Superheated Vapor

Fig. 37-27 Illustration of Expansion Valve Operation. Source: The Trane Company through the valve. The pressure reduction causes the liquid to boil, or flash. This heat absorbing process sensibly cools the liquid to the saturation temperature of 40°F (4°C), which corresponds to 69 psig (5.8 bar). The greatest limitation of a TXV is that its control can be unstable at low loads or under conditions of low-pressure differential across the valve.

An electronic expansion valve (EXV) performs the same function as a TXV, except it is operated with a microprocessor-based controller. The controller can be dedicated to the EXV or integrated into the microprocessor controlling the entire chiller operation. When integrated, EXV operation can improve low load and overall efficiency, based on the operating conditions of all chiller components.

A capillary tube, which is typically used in household refrigerators and small window air conditioning units, is simply tubing whose length and bore are precisely designed to control refrigerant flow to the evaporator. Pressure reduction between the condenser and the evaporator results from the friction loss, or pressure drop, in the long, small-diameter passage. One or more tubes are required, depending on the evaporator design. The use of capillary tubes is largely limited to pre-assembled units because the bore diameter and length are critical to its efficiency.

A float valve is used with flooded evaporators. Its assembly is a variable orifice device located in a chamber connected to the evaporator. It is actuated by a float immersed in a liquid container. A low-side float valve is operated by low-side level. It opens at a low level and closes as the liquid refrigerant, under pressure, forces the float to rise. A high-side float valve is operated by high-side level. The valve opens when the liquid level increases to the liquid level in the float chamber and admits liquid to the low side. Float valves maintain the evaporator refrigerant over a wide range of operating conditions and are commonly used in ammonia systems in industrial refrigeration applications and with large flooded evaporators. Typically, these are applied in chilled water systems with capacities in excess of 150 tons (500 kW^).

Fixed orifice type metering devices are typically fixed hole plates, or a hole in a series of plates, in the liquid refrigerant line entering the evaporator. The amount of liquid refrigerant passing through the device is based on the level of refrigerant in the condenser line and the pressure differential between the evaporator and the condenser. Fixed orifices have no moving parts and are, therefore, less subject to wear than float valves. However, part-load performance is not as good. Similar to float valves, fixed orifices are typically applied in chilled water systems with capacities in excess of 150 tons (500 kW^).

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