Valves expansion

The purpose of the expansion valve is to control the flowof refrigerant from the high-pressure condensing side of the system into the low-pressure evaporator. In most cases, the pressure reduction is achieved through a variable flow orifice, either modulating or two-position. Expansion valves maybe classified according to the method of control. 

Such a valve is a metering device and may not provide positive shut-off when the compressor is stopped. Under these circumstances, refrigerant will continue to leak into the evaporator until pressures have equalized, and the liquid level might rise too close to the suction outlet. To provide this shut-off, a solenoid valve is needed in the liquid line. 

The commonest form of level detector is a metallic float carrying an iron core which rises and falls within a sealing sleeve. An induction coil surrounds the sleeve and is used to detect the position of the 

Should a float control fail, the level in the shell may rise and liquid pass into the compressor suction. To warn of this, a second float switch is usually fitted at a higher level, to operate an alarm and cut-out. 

Where a flooded coil is located in a liquid tank, the refrigerant level will be within the tank, making it difficult to position the level control. In such cases, a gas trap or siphon can be formed in the lower balance pipe to give an indirect level in the float chamber. Siphons or traps can also be arranged to contain a non-voIatile fluid such as oil, so that the balance pipes remain free from frost. 

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Work that could be obtained in turbine is small, and iturbine is substituted for an expansion valve. For the reasons of proper compressor function, wet compression is substituted for an compression of dry vapor. 

Becker also analyzed a similar cycle in which the expansion valve was replaced by a turbine and obtained an efficiency of 48.1%. Analysis of Becker s single-expander cycle gave an efficiency of about 40%. 

Refrigerant circulation or kettle or Chiller 2. Expansion valve capacity. 

This formulation applies both to the use of a semi-continuous cycle with an expansion valve and to a discontinuous cycle (such as that in the solar refrigerator) using a flooded evaporator in which the warm condensate must first cool itself before it can cool the load. 

Flash chamber A chamber provided to al low the burning of a flammable gas in a process. In a refrigeration system, it is the separating tank between the expansion valve and the evaporator. 

The gas is routed through heat exchangers where it is cooled by the residue gas, and condensed liquids are recovered in a cold separator at appro.ximately -90°F. These liquids are injected into the de-methanizer at a level where the temperature is approximately -90°F. The gas is (hen expanded (its pressure is decreased from inlet pressure to 22.3 psig) through an expansion valve or a turboexpander. The turboexpander Lises the energy removed from the gas due to the pressure drop to drive a compressor, which helps recompress the gas to sales pressure. The cold gas f-)50°F) then enters the de-methanizer column at a pressure and temperature condition where most of the ethanes-plus Lire in the liquid state. 

Refrigerating Effect. This is the heat absorbed in the evaporator per lb of refrigerant. It is determined by the difference in enthalpy of a lb of refrigerant vapor leaving the evaporator and that of a lb of liquid just upstream (ahead) of the expansion valve at the evaporator. From Figure 11-48A,... 

Comparison of Effect of System Cycle and Expansion Valves on Required Horsepower... 

Two compressors (one dual-effect type) Individual expansion valves 47.3 10.2... 

Three individual compressors Individual expansion valves 45.8 13.1... 

Compound compressors and intercoolers Individual expansion valves 45.2 14.2... 

A phial senses the temperature of the outlet suction line to the compressor and controls the expansion valve opening to maintain a constant temperature in the coil. 

Larger condenser 1. Lower condensing temperature and colder liquid entering the expansion valve, giving more cooling effect 2. Lower discharge pressure, so a lower compression ratio and less power. 

Condensers are sized so that they can reject the system heat load under maximum conditions of air or water temperature. In colder weather, the condensing temperature will fall with that of the cooling medium and this may cause difficulties in correct operation of the plant. In particular, the pressure across the expansion valve (see also Chapter 8) may be too low to circulate the required mass flow of the refrigerant. Under such circumstances, artificial means must be used to keep the head pressure up, always remembering that the condensing pressure should be kept as low as practical for power economy. 

The total refrigerant charge required in a circuit will vary with different operating loads and ambients, and must be sufficient at all times so that only liquid enters the expansion valve. This implies... 

The purpose of the evaporator is to receive low-pressure, low-temperature fluid from the expansion valve and to bring it in close thermal contact with the load. The refrigerant takes up its latent heat from the load and leaves the evaporator as a drygas. Evaporators are classified according to their refrigerant flow pattern and their function. 

Expansion valves for such circuits embody a mechanism which will detect the superheat of this gas leaving the evaporator (Figure 8.5). Refrigerant boils in the evaporator at Tg and p, until it is all vapour, and then superheats to a condition Tj, p, at which it passes to the suction line. A separate container of the same refrigerant at temperature Tj would have a pressure and the difference p is a signal directly related to the amount of superheat. 

Figure 8.6 Thermostatic expansion valve, (a) Circuit, (b) Cross-section (Courtesy of Teddington Controls Ltd)...

Where an evaporator coil is divided into a number of parallel passes, a distribution device with a small pressure loss is used to ensure equal flowthrough each pass. Pressure drops of 1-2 bar are common. There will nowbe a much larger finite difference between the pressure under the diaphragm and that at the coil inlet. To correct for this, the body of the valve is modified to accommodate a middle chamber and an equalizing connection which is taken to the coil outlet, close to the phial position. Most thermostatic expansion valves will have provision for an external equalizer connection (see Figure 8.8). 

The thermostatic expansion valve is substantially an undamped proportional control and hunts continuously, although the amplitude of this swing can be limited by correct selection and installation, and if the valve always works within its design range of mass flow. Difficulties arise when compressors are run at reduced load and the refrigerant mass flow falls below the valve design range. It is helpful... 

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