By Greg Scrivener
Evaporator pressure regulators are pressure regulators used in the suction line, leaving an evaporator to control the pressure. This means that they are inlet pressure regulators. They take the pressure input from the inlet piping and open/close the valve to maintain a set pressure. The valve outlet pressure always has to be lower than the inlet pressure otherwise there will be no refrigerant flow. There are a couple of reasons why you may want to control the suction pressure in an evaporator.
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In commercial refrigeration, these valves are most commonly referred to as evaporator pressure regulator (EPR) valves and occasionally as ORITs (based on a common model nomenclature). In industrial refrigeration, they are usually only referred to as suction pressure regulators or simply suction regulators.
Recall, of course, that by controlling the pressure in the evaporator, we are controlling the temperature in that evaporator. This type of control may be desirable or necessary for several reasons, including preventing ice formation on an air evaporator and minimizing defrost, preventing freezing in a chiller, or controlling the temperature of the evaporator to minimize product impact.
First, let’s take a look at the simple refrigeration system in Figure 1. In this system, the evaporator pressure, and therefore the temperature, is only a factor of the refrigeration system balance. If there is a decrease in load at the evaporator, less refrigerant will boil. The compressor has to move the same amount of refrigerant as the evaporator boils for the pressure to stay constant. That way if the evaporator reduces how much refrigerant boils, the pressure will drop until the flow balances. This happens because as the refrigerant pressure falls its density also decreases. Since the compressors we use are constant volume pumps, a lower density means less mass is moved through the compression each stroke or cycle. The condenser will reject the heat that was absorbed in the evaporator combined with the heat that was added during the compression.
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Special cases
In many refrigeration systems, the basic system described above works perfectly fine because as soon as there isn’t a load at the evaporator, the temperature is low enough that the system shuts down (or that evaporator shuts down at least). However, in some cases, this decrease in pressure and temperature is not acceptable. Consider an application with sensitive products like flowers. In some of these applications, we want to make sure that the air temperature off of the evaporator never decreases below a certain point. In order to do that, we can add a pressure regulator in the suction line, as shown in Figure 2.
This pressure regulator functions by adjusting to match the inlet pressure setpoint. Mechanical valves use a spring and bellows so that when the pressure drops below the setpoint, the valve is completely closed. As the pressure increases above the setpoint, the valve opens, depending on its sensitivity, until it is fully open. Using R507 as an example, if we set our EPR from Figure 2 at 76 PSIG, we would be assured that the evaporator coil never reached a temperature below 32 F. In fact, it would be at almost exactly 32 F whenever it was running, and this provides very consistent air temperature leaving the coil. Consider a water chiller as opposed to an air coil in this application. The evaporator temperature mustn’t go low enough to freeze the water. Water chillers also often must operate in a very large range of capacities and have compressors with unloaders and other capacity control. This means they are at fairly high risk of being in a situation where there is an imbalance between the compressor and evaporator that causes the pressure to decrease.
Frost levels
Similarly, it is usually desirable to reduce the amount of frost that builds up on an evaporator. In the case of an air conditioner that isn’t designed to handle any frost, an EPR can be used to maintain adequate suction pressure to prevent frost from forming at all. Employing an EPR in this way for air conditioning is not typical since most air conditioning systems can operate fine without this extra control but they are used on some specialty air conditioners. It is more common to see EPRs used on a system with multiple evaporators operating at different temperatures as shown, in Figure 3.
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It is possible to refrigerate a space to 35 F using a -20 F coil. However, the amount of frost that collects on a coil increases with a decrease in the coil temperature, and coincidently, this also results in low humidity in the space since more water is removed. In order to prevent this, an EPR can be used to maintain a higher temperature in the coil.
Drawbacks
Other than the obvious fact that using an EPR increases the cost of the system, there are two main drawbacks to using an EPR. The first is that there is always an energy penalty when you use a pressure regulator on a suction line. At full load, you have introduced a pressure drop that wouldn’t otherwise exist in the line, and if you recall the rule of thumb, for each 1 F SST equivalent in pressure drop, there is about a two per cent energy penalty. For our R507 example, that means if this regulator introduced a 1.5 psi pressure drop (which it likely would), there would be a two per cent energy penalty. The other drawback is a bit more subtle and applies mainly to systems with high peak loads. It is sometimes desirable to float suction pressures down to increase the refrigerated capacity of the evaporator. This can be done in modern refrigeration controls by decreasing suction pressure set points, which allows more compression (either with capacity control on a single compressor or by adding compressors). This option is not possible with standard EPR valves.
EPR valves are available with many options. Probably the most common is a combination EPR/suction stop valve that is used for hot gas defrost. These valves include a solenoid that, when energized, causes the valve to close so that hot gas can flow in the opposite direction up the suction line. Hot gas valve groups will be a topic of a future article since there is not enough time to go into detail here.
Strategically employing EPR valves can improve the operation of complex refrigeration systems; while using EPR valves on every circuit is common practice in both industrial and commercial applications, there is a substantial energy penalty that ought to be considered. There are additional complications when using these types of valves with pumped liquid; things can become even more challenging when electronic suction regulators are used since they behave differently in power outages. As is often the case, there is a delicate balance between operation and energy and lots of things to consider.
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