Description
APPLICATIONS
The Thermostatic expansion valves in series 221X, 222X, and 223X illustrated in this chapter are designed to work with the interchangeable orifice assembly, for flexible selection of capacity, and can be used in a wide range of applications as listed below:
• Refrigeration systems (display cases in supermarkets, freezers, ice cream and icemaker machines, refrigerated shipping, etc.).
• Air conditioning systems
• Heat pump systems
• Liquid chillers That use the following refrigerant fluids:
• HCFC (R22)
• HFC (R134a, R404A, R407C, or R507)
Mechanical thermostatic valves, designed for R22, R407C, R404A, R134a e R507 with liquid or MOP charge.
There are two different evaporation temperature ranges : -40 +15°C and -60 -25°C.
OPERATION
Thermostatic expansion valves regulate the flow of liquid refrigerant in evaporators. The superheating of the refrigerant fluid controls liquid injection.
Thermostatic expansion valves act as throttle between the high and lowpressure sides of refrigeration systems and ensure that the refrigerant flow rate into the evaporator matches the rate of evaporation of liquid refrigerant in the evaporator. If the actual superheating is greater than the set point, the valve feeds the evaporator more liquid refrigerant. If the actual superheating is lower than the set point, the valve decreases the flow of liquid refrigerant to the evaporator. This ensures that the evaporator is utilized fully and that no liquid refrigerant can reach the compressor.
CONSTRUCTION
The thermostatic expansion valve consists of two parts that must work together. The first is the body, which acts as the regulating system actuator. The second is the orifice, which contains the real regulator and performs the expansion of the refrigerating fluid.
Body assembly:
This consists of two parts: the thermostatic element and the body with its inner components. The thermostatic element is the valve motor. A sensing bulb is connected to the diaphragm assembly by 1.5 m capillary tubing, which transmits the bulb pressure to the top chamber of the valve’s diaphragm. The sensing bulb pressure is directly correlated to the temperature of the thermostatic charge, which is the gas mixture within the bulb.
The body is made from hot forged brass with right-angle connections. The interchangeable orifice assembly can be replaced through the inlet connection. A steel rod slides inside the body and transfers the diaphragm movement to the shutter inside the orifice assembly. When the thermostatic charge pressure increases, the diaphragm will be deflected, transferring this motion to the shutter, which lifts from its seat and allows the liquid to pass through the orifice. A spring opposes the force underneath the diaphragm and a side set screw can adjust its tension. Static superheating is increased by turning it clockwise and decreased by turning it counter-clockwise.
The thermostatic element is solidly connected by brazing to the forged brass body to avoid any leakage. The body assembly can be supplied with an internal or external equalizer. Both types can be supplied either with SAE Flare connections or with solder connections (outlet and external equalizer, if present). Both SAE Flare unions, required for threaded connections, and inlet SAE/ ODS connection, required for solder connections, must be ordered separately. Each body assembly is supplied with a strap unit, part no. G9150/R61, that allows the bulb to be fixed to the pipe.
Orifice assembly:
Interchangeable orifice assembly provides a wide capacity range, from 0.5 to 15.5 kW (nominal capacity with R22). The external cartridge contains the following elements: housing, shutter (metering device), seat, spring and filter. The solid construction of the orifice assembly and its internal components ensure that shutter and seat can withstand all types of critical stresses (hammering, cavitation, sudden pressure variations at temperature, or contaminants).
The spring holds the shutter firmly in contact with the seat to minimize leakage through the valve. To guarantee complete shut-off, a solenoid valve is required upstream from the thermostatic expansion valve.
Orifice assemblies are available in these two solutions:
• With conical flanged filter, for valves with SAE Flare threaded connections.
• With flat flanged filter, for valves with ODS solder connections, to use with adapters in series 2271. Orifice assembly filters can be cleaned or replaced. In the event of replacement, the following two types of filters are available for separate order:
• Filter 2290, for valves with SAE Flare threaded connections.
• Filter 2290/S, for valves with ODS solder connections.
THERMOSTATIC CHARGES
Liquid charge: it the behaviour of valves with liquid charge is exclusively determined by the temperature changes at the bulb and they are not subject to any environmental interference. They feature fast response time, reacting quickly in the control circuit. Castel thermostatic expansion valves with liquid charge cannot incorporate MOP functions.
Gas charge: the behaviour of valves with gas charge will be determined by the lowest temperature at any part of the expansion valve (thermostatic element, capillary tube or bulb). If any parts other than the bulb are subjected to the lowest temperature, the malfunctioning of the expansion valve can occur (charge migration). Castel thermostatic expansion valves with gas charge always feature MOP function and include ballast bulb. The ballast has a damping effect on the valve regulation and leads to slow opening and fast closure of the valve.
MOP (Maximum Operating Pressure): this function limits the evaporator pressure to a maximum value to protect the compressor from the overload condition. MOP is the evaporating pressure at which the expansion valve will throttle liquid injection into the evaporator, preventing the evaporating pressure from rising. The expansion valve acts as superheating control in the normal working range and acts as a pressure regulator within MOP range. The MOP point will change if the factory superheating setting of the expansion valve is changed. Superheating adjustments influence the MOP point as follows:
• Increase superheating → Decrease MOP
• Decrease superheating → Increase MOP
Superheating:
This is the controlling parameter for the expansion valve. Superheating, measured at the evaporator outlet, is defined as the difference between the actual bulb temperature and the evaporating temperature, deduced from evaporator pressure. In order to prevent liquid refrigerant from entering the compressor, a certain minimum superheating value must be maintained. In expansion valve operation, the following terms are used:
• Static superheating: this is the superheating above which the valve begins to open. Castel thermostatic expansion valves are factory pre-set to the following static superheating values:
• 5 °C for Castel valves without MOP
• 5 °C for Castel valves with MOP Under nominal operating conditions
• Opening superheating: this is the superheating, above the static superheating, required to produce a given valve potential
• Operating superheating: this is the sum of static and opening superheating.
Subcooling:
this is defined as the difference between the condensing temperature (deduced from condensing pressure) and the actual temperature at inlet valve. Subcooling generally increases the refrigeration system potential and must be accounted for when dimensioning an expansion valve. Depending on system design, subcooling may be necessary to prevent gas bubbles from forming in the liquid line. If gas bubbles form in the liquid line, the potential of the expansion valve will be reduced significantly. All potential tables, provided in this chapter, are calculated for a subcooling value of 4 °C. If the actual subcooling value is higher than 4 °C, the valve potential is taken from the evaporator demand divided by the correction factor shown in the tables below each potential table.
SELECTION
To dimension a thermostatic expansion valve for a refrigerating system correctly, the following design parameters must be available:
• Type of refrigerant
• Evaporator capacity, Qe
• Evaporating temperature/pressure, Te / pe
• Minimum condensing temperature/pressure, Tc / pc
• Liquid refrigerant temperature at valve inlet, Tl
• Pressure drop in the liquid line, distributor and evaporator, Δp
More detail about Arctic Refrigeration Components Supply Sdn Bhd
