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MOP: Maximum Operating Pressure
Jan 21, 2021
On our refrigeration unit, the Thermal Expansion Valve has MOP function. What is MOP function? How to achieve it? What should be paid attention to in its application? The following is a brief analysis.

MOP: Maximum Operating Pressure
The opening of Thermal Expansion Valve is regulated by three forces, Pb: temperature sensing bag (valve opening force), PE: Evaporation pressure (closing valve force), PS: Spring force (closing valve force). As shown in the figure
Because the current of the compressor is closely related to the suction pressure, controlling the suction pressure of the compressor can prevent the compressor from overload. The MOP function of the expansion valve is to limit the liquid supply of the evaporator and avoid the high pressure to protect the compressor.

The realization process is mainly through the control of the expansion valve temperature chamber working fluid charge.When the evaporation pressure (1) rises to a certain value (MOP point), the refrigerant in the temperature sensing bag will be vaporized completely (after that, the change of the temperature sensing bulb pressure meets the gas state equation: P1 / P2 = T1 / T2). When the suction pressure increases again, the temperature sensing bulb pressure changes slowly (dotted line after MOP point), that is, the change of Pb in the figure above is not obvious, but when Pe increases, the valve will close, reducing the refrigerant flow and limiting the increase of evaporation pressure.
The filling amount of working fluid in the temperature wrap with MOP function is very small. Take Danfoss TX2 Series B valve with MOP function as an example, the volume of temperature sensing bag is about 0.008L. According to the temperature at MOP point of - 20 ℃, R22, V ''= 92.84 (L / kg), the filling volume of temperature sensing bag is about 86.17mg. (And this is why it is often said that the filling capacity of the temperature sensitive package of the valve with MOP function is only tens of milligrams. Of course, the filling amount of working fluid in the temperature sensitive package is different when the temperature of MOP point is different).

Valve application with MOP function:
1. It is mainly used for quick freezing and low temperature cold storage.
2. The application of two-stage compressor system is common.

Application notes:
Due to the small amount of working fluid filling in the temperature sensing package, the liquid part must be kept in the temperature sensing package during operation, and can not be moved to the valve head part --- the expansion valve should be installed on the periphery of the cold storage as far as possible, and the valve head should be insulated. The temperature sensitive package should be installed horizontally as far as possible, and the capillary tube should be upward.

Note 1: in fact, it is not the Evaporation pressure, but the temperature of the suction pipe is sensed by the temperature sensing package. The superheat of the expansion valve is also covered here, so the introduction is more complicated
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