Refrigeration Plant
A pressure-reducing device must be placed in the liquid line before the evaporator. In air conditioning, three such devices are in use: the thermostatic expansion valve, the electronic expansion valve and the capillary tube. Expansion valves have to be protected from dirt and moisture by properly sized strainers, filters and driers on their upstream side.
(a) The thermostatic expansion valve
The primary function of a thermostatic expansion valve is to reduce the pressure from the high value prevailing in the condenser to the low one in the evaporator. Its secondary function is to meter the flow of the refrigerant so that the mass flow pumped by the compressor equals that fed through the evaporator. It also meters the flow of refrigerant so that the vapour leaving the evaporator is somewhat superheated, the purpose being to ensure that only gas is pumped and that no liquid enters the compressor (where it would cause damage) under any circumstances.
Figure 12.1 is a diagram of a thermostatic expansion valve. The downward force exerted above the diaphragm, D, corresponds to the saturation vapour pressure exerted by the fluid refrigerant in the partly filled bulb, B, located at the suction line leaving the evaporator. The upward force acting beneath the diaphragm is the sum of that exerted by the adjustable spring, S, and that corresponding to the evaporator pressure. If the vapour leaving the evaporator is superheated the temperature at the bulb will exceed that in the evaporator. Hence the saturated vapour pressure above D will be greater than the evaporator pressure beneath it. The imbalance is met by S, which can be adjusted manually to give any amount of superheat required. If the load on the evaporator falls (say if the enthalpy of the air on to the cooler coil reduces) insufficient heat is absorbed to vaporise all the refrigerant and provide the superheat wanted. The latter therefore diminishes, the temperature sensed by B falls and the saturation vapour pressure above D, exerted by B, also reduces. An imbalance then exists across D which is rectified by the spring, S, partly closing the valve and lessening the refrigerant flow. A new balance is established with the upward force at D, exerted by the sum of the evaporating pressure and the spring, matched by the downward force produced by the saturated vapour pressure from the bulb B. The value is usually set in the factory so that it begins opening when the superheat is about 5° (see point 1 in Figure 12.1(d)). The valve is arranged to deliver 100 per cent rated capacity when the superheat is increased by above another 3°, corresponding to its proportional band (see section 13.2), giving a total superheat of approximately 8°C (points 0 to 2 in Figure 12.1(d)). Expansion
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Thermostatic expansion valve |
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Fig. 12.1 The thermostatic expansion valve, its connection and performance. |
Valves are commonly rated at an evaporating temperature of about 5°C, this being necessary because capacity for a given superheat setting decreases with pressure, owing to the relationship between saturation temperature and pressure. It is not worthwhile reducing the superheat setting in an attempt to increase the capacity of an evaporator: the gain is insignificant. The valve is often big enough to give a further 20 per cent or so of capacity but this should not be used when sizing it. If the valve selected is too large proportional control will tend to be lost at partial load and the valve will hunt with the risk of allowing liquid into the compressor. Hunting is not only the result of over-sizing the expansion valve: it may also be because of overshoot and undershoot caused by the time lag of the control loop (see section 13.3). If the valve is too small not enough liquid passes through when fully open to meet the load, system capacity is reduced and the suction pressure will fall. It is important that condensing pressure be controlled at a high enough value to give a sufficient pressure difference across the valve for the correct flow of refrigerant to meet the duty. If condensing pressure is allowed to drop too low the suction pressure will also fall and reduced capacity will ensue; there will be the added risk of hermetic motor burnout because of the lowered suction density.
The bulb should be clamped on top of a horizontal part of the suction line, between the two o’clock and ten o’clock positions, as close as possible to the evaporator outlet and before the equalising line connection. Although some expansion valves have internal pressure equalisation between their downstream side and the space beneath the diaphragm it is essential to use an external equalising line (see Figure 12.1) when there is an appreciable pressure drop between the expansion valve and the place in the suction line where the bulb is located. If an external equalising connection is not used with an evaporator circuit having a large pressure drop then, for the same evaporating pressure as with external equalisation, the closing pressure under the diaphragm will be that much larger. To achieve a balance, the pressure above the diaphragm exerted by the bulb will have to increase. This will be achieved by the expansion valve throttling the refrigerant flow until a higher superheat is developed at the evaporator outlet. The reduced refrigerant flow associated with this abnormally high superheat will give a serious fall in cooling capacity.
It is not possible to predict with any accuracy the performance of an expansion valve at partial duty because the valve is then over-sized and because of the influence of other factors, such as the design of the suction line, the location of the bulb, etc.
When sizing an expansion valve allowances must be made for the frictional losses through the condenser, the liquid line and its accessories (fittings, filters, driers, solenoid valves, isolating valves, distributor), the evaporator, the suction line and its accessories. Static head in the liquid line must also be taken into account. It is to be noted that pressure drop through the expansion valve is approximately proportional to the square of the mass flowrate of refrigerant and hence an increase in the condensing pressure will not give a proportional increase in flowrate and capacity.
A thermostatic expansion valve does not operate smoothly at less than about 50 per cent of its rated capacity.
Thermostatic expansion valves should not be used with flooded evaporators because a significant fall in capacity results from a few degrees of superheat.
(b) The electronic expansion valve
The electronically controlled, motorised expansion valve is a most efficient alternative to the thermostatic expansion valve. Four methods of providing power for actuating the valve movement are in use:
(i) Heat operated. An electrically heated coil, wound round a bimetallic element, receives an electronic signal from a controller, via a processor, and changes the temperature of the element, modifying its flexure. The element is connected mechanically to the valve stem and hence the valve movement is related to the electronic signal received. An alternative is to use a volatile material, encased in an electrically heated chamber, as the driving force. The expansion of the material is translated into a linear movement to operate the valve in response to the electronic signal.
(ii) Magnetically modulated. Valve movement responds to the pressure of a spring, actuated by the position of an armature that is smoothly modulated by a direct current electromagnet. The movement of the armature is related to the current flowing in the electromagnet which, in turn, depends on the electronic signal received from the controller.
(iii) Modulated pulse width. The expansion valve is a two-position solenoid valve with an exceedingly long cyclic life of millions of operations. The short durations of the solenoid valve in the open or closed positions are dictated by the load, through the electronic signal received.
(iv) Multiphase step motor. The rotary motion of the motor is translated into a linear movement to operate the valve, which can be very accurately positioned to open or close, in small increments, according to the electronic signal received. The quality of the valve is better than that of conventional expansion valves and its stroke is significantly longer, giving closer control.
Since it is easier to modify the electronics than to try and adapt the valve, electronic expansion valves readily accept the new refrigerants and seem able to get the best performance from a refrigerant that exhibits glide. The quality of control depends on the electronics, rather than the design of the valve.
The fact that a valve is motorised permits the addition of integral and derivative actions (see section 13.11) to the simple proportional control offered by the self-acting, thermostatic expansion valve. When coupled with electronics and small, dedicated computers, this opens the way for much more effective control over reciprocating refrigeration plant, used to chill water.
A self-acting expansion valve operates with about 8 degrees of superheat, when fully open, measured at the outlet from the evaporator. With the motorised valve, the amount of superheat can be measured by the temperature difference between two thermistors (see section 13.4), one located in the evaporator to sense the saturated evaporating temperature, and the other positioned in a passage within the compressor, just before the suction valve. Since, with the hermetic compressors normally used (see section 12.6), the suction gas rises in temperature by about a further 11 degrees as it flows through the motor driving the compressor, it follows that the gas entering the compressor could be as much as 19 degrees above the evaporating temperature. There is no virtue in having superheat, other than as a safety measure to prevent liquid entering the compressor under reduced load conditions. Hence, with a motorised, electronically controlled, expansion valve, its movement can be continuously monitored and its position adjusted so as to give zero superheat at outlet from the evaporator, whilst still maintaining the necessary, safe amount of superheat at entry to the compressor. The benefit of this is that all the evaporator surface can be used for evaporation, none being required for superheating, with a consequent significant increase in cooling capacity.
Tracking the valve position from an electronic processor that receives information regarding temperature from several thermistors has several other advantages, some of which are:
(i) With air-cooled machines the condensing pressure can be reduced under partial load by allowing some or all of the fans to continue running. This lowers the compression ratio, improves the volumetric efficiency and reduces the power absorbed by the compressor. (Water-cooled condensers, using conventional methods of pressure control, cannot achieve this in the same way.)
(ii) The evaporating pressure can be increased for design load conditions, as a direct consequence of being able to reduce the amount of superheat. This also helps to lower the compression ratio.
(iii) Chilled water return temperature can be measured and, since it is representative of the load on the system, used to raise the set-point of the temperature sensor controlling the flow temperature of the chilled water from the evaporator, under partial load conditions. This also raises the evaporating pressure and helps to minimise energy consumption. The re-set value of the chilled water flow temperature can be related to other control variables but it must be remembered, however, that even though the cooling load for the whole building may have reduced, there might still be components in the total load that need chilled water at the lower design temperature. This would impose a limitation.
(iv) A consequence of the above and the quick response of the electronic control system is that refrigeration capacity can be reduced by cylinder unloading (see section 12.6) in a manner that gives closer control over the chilled water flow temperature than is possible with conventional, electro-mechanical methods using step-controllers actuated from return chilled water temperature alone.
(v) Measurement of the rate of change of chilled water return temperature allows the rate at which cylinders are loaded or unloaded to be varied, in order to match the rate at which load changes occur during start-up and shut-down.
This is a small bore tube, principally used in domestic refrigerators and freezers. The diameter and length of the tube relate to the pressure difference between the condenser and the evaporator and restrict the flow of refrigerant while the compressor is running. Cutting the tube to a particular length provides the required pressure drop. When the compressor stops the refrigerant continues to flow until the pressures equalise. The capillary is additionally used as a restrictor to meter refrigerant flow in small, air-cooled, room air conditioning units. It has also been effectively adopted for water-loop, air conditioning/heat pump units. The flow of refrigerant is reversed when the units change their mode of operation, the roles of the evaporator and condenser being exchanged. This is acceptable to a capillary tube but would be impossible for an expansion valve.
EXAMPLE 12.1
Refrigerant 134a evaporates at 2°C in a direct expansion (abbreviated DX) cooler coil. What surplus pressure must the spring in the thermostatic expansion valve exert to maintain 8°C of superheat?
Answer
From Table 9.1 for the properties of saturated Refrigerant 134a we find that the saturation vapour pressure at 2°C is 314.5 kPa absolute and at 10°C (= 2° + 8 K) it is 414.99 kPa. The spring must therefore be adjusted so that it exerts a force corresponding to a pressure surplus of 414.99 — 314.5 = 100.49 kPa.
Posted in Air Conditioning Engineering