Valve expansion, thermostatic

EXPANSION VALVE, THERMOSTATIC - Control valve operated by temperature and pressure within evaporator. It controls flow of refrigerant. Control bulb is attached to outlet of evaporator. 

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... 

Figure 8.8 Thermostatic expansion valve with external equalizer...

The thermostatic expansion valve can also be used to maintain a liquid level. The phial and a heater element are both clamped to a bulb at the required liquid level. Illiquid is not present, the heater warms the phial to a superheat condition and the valve opens to admit more liquid. 

Systems are normally designed to work satisfactorily during maximum amhient conditions, and the condenser will he sized for this. In colder weather, the condensing temperature and pressure will fall and the resulting lower pressure difference across a thermostatic expansion valve may lead to malfunction. A drop of pressure difference to half the normal figure may reduce mass flow helow that required, and it will he necessary to prevent the condenser pressure from falling too low. 

Thermostatic expansion valves or electronic expansion valves for most dry expansion circuits. 

Troubles arise with the selection of thermostatic expansion valves, since this is the type generally used in custom-built systems and, for these, selected outside a factory. 

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 maybe unstable and hunt , with slight loss of evaporator efficiency Since the required duty is less, this is of no great importance. It is possible to fit two expansion valves in parallel, one for the minimum load and both for the full load, but this arrangement is not usually necessary... 

Where fluid pressure is carried by a capillary tube, such as with the thermostatic expansion valve or pressure switches, the tube should be installed with due attention to the risk of it chafing against metal edges and wearing through. Tubes to manometers are usually in plastic, but may be copper. These must be carefully tested for leaks, as they are transmitting very low pressures. 

Halocarbon systems are almost invariably controlled by mechanical or electronic thermostatic expansion valves, requiring a superheat signal to operate the control. The superheating of the suction gas into the compressor will cause it to expand, resulting in a lower mass flow for a given swept volume. Reduction of the superheat setting of the expansion valve will therefore result in better use of the compressor. The limit will be reached when there is insufficient signal to work the expansion valve. 

The required refrigerating equipment is divided into individual circuits as described above between the cooling circuits and the operating groups. It includes the connecting piping and valves such as liquid magnetic valves and thermostatic expansion valves (provide manufacturer information). 

EXTERNAL EQUALIZER - Tube connected to low-pressure side of a thermostatic expansion valve diaphragm and to exit end of evaporator. 

NON-FROSTING EVAPORATOR - Evaporator which never collects frost or ice on its surface. Uses only thermostatic expansion valves. 

PHIAL -Term sometimes used to denote the sensing element on a thermostatic expansion valve. 

A thermo-expansion FW regulator is employed on larger WT boilers when FW pumps run continuously. The thermo-expansion regulator uses a thermostat located at the steam-water interface NOWL. The device expands when the water level drops because it is surrounded by (higher temperature) steam. This expansion moves a mechanical linkage, which opens the valve. 

The flow method ts the simplest and the most straightforward. In the flow method, the solvent fluid is supplied to a compressor by a pressure cylinder. At the desired pressure, the fluid passes into the thermostatted extractor cell that contains the solute present in appropriate matrix (e.g., multiple layer of glass wool). The fluid dissolves the solute in the extractor and, on expansion through a heated metering valve, precipitates solute into a series of collection vessels to be measured gravimetrically. The volume of the decompressed fluid is totaled by a wet or dry gas test meter. Static or equilibrium solubility measurement methods are used to eliminate the need to sample the supercritical fluid solution. A high-pressure flow cell is placed in the flow circuit to monitor the dissolution process by spectrophotometry. 

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