March 2011 / BULLETIN 10-9 Thermostatic Expansion Valves Theory of Operation, Application, and Selection
Page 2 / BULLETIN 10-9 THERMOSTATIC EXPANSION VALVES 10 Outstanding Features & Benefits of Sporlan Thermostatic Expansion Valves • SELECTIVE THERMOSTATIC CHARGES Designed to provide optimum performance for all applications—air conditioning and heat pump, medium and low temperature refrigeration. • THERMOSTATIC ELEMENT DESIGN Long lasting and field proven stainless steel diaphragm and welded element construction. • DIAPHRAGM DESIGN Large flat diaphragm permits precise valve control.
BULLETIN 10-9 / Page 3 SPORLAN THERMOSTATIC EXPANSION VALVES The thermostatic expansion valve (TEV) controls the flow of liquid refrigerant entering the direct expansion (DX) evaporator by maintaining a constant superheat of the refrigerant vapor at the outlet of the evaporator. Superheat is the difference between the refrigerant vapor temperature and its saturation temperature.
Page 4 / BULLETIN 10-9 valve’s diaphragm. The sensing bulb, capillary tubing, and diaphragm assembly is referred to as the thermostatic element. The thermostatic element on all standard Sporlan TEVs is replaceable. The diaphragm is the actuating member of the valve. Its motion is transmitted to the pin and pin carrier assembly by means of one or two pushrods, allowing the pin to move in and out of the valve port. The superheat spring is located under the pin carrier, and a spring guide sets it in place.
BULLETIN 10-9 / Page 5 P4 becomes more significant to TEV operation the greater the port area to effective diaphragm area ratio, and the greater the pressure drop varies across the valve port. Table 1 Evaporating Temperature °F Refrigerant Balanced Port TEVs 40 20 0 – 20 Sporlan introduced the concept of the balanced port thermostatic expansion valve in 1946 on large tonnage Types T and W valves.
Page 6 / BULLETIN 10-9 Figure 4 shows the same internally equalized valve on a system having the same evaporator pressure at the sensing bulb location. The evaporator coil, however, now has a pressure drop of 6 psi. Since an internally equalized valve senses evaporator pressure at the valve outlet, the total pressure in the closing direction becomes 58 psig plus the 12 psi spring pressure, or 70 psig.
BULLETIN 10-9 / Page 7 The adsorption charge consists of a noncondensable gas and an adsorbent material located in the sensing bulb. As the temperature of the bulb increases, gas is expelled (desorbed) from the adsorbent material increasing bulb pressure. Conversely, as the temperature of the bulb decreases, gas is adsorbed thus decreasing bulb pressure. Like the liquid and liquid-cross charges, the adsorption charge does not provide an MOP, and it will not migrate.
Page 8 / BULLETIN 10-9 ing the evaporator, which is usually termed hunting or cycling. The amount of hunting in a system is influenced by the design of the evaporator coil, suction line piping at the valve’s sensing bulb location, and the variability of the heat load on the evaporator. Hunting may cause a reduction in total system capacity, and a noticeable variation of evaporator pressure on systems having one evaporator. If hunting is severe, occasional floodback may result.
BULLETIN 10-9 / Page 9 floodback during compressor startup, reduces the load on the compressor after startup, and permits rapid pulldown. Since the majority of low temperature systems operate at or near a specific evaporating temperature, the TEV can be set for optimum superheat at the design temperature permitting the system to operate as efficiently as possible. The Types ZP Charges (FZP, VZP, SZP, RZP, and PZP) are gas-cross charges having the same operating range as the Type Z Charges.
Page 10 / BULLETIN 10-9 loads for long periods of time, and if slightly higher than normal superheats can be tolerated at full load conditions. Distributor Sizing — The proper sizing of the distributor is extremely important for systems using methods of capacity reduction. The function of the refrigerant distributor is to evenly distribute refrigerant to a multi-circuited evaporator. If the distributor cannot perform its function at all load conditions erratic TEV operation can be expected.
BULLETIN 10-9 / Page 11 The Types (E)BF and EBS valves feature a single pushrod which extends through the port of the valve. See Figure 8. The port and pushrod cross sectional areas are identical so that the opening force created by pressure drop across the port is canceled by the pressure drop across the pushrod. Furthermore, excellent pin and port alignment is provided by this design. Refer to the section, Effect of Pressure Drop Across the Valve Port, on Page 4 for additional information.
Page 12 / BULLETIN 10-9 Hot Gas Bypass and Desuperheating TEVs Systems which are required to operate at load conditions below the unloading capabilities of their compressors present an additional design problem. To balance the system under these conditions, bypassing a controlled amount of hot gas to the suction side of the system provides a practical solution. Bypassing hot gas is accomplished with a modulating control valve known as a discharge bypass valve.
BULLETIN 10-9 / Page 13 The Rapid Pressure Balancer (RPB) Feature — The thermostatic expansion valve with the Rapid Pressure Balancer (RPB) feature was developed by Sporlan in response to an industry demand for a TEV which would equalize system pressures during off-cycle more rapidly than a TEV with a permanent bleed port. In some cases, the bleed port has proved somewhat slow at equalizing system pressures creating restart problems for low starting torque compressor motors.
Page 14 / BULLETIN 10-9 serve as a secondary orifice to reduce pressure drop across the valve port. Thermostatic Charges for Ammonia Valves Thermostatic charges C, Z, and L are available for the Type D thermostatic expansion valve. The Type L thermostatic charge is the only charge available for the Type A valve. The Types C and Z thermostatic charges provide operating advantages for systems that cycle in response to a suction pressure switch or thermostat.
BULLETIN 10-9 / Page 15 When a Sporlan TEV is properly selected and applied, the factory superheat setting will usually provide an operating superheat in the range of 8 to 12°F. A precise determination of the valve’s operating superheat from a factory setting is not possible since factory settings are determined on the basis of static superheat, and opening superheat of the valve is influenced by several design factors within the system.
Page 16 / BULLETIN 10-9 Pressure losses in the liquid line result from friction and static pressure losses. Minimizing these pressure losses as much as possible is necessary for proper system design. Friction losses may be minimized by properly sizing the liquid line and liquid line accessories such as a solenoid valve and filter-drier. Static pressure losses, however, are solely the result of the weight of the vertical height of refrigerant liquid.
BULLETIN 10-9 / Page 17 The SVE-2 has valve capacity of: 2.00 x 1.06 = 2.12 tons at 40°F evaporating temperature, 100 psi pressure drop, and 90°F liquid temperature.
Page 18 / BULLETIN 10-9 FTP PipeFlange SAE Flare ODF SolderFlange ODF Solder SPORLAN SELECTIVE CHARGES ENGINEERED for PEAK PERFORMANCE for EACH SPECIFIC APPLICATION Recommended Thermostatic Charges* Refrigerant Air Conditioning or Heat Pump Commercial Refrigerant 50°F to –10°F Low Temperature Refrigerant 0°F to –40°F Extreme Low Temperature Refrigerant –40°F to –100°F 12 FCP60 FC FZ & FZP — 22 VCP100 and VGA VC VZ & VZP40 VX 134a JCP60 JC — — 404A SCP115 SC SZ &SZP SX 410A ZN
32011 / Bulletin 10-9 © 2011 Parker Hannifin Corporation Parker Hannifin Corporation Sporlan Division 206 Lange Drive • Washington, MO 63090 USA phone 636 239 1111 • fax 636 239 9130 www.sporlan.
