the valve.
        a. Δp,valve = pc – (pe + Δp1 + Δp3 + Δp4)
        b. The pressure drop across the liquid line and the distributor system must be calculated on the basis of the valve’s max. capacity, as the valve operates with pulse-width modulation.
     
  3. Correction for sub cooling
        a. Zeas Condensing unit and Conveni-pack unit provides a higher undercooled liquid to the evaporators than traditional         systems. This needs to be taken into account when selecting the correct expansion valves.
        b. When a booster unit is used, the liquid temperature can be calculated by using following formula:
            Liquid temperature to freezer cabinet = Evaporation temperature on MT side + 10 K 4. 

  4. Correction for evaporating temperature
        a. Depending on the application, the valve should have an overcapacity enabling it to cope with the extra amount of         refrigeration needed during certain periods.
        b. Qevap,corrected = Qevap x correction coeff 

  5. Determination of valve size
     
  6. Correctly dimensioned liquid line
        a. The liquid flow rate should not exceed 1 m/sec. Higher flow rate will cause pulsations in the liquid line!
        b. The dimensioning of the liquid line must be based on the capacity of the valve at the pressure drop with which it is         operating (cf. capacity table), and not on the evaporator’s capacity.
        c. The diameter of the liquid line from the refnet till the refrigerated cabinet/evaporator must be increased, if the         capacity of the valve exceed the maximal allowed capacity flow through the selected pipe diameter.