THERMOSTATIC EXPANSION VALVES

Direct expansion circuits must be designed and installed so that there is no risk of liquid refrigerant returning to the compressor. To ensure this state, heat exchange surface in the evaporator is used to heat the dry saturated gas into the superheat region. The amount of superheat is usually of the order of 5 K.

Thermostatic expansion valves (TEVs) for such circuits embody a mechanism which will detect the superheat of this gas leaving the evaporator (Figure 8.1). Refrigerant boils in the evaporator at Te and pe, until it is all vapour, point A and then superheats to a condition Ts, pe, at which it passes to the suction line, point B. A separate container of the same refrigerant at temperature Ts would have a pressure ps, and the difference ps — pe represented by C-B in Figure 8.1 is a signal directly related to the amount of superheat.

The basic thermostatic expansion valve (Figure 8.2) has a detector and power element, charged with the same refrigerant as in the circuit. The pressure ps generated in the phial by the superheated gas passes through the capillary tube to the top of the diaphragm. An adjustable spring provides the balance of ps — pe at the diaphragm, and the valve stem is attached at the centre. Should the superheat fall for any reason, there will be a risk of liquid reaching the com­pressor. The Ts will decrease with a corresponding drop in ps. The forces on the diaphragm are now out of balance and the spring will start to close the valve.

Conversely if the load on the evaporator increases, refrigerant will evapor­ate earlier and there will be more superheat at the phial position. Then ps will increase and open the valve wider to meet the new demand. 103

All liquid Superheat

TOC o "1-5" h z Evaporated ^

Te, Pe i Superheating gas pe _ Suction

^ . . w w ^ ‘ s> . „ ^ .

C

подпись: c^ * Y////////A ^

C

подпись: c

C

подпись: c

Expansion Te, pe valve

подпись: expansion te, pe valve

From

Condenser

подпись: from
condenser
Ts, ps

THERMOSTATIC EXPANSION VALVES

Temperature

Figure 8.1 Superheat sensor on direct expansion circuit

The phial must be larger in capacity than the rest of the power element or the charge within it may all pass into the valve capsule and tube, if these are colder. If this happened, the phial at Ts would contain only vapour and would not respond to a position Ts, ps on the T-p curve.

Use can be made of this latter effect. The power element can be limit charged so that all the refrigerant within it has vaporized by a predetermined temperature (commonly 0°C). Above this point, the pressure within it will cease of follow the boiling point curve but will follow the gas laws as shown in Figure 8.3 :

Јl = T

P2 T2

THERMOSTATIC EXPANSION VALVES

(a)

Condenser

подпись: condenser
 
(b)

Figure 8.2 Thermostatic expansion valve. (a) Circuit. (b) Cross section (Danfoss)

THERMOSTATIC EXPANSION VALVES

Temperature

Figure 8.3 Detector pressure for limit charged valve

And the valve will remain closed. This is done to limit the evaporator pressure when first starting a warm system, which might overload the drive motor. This is termed limit charging or maximum operating pressure. Such valves must be installed so that the phial is the coldest part.

The slope of the T—p curve is not constant, so that a fixed spring pressure will result in greater superheat at a higher operating temperature range. To allow for this and provide a valve which can be used through a wide range of applications, the phial may be charged with a mixture of two or more volatile fluids to modify the characteristic curve.

Some manufacturers use the principle of the adsorption of a gas by a porous material such as silica gel or charcoal. Since the adsorbent is a solid and cannot migrate from the phial, these valves cannot suffer reversal of charge.

Posted in Refrigeration and Air Conditioning