ACHRNEWS

Here's a closer look at EPR valves

March 29, 2000
Evaporator pressure regulators (EPRs) are a common sight in the machine room. Typically, you will find one on each and every suction line circuit on the parallel refrigeration system. EPR valves are also known as inlet, upstream, and back-pressure regulators.

Lets first review the basic designs, principles of operation, and application of these valves.

Valve design and operation

There are two basic types of inlet pressure regulators, the direct operated and the pilot operated.

The inlet pressure acts on the underside of the diaphragm; when the force created by the upstream pressure exceeds the force of the range spring, the diaphragm is lifted off of the pilot seat and flow occurs from the regulator inlet to the outlet.

Increased inlet pressure lifts the diaphragm farther, allowing increased flow. A decrease in inlet pressure causes the diaphragm to move closer to the pilot seat, thus reducing the flow. Once the inlet pressure drops below the regulator setting, the diaphragm closes against the pilot seat, thereby shutting off flow.

The diaphragm in the direct-operated valve (as illustrated in Figure 1) has a stroke of 0 to 0.015 in., and is 10 times the size of the pilot seat area, which results in the sensitivity of the valve. This is practical for small-capacity valves but not for larger valves.

If we were to design a direct-operated valve with a port size of 1 3/8 in., the diaphragm would have to be at least 14 in. dia. This would not be very practical to manufacture or install on a system because of the physical size and weight of the valve.

The alternative is to design a valve that is pilot operated. There are two common types of pilot-operated inlet regulators used specifically for commercial applications. One valve type uses high-pressure discharge gas to pilot the main valve, while the other uses upstream pressure for the same purpose.

The discharge gas method uses a hot gas header, which runs parallel to the suction header, to supply discharge gas to operate each EPR valve. This method will add 150 to 250 additional high-side joints to each supermarket machine room. Each valve requires at least four high-side joints.

Refrigerant costs have risen dramatically since the phaseout of the once low-priced refrigerants (R-12 and -502). The new refrigerants still cost more, and this has made the manufacturers design systems that use the least amount of joints, which helps reduce possible gas loss. It just makes good sense to reduce as many joints in the system, whenever possible.

The other method is to pilot the regulator with the upstream pressure internally transmitted through the valve body. For example, the Sport II® valve (manufactured by the Refrigerating Specialties Division of Parker Hannifin Corp.) operates by two pressures, the same as the direct-operated valves: the upstream and the range spring pressures. This method helps reduce the bleed rate of discharge gas through the valve to the suction side of the refrigeration system.

Throttling point

In all cases, a throttling point serves to increase or decrease the rate of delivery of the pilot stream from the upstream side of the valve to the space on top of the piston, from which it is bled to the outlet of the valve via both a bleed hole through the piston and the clearance between piston and bore.

An increased pilot stream flow increases the pressure on top of the piston that pushes the modulating plug down. Conversely, a decreased pilot stream flow reduces the pressure on top of the piston and permits the closing spring to push the modulating plug up, reducing the flow area available at the port. The pilot portion of the valve is devoted to administering this pilot stream flow, thus effectively controlling the main valve opening.

The inlet pressure is applied via a passage to the underside of the diaphragm at the chamber. Con-sidering a valve that is initially closed as the inlet pressure rises, the diaphragm exerts a force upward against the range spring.

When that force developed by the inlet pressure is high enough, it will equal the spring force at that point of adjustment and the diaphragm will rise off its seat, permitting flow from the upstream to the chamber, thus raising the pressure on top of the piston, causing the modulating plug to move down, and opening the port.

Should the system conditions cause the upstream pressure to decrease, the diaphragm will return to a position closer to the pilot seat, reducing the pilot stream flow, and allowing the pressure in the chamber to bleed to the downstream side of the valve, which permits the closing spring to move the modulating plug up, thus closing the port.

A valve with the “S” feature will act to regulate upstream pressure in its normal fashion when its solenoid coil is energized, and shut tightly when the solenoid coil is de-energized. The pilot solenoid is arranged so that when closed, it will intercept the pilot stream before it reaches the pilot section of the regulator, thus ensuring that the pressure on top of the piston is the same as downstream pressure and permitting the closing spring to close the main valve. When the solenoid is energized, it permits the full upstream pressure to be delivered to the top of the piston.

The “S” feature’s primary function is to give the valve the capability of a suction stop, in order to utilize reverse gas defrosts. This feature can be added in the field at a later date due to the modular design of the valve.

The “B” feature’s primary function is to give the valve the capability of controlling a circuit at dual temperatures. This type of valve is used on low-temperature refrigeration systems by operating the valve in the wide-open position. This is accomplished when both solenoids are energized.

When the bypass coil is de-energized and the shut-off coil is energized, the circuit temperature will rise to a desired medium temperature range. This can also be added in the field at a later date.

Sizing EPRs

There are two different methods of sizing EPR valves. The valves can be sized to have minimal pressure drop or a higher fixed pressure drop by reducing the effective port size of the valve.

Sizing for maximum pressure drop, the reduced port size will compensate for the pressure drop of the range spring. For example, if the evaporator pressure is 10 lb higher than the suction pressure, you might select a valve with a 7-lb pressure drop at full capacity. This will allow you to select a valve with a port size two or more sizes smaller than the suction line size.

The range spring will control the valve the 3-lb difference in the setting to achieve the desired evaporator pressure. This method allows you to reduce the size of the valve, which will lower the initial cost of the smaller valves that are selected.

This is practical to apply because capacities of valves increase with the increase in pressure drop according to this formula: Capacity 2 = capacity 1 (delta P2 / delta P1) 1/2.

Examples: Four times the pressure drop doubles capacity, nine times the pressure drop triples capacity, and 16 times the pressure drop quadruples capacity.

The minimal-pressure-drop sizing method has all of the valves selected at the minimal pressure drop capacity for each and every application. The port size, typically will be the same size as the suction line size of each circuit.

First cost vs. benefits

There are several advantages to selecting the low-pressure-drop valves with their initial higher cost.

The low-pressure-drop design has two immediate benefits for the operation of the refrigeration system. Valves that are the lowest evaporator temperature circuits on the system will not affect the suction temperature setting. And the most important advantage is that there will always be a faster recovery of case or walk-in temperature after every defrost cycle.

There are two types of EPR valves to chose for refrigeration systems, the discharge gas or the inlet-piloted-operated regulators to control the temperature in the cases. There also are two different methods of sizing the valves to comply with each specific refrigeration requirement.

The options are there for you, so look at all of them closely.

Sidebar: A word on modulating plugs

There are several different designs of modulating plugs used in regulators. There are flat and parabolic plugs used in commercial refrigeration and characterized type plugs in industrial refrigeration.

The parabolic and characterized plugs develop a linear relationship between stroke and flow area, which results in close control. This plug design gives the valve accuracy to hold its setpoint over a broad range of flows.

Sidebar: Take the quiz!

1. What are EPR valves also known as? ___________________ ___________________ ___________________

2. The inlet pressure acts on the (topside, underside) of the diaphragm.

3. It makes good sense to (reduce, increase) as many joints as possible in the system.

4. A throttling point serves to (increase; decrease; increase or decrease) the rate of delivery of the pilot stream from the upstream side of the valve to the space on top of the piston.

5. When the bypass coil is (energized, de-energized) and the shut-off coil is (energized, de-energized), the circuit temperature will rise to a desired medium temperature range.

Answers: 1. inlet regulators, upstream regulators, and back-pressure regulators; 2. underside; 3. reduce; 4. increase or decrease; 5. de-energized; energized.