TEV with MOP: how to keep valve body temp above bulb temp?

I am using a small TEV on a liquid chiller as the metering device. This TEV (Danfoss TEN2, orifice 0X) is equipped with MOP point at 0 °C (32 °F). The TEV is feeding a coaxial heat exchanger. I chose a MOP type valve to reduce compressor overloading during pulldown.

Now, I have read in Sporlan documentation that the thermostatic element on the valve body and capillary tube must be at a higher temperature than the bulb to prevent charge migration from the bulb to the element.

What happens at my system is this: during pulldown the refrigerant starts evaporating directly after the valve needle, cooling not only the evaporator, but also the valve itself. Since coolant temperature is still high, the bulb temperature is soon higher than the valve body temperature.

Of course, this can be prevented by heating the thermostatic element on top of the valve just enough to keep it's temperature above bulb temperature under all circumstances. But what is the normal method of solving this problem?

Most TEVs with an MOP (maximum operating pressure)-type thermostatic charge use a gas charge which condenses in the desired operating range of the valve. If bulb temperature goes above this operating range, the thermostatic charge go back to its vapor state, giving us an MOP.

So why does a thermostatic charge in its vapor state give us an MOP? Consider when you raise the temperature of a vapor, you get a relatively small increase in pressure (which can be approximated by the ideal gas law), compared to raising the temperature of saturated liquid. So once the bulb temperature is warm enough to place the charge in its vapor state, then additional rise in bulb temperature will have less effect on opening the valve, compared to having liquid present in the bulb.

The condensed portion of an MOP charge, however, will migrate to the lowest temperature, just as liquid refrigerant does in an inactive refrigeration system. This may or may not be a problem, and it will depend on the temperature of the thermostatic element (power head). If the thermostatic element is at a temperature well above evaporating temperature, as one might expect during a pulldown, then there will be no problem. The TEV will "sense" high superheat, and the valve will open. However, if the thermostatic element is at a temperature near evaporating temperature, and it is also lower than the vapor temperature leaving the coil, then the TEV will starve.

Charge migration is more of a problem with medium temperature applications using internally equalized valves. Liquid temperature entering the TEV (a heat source for the valve) is typically closer to evaporating temperature, than for low temperature applications. Think of the difference between liquid temperature entering the TEV and evaporating temperature as a heat source... the greater the difference, the less likely you will have problems with charge migration.

An externally equalized TEV can also be effective in preventing charge migration. Consider that an internally equalized valve senses evaporator pressure thru an internal passageway. This passageway connects the downstream side of the valve, which will be at evaporator temperature, to the underside of the valve diaphragm. An externally equalized TEV senses evaporator pressure via the equalizer line which is normally connected at the outlet of the evaporator. This arrangement prevents the thermostatic element from further cooling.

I wonder if MOP valves are often misapplied. For example, I have seen them on self-contained equipment that also has a CPR valve. Also, I see them applied to equipment connected to parallel racks. Each of these examples cause confusion in a technicians observations.

I am not fond of MOP valves in such applications.

I have also seen icing on the valve or powerhead that causes the valve to apparently misfunction but only because of oversights from the equipment designers.... such as not defrosting the area where the distributor tubes and valve is located during defrost.

Actually, I am not fond of MOP valves in any application.:)

I added a small (24V/2W) resistive heater to the top of the valve. This seems to improve performance down to a coolant temperature of approximately 5 °C quite a bit. This heater is connected to the DC fans, which are controlled by a simple proportional regulator. Thus, the heater only works when nescessary.

I miss a large part of the TEV heating provided by the liquid refrigerant since the liquid line<->suction gas heat exchanger removes a lot of heat from the liquid refrigerant.

Dan:

I chose to use a MOP valve because my compressor is rated from -30 &deg;C (-22 &deg;F) to -10 &deg;C (14 &deg;F) evaporation temp. Without MOP the suction pressure would be high during pulldown, and the compressor is overloaded for quite a long time. Now with MOP, suction pressure is limited and compressor overload is less severe. It also reduces peak load on the condenser.

In this case, a MOP type valve seemed to have advantages. Am I wrong?

BTW: in fact a MOP type valve was not nescessary. The TEV maxes out at about 450W cooling (nominal capacity ~350W @ 8 bar pressure drop). The limited flow alone would have been sufficient to protect the compressor from severe overloading.

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Actually, I am not fond of MOP valves in any application.

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You may think of an MOP-type TEV as a low cost means of compressor overload protection. :p

I agree Dabit, it was a good idea. And professor, my supermarket background has distorted my opinion of MOP valves. Hussmann actually designs self-contained cases with compressors that require some form of overload protection. Why Hussmann uses both an MOP valve and a CPR valve has always befuddled me. Belt and suspenders?:)