Selecting a thermostatic expansion valve

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Exercise - Select a Thermostatic expansion valve

Select a TU thermostatic expansion valve for a dry expansion system with R448A with the following operating conditions:

  • Cooling capacity: 2.5 kW
  • Evaporator dew point temperature: -8 °C
  • Useful superheat: 8 K (Default)
  • Condenser bubble point temperature: +45 °C
  • Subcooling: 3 K
  • Load: 100%
  • Distributor pressure drop: 1.0 bar

a) Which valve is suggested by Coolselector®2?
b) What is the nominal capacity of the suggested valve?
c) Is it possible to run the system at 20 °C condensation temperature with this valve size?
d) What is the load at 45 °C and 20 °C, respectively?
e) What is the load a TXV should typically be selected at?

Solution for selecting a Thermostatic expansion valve

With a commercial refrigeration system, we usually talk about a dry expansion system. So, in Coolselector®2, you first set the System to ‘Dry’ and then select the Liquid line:

The operation (system) condition values are required to calculate the mass flow circulating through the expansion valve. Part of this information is the pressure losses between the condenser outlet and the valve inlet, as well as the pressure drop between the outlet and the evaporator inlet. Those pressure drops are reflected in the value of “Distributer dp”. The pressure drop across the valve therefore is equal to pc – (pe + dp distributer).

a). The suggested valve is a TU orifice size 5 (TU - 5):

b). The suggested valve has a nominal capacity of 2.541 kW:

c). Yes, it is possible to run the valve at +20 °C condensation temperature. To prove this in Coolselector®2, change the bubble point temperature of condensation to +20 °C:

You will now see that TU – 5 is no longer the suggested best match by Coolselector®2, however, it is still in the list with a warning that the capacity of the expansion valve is too small:

A thermostatic expansion valve has a nominal capacity at a defined opening super heat, but with higher opening super heat a higher capacity is given. However, typically, such a valve can be used up to 150% of its nominal capacity. Due to that, a TXV has a so-called “over capacity”, which is indicated by the Performance diagram with a curve that does not end at 100% nominal capacity:

Since the valve, for a limited period of time, would run at the higher load, it is not the recommended selection in Coolselector®2. However, in reality, the valve is an option.

d). At 45 °C condensation the load is about 98% and at 20 °C condensation the load is about 110% of the nominal capacity.

45 °C:

20 °C:

e). A TXV should never be over sized but should be selected at 100% of its nominal capacity. To reach the required working super heat built in to the system, the static super heat should be re-adjusted rather than over sizing the valve. In this way, the bandwidth will be utilized better and the valve will work with higher stability.

 

Documents

Documents
Type Name Language Valid for Updated Download File type
Promotion material Coolselector®2 Exercises English Multiple 03 Jun, 2019 13.2 MB .pdf
Installation guide Coolselector®2 Server Installation Guide English Multiple 17 Nov, 2020 1.2 MB .pdf
User guide Coolselector®2 User Guide French France 04 Mar, 2020 5.5 MB .pdf
User guide Coolselector®2 User Guide Chinese (CN) Switzerland 15 Apr, 2020 25.1 MB .pdf
User guide Coolselector®2 User Guide English Multiple 03 Jun, 2019 4.3 MB .pdf
User guide Coolselector®2 User Guide Portuguese Multiple 02 Jan, 2020 5.3 MB .pdf
User guide Coolselector®2 User Guide Russian Russia 24 Feb, 2020 4.4 MB .pdf
User guide Coolselector®2 User Guide Spanish, Castilian Spain 03 Jun, 2019 5.8 MB .pdf

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