How a Thermostatic Expansion Valve Works
A short summary of the Fujikoki product catalog: the three forces on the diaphragm, superheat, internal and external equalization, choosing a valve with sub-cool, and how to install it.
1. Three forces on the diaphragm
A thermostatic expansion valve works on the balance of three forces:
- PB — pressure inside the thermal sensing bulb, acting on the upper side of the diaphragm: it opens the valve.
- PL — pressure inside the evaporator, acting on the lower side: it closes the valve.
- F — force of the adjusting spring, through the push rod: it closes the valve.
When the system stops and the bulb is at the same temperature as the refrigerant, PB equals PL, so only the spring keeps the valve closed. When the compressor runs, PL falls and superheat at the evaporator outlet rises; the bulb pressure PB rises with it and the valve opens until the refrigerant supply is right.
2. Superheat
- Static superheat (S.S.H) — superheat needed before the valve starts to open. The adjusting spring sets this point.
- Superheat change (S.H.C) — additional superheat needed for the valve to reach the proper opening.
- Total superheat (T.S.H) — static superheat plus superheat change. This is what is generally called “superheat”.
Car A/C valves (C, G and V types) are regulated internally: the superheat is set at the Fujikoki plant, so please state your required superheat setting when you order.
3. Internal or external equalization
Internal equalization
- Simple structure with a cost advantage.
- For small-capacity evaporators with little pressure loss.
- Example: all C type models.
External equalization
- Controls accurately to the set superheat.
- For large-capacity evaporators with pressure loss.
- F and J series: models with “-E” use an external equalizing tube from the evaporator outlet.
General guidance: use external equalization when the pressure loss in the evaporator is 0.02 MPa or more. With internal equalization on such an evaporator, the static superheat becomes extraordinarily high.
4. Choosing a valve when there is sub-cool
Divide the evaporator capacity by the correction factor for your condensing temperature and sub-cool, then pick a model whose capacity is equal to or larger than the result. Example from the catalog:
- ConditionsR404A · evaporator 4.0 kW · condensing 35°C · evaporating −20°C · sub-cool 25°C
- Correction factorFor condensing 35°C and sub-cool 25°C the factor is 1.45.
- Corrected capacity4.0 ÷ 1.45 = 2.76 kW
- SelectIn the R404A capacity chart for the F types, the 3.5 kW nominal model gives 3.19 kW at these conditions — select it.
Capacity charts and correction factors are in the Fujikoki catalog. Send us your conditions and we will check the model with you.
5. Installation tips
Thermal sensing bulb
- Mount it on the suction pipe between the evaporator outlet and the compressor inlet, on a flat, smooth, clean surface.
- Best: parallel to a horizontal suction pipe, in tight contact. On pipes of 3/4 inch or more, mount it between the horizontal plane and 45° below the pipe centre.
- Fix it tightly with the straps supplied and cover it with non-absorbent insulation.
- Do not mount it on a trap, where liquid and oil collect.
- With external equalization, mount the bulb closer to the evaporator outlet than the equalizing pipe connection.
- On an evaporator with a header, mount it on the compressor side beyond the header.
Valve body
- Do not install the valve with its outlet facing upward.
- Avoid installing it upside-down (diaphragm at the bottom) if at all possible.
- Both positions let foreign matter from the refrigerant oil collect inside the valve and may cause functional failure.
Source: FUJIKOKI Product Catalog (2016), pages 5, 6 and 45. Summarised; see the catalog for full details.
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