The expansion valve is a passive orifice through which the liquid refrigerant is forced by the pressure difference between the condensing and the evaporating conditions. Capacity ratings are given in the catalogues of manufacturers and suppliers. Types in general use are:
1. Capillary tubes, for small hermetic systems. These are factory selected and cannot be adjusted.
2. Solenoid valves with liquid level sensors or liquid level valves for flooded evaporators.
3. High-pressure float valves plus handset throttle valves for some flooded and low-pressure receiver circuits.
4. Thermostatic expansion valves or electronic expansion valves for direct expansion circuits.
Troubles can arise with the selection of thermostatic expansion valves, the type generally used in custom-built systems. It is usual to select a thermostatic expansion valve for the maximum duty and at the summer condensing condition, taking into account the pressure drop through a liquid distributor in the case of a multiple-feed coil. Valve ratings are given for a range of pressure differences, i. e. for a range of condensing conditions for a specific refrigerant and evaporating condition, as in Table 10.3 . It might be thought that the duty varies with pressure difference according to fluid flow laws, but this will not be the case because phase change occurs as the pressure falls. This means that the valve may be able to pass more liquid at low condensing pressures. Conversely, if the valve is selected at a lower pressure difference (possibly corresponding to a condensing condition in the UK of 20-25°C), the valve will not be grossly oversized at the maximum summer condition. Although no standards exist for valve rating conditions it is conventional to state a nominal capacity at 38°C condensing, 4°C evaporating with 1 K sub-cooling and 3-4 K superheat. Sometimes the data takes the form of nominal capacity and tabulated correction factors.
Table 10.3 Typical expansion valve capacities for specific evaporating temperature
Unless a thermostatic expansion valve is very tightly rated, the system will operate satisfactorily at a lower condensing condition in cool weather, with a gain in compressor duty and lower power input. A growing awareness of energy economy is leading to more careful application of this component. Suppliers are ready to help with advice and optimum selections.
A greater difficulty arises where the compressor may go down to 33% or 25% capacity and the thermostatic expansion valve is called upon to control a much reduced flow. Under such conditions, the thermostatic expansion valve may be unstable and ‘hunt’, with slight loss of evaporator efficiency. Since the required duty is less, this may appear to be of no great importance but should avoided. It is possible to fit two expansion valves in parallel and isolate one at part load, but this arrangement is not usually necessary.
Low condensing pressure operation should present no problem with float or electronic expansion valves, since these can open to pass the flow of liquid if correctly sized.
Posted in Refrigeration and Air Conditioning