Thermostatic Expansion Valves

Thermostatic Expansion Valves

Thermostatic expansion valves must be selected and applied in accordance with the manufacturer´s instructions. Either internally equalized or externally equalized type, the external equalizerline must be connected, preferably at a point beyond the expansion valve thermal bulb. Do not cap or plug the external equalizer connection as the valve will not operate without this connection.

Valve superheat should be preset by the valve manufacturer, and field asjustment ahould be set to provide 5º F. to 10º F. superheat at the thermal bulb location. Too high a superheat setting will result in starving the evaporator, and can cause poor oil return. Too low a superheat setting will permit liquid floodback to the compressor.

A minimum of 15º F. superheat at the compressor must be maintained at all times to insure the return of dri gas to the compressor suction chamber, and a minimum of 20º F. superheat is recommended. Note that this is not superheat at the expansion valve, but should be calculated from pressure measured at the suction service valve and the temperature measured 18´´ from the compressor on the bottom of a horizontal run of suction line tobing. Lower superheat can result in liquid refrigerant flooding back to the compressor during variation in the evaporator feed with possible compressor damage as a tinually returing to the compressor wear, as well as resulting in a loss of capacity.

It is important that users realize that flash gas in the liquid line can seriously affect expansion valve control. So long as a head of pure liquid refrigerant is maintained at the expancionvalve, its perfonmance is relatively stable. but if flash gas is mixed with liquid refrigerant fed to the valve, a larger orifice opening is required to feed the some weight of liquid refrigerat. The onlu way the orifice opening can be increased is by an increase in superheat, and as the persentage of flash gas increases, the superheat increases, the valve opens wider, and the evaporator is progressively more starved.

If the valve has been operating with a large percentage of flash gas entering the expansion valve, and a head of pure liquid refrigerant is suddenly restored, the orifice opening will be larger than required for the load, and liquid will flood throung the system to the compressor until the valve again regains control. Conventional expansion valves with the thermal bulb strapped to the suctionline may be somewhat sluggish in response, and it may be several minutes before control can be restored to normal.

Tipically, changes in the qualit of liquid refrigent feeding the expansion valve can occur quickly and frequently because of the action of head pressure control devices, sudden changes in the refrigeration load, hunting of the expansion valve, action of an unloading valve, or rapid changes in condensing pressure.

On systems with short suction lines and low superheat requirements, quick response thermal bulbs or wells in the suction line may be essential to avoid periodic floodback to the compressor.

Temperatures and pressure alone may not give a true picture of the actual liquid refrigerant control in a system. Excessive oil circulation has the effect of increasing the evaporating temperature of the refrigerant. The response of the expansion valve is based on the saturation pressure and temperature of pure refrigerant. In an operating system, the changed pressure-temperature characteristicof the oil rich refrigerant will give the expansion valve a false reading of the actual seperheat, and can result in a somewhat lower actual supeheat than apparently exists, causing excessive liquid refrigerant floodback to the compressor. the only real cure for this condition is to reduce oil circulation to a minimum. Normally excessive oil in the evaporator can only result from an excessive system oil charge or other factors which could cause excessiveoilcirculation, or from low velocities in the evaporator which result in oil logging. In low temperature applications where proper oil circulation cannot be maintained, an oil separator may be required.

Vapor charged valves are sastifactory for air conditioning usage, and are desirable in many cases because of their inherent pressure limiting characteristic. For all refrigeration applications, liquid charge valves should be used to prevent condensation of the charge in the head of the valve and the resulting loss of control in the event the head becomes colder than the thermal bulb.

A pressure limiting type valve may be helpful in limiting the compressor load, and also prevents excessive liquid refrigerant floodback on star-up on systems using hot gas defrost, the defrost load is normally greater than the refrigeration load, and some other means of limiting the compressor power input must be used if required.

The thermostatic expansionvalve must be sezed properly for the load. Although a given valve normally has a wide operating capacity range, excessively indersized or oversized valves can cause system malfuctions. Undersized valves may starve the evaporator, and the resulting excessive superheat may adversely affect the system perfonmance. Oversized valves can cause hunting, alternately starving and flooding the evaporator, resulting in extreme fluctuations in suction pressure.