TEVs: Fine points of maximum operating pressure

April 20, 2000
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The thermostatic expansion valve (TEV) is often one of the most misunderstood components in a vapor compression refrigeration system.

To further complicate matters, the feature known as the MOP (maximum operating pressure) is often included on TEVs. What does MOP really mean? What is its purpose in the system? How does it work?

These are a few of the questions that will be answered in this article.

What's MOP?

As stated above, MOP is an acronym for maximum operating pressure. Some manufacturers also promote the MOP as motor overload protection, with the motor understood to be the system compressor.

Also, it might be appropriate to refer to this feature as the “MOT,” as this is very much a temperature-driven response to system conditions. The temperature relation to the MOP feature will become apparent later in the article.

The MOP is normally used to prevent system flooding, compressor overload, or to limit flow at startup under lightly loaded conditions. In some ways it is appropriate to compare the MOP of a TEV to the function of the traditional crankcase pressure-regulating valve, or the mechanical pressure-limiting expansion valve elements.

A thermostatic charge with the MOP feature causes the TEV to appear to close above a predetermined evaporator pressure. This limits the maximum evaporator pressure at which the system can operate.

Air conditioning and heat pump applications usually require a pressure-limiting, MOP-type thermostatic charge to limit compressor loading during system pulldown. In this application, the pressure-limiting charge appears to cause the TEV to remain closed until the system evaporator pressure is reduced below the MOP of the charge. This enhances the pulldown capabilities of the system compressor. (See Figure 1.)

How it works

At a predetermined temperature, the gas charge in the expansion valve’s bulb becomes superheated. This limits the opening force exerted on the valve’s diaphragm assembly.

A high superheat condition now occurs at high evaporator pressures. This gives the appearance of a closed or starving TEV, when in fact the valve does continue to feed refrigerant, although it now opens at a much lower rate. This starts to occur when the temperature sensor, the bulb, reaches a somewhat predetermined maximum temperature.

Since the MOP charge is designed to limit suction pressure beyond a specific bulb temperature, let’s discuss how this happens.

The thermostatic element on the expansion valve consists of two volumes and the charge mix usually consists of at least two components. In the general industry, the thermostatic element is sometimes referred to as a power element or power head. The sensing bulb and the element head or diaphragm section comprise the two volumes. (See Figure 2.)

Regarding the charge components, one is normally a refrigerant that is selected to allow the valve to control the proper range of superheats within a specified evaporator temperature range.

The second charge component is normally a gaseous substance that is “noncondensable.” This means at relatively normal temperatures and pressures, the substance will not condense into a liquid. The noncondensable is used to fine tune the charge and shift the superheat characteristics as necessary. Consider that it acts like a mechanical spring.

The two volumes (let’s call them V1 and V2) are charged with the specific components that were discussed earlier (Figure 2). As the suction line temperature increases, the bulb temperature also increases. This has a tendency to drive the refrigerant charge component to the head of the expansion valve; this is V2.

Once most of the refrigerant charge component is in the head of the thermostatic element, all that remains in the sensing bulb, V1, is the noncondensable constituent. As discussed earlier, the noncondensable is in a gaseous state.

In fact, by this time so little refrigerant component is in the bulb, the noncondensable charge component acts very much like an ideal gas and allows the superheat to increase while controlling the suction pressure relative to the sensing bulb temperature. This is the MOP effect.

What really happens

Consider a situation with a load on the system at startup.

1. The bulb of the TEV is warm and drives the valve to a more open position in response to superheat.

2. The suction line temperature increases as a result of the increased flow and the current load.

3. This, in turn, causes the TEV bulb temperature to increase. The refrigerant component of the thermostatic charge changes state from a liquid to a gaseous condition.

4. This component of the charge now migrates to the head of the valve. The noncondensable component remains in the bulb and acts as an approximation of an ideal gas, limiting the opening force applied to the diaphragm assembly and, hence, the valve flow.

The valve is not closed at this point. It simply does not open as would a valve without the MOP feature given these conditions.

The valve will continue to open as higher temperatures are applied to the sensing bulb. The Ideal Gas Law predicts 1-psi change in outlet pressure for every 7°F increase in bulb temperature. (See Figure 3.)

MOP numbers

The MOP designation marked on the valve indicates the downstream pressure controlled by the TEV under specific test conditions.

Usually the test conditions include a suitable test orifice and inlet pressure specified for the product by the manufacturer. These conditions are usually established by the manufacturer to achieve a static superheat condition with a specific inlet pressure and flow.

The MOP designation is established at a point on the curve where the refrigerant charge component has totally migrated to the head of the valve. (See Figure 4.)

There is generally an 8- to 10-psi differential between the MOP designated on the valve and the actual MOP experienced by the system. (Again, refer to Figure 4.)

The MOP designation on the valve is established at a relatively high bulb temperature. This ensures complete migration of the refrigerant component of the charge. The actual system temperatures are generally much lower than the production test temperature. However, system conditions can achieve temperatures that engage the MOP.

Factors that affect the MOP

Valve stroke, bulb temperature, and compressor flow rate all impact the overall MOP effect.

Remember Figure 1 on page 40? As the valve strokes, the element volume changes. This would typically allow the flow and resulting outlet pressure to increase. This means the MOP increases.

Valve stroke, bulb temperature, and compressor capacity (system mass flow rate) all directly affect the MOP. Once you accept the fact that the MOP is merely an outlet pressure controlled by the TEV, you will also understand the effect of initial valve adjustment or setting.

The higher the initial flow through the TEV, the higher the resulting MOP. (See Figure 5 to see the impact of different initial adjustments on the MOP.)

Summing it up

  • The MOP is a downstream pressure controlled by the TEV under certain conditions.
  • The MOP does not achieve the sharp control of a pressure-regulating valve, solenoid valve, or electrical switch.
  • The TEV does continue to move to a more open position if the bulb temperature continues to increase, although at a much slower rate than it did before reaching the point where the MOP will engage.

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