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Different Metering Devices and How They Work

July 30, 2020

by Interplay Learning

Metering devices are crucial to all HVAC systems’ operation. In fact, there are several metering devices and valves governing the flow of refrigerant through these systems to help regulate pressure. Here, we’re going to cover some basics about how metering devices are fitted to a line, their purpose and function, and how to identify problems with them.

Core Functions of a Metering Device

There are several metering devices in a typical system, including the thermostatic expansion valve (TEV or TXV), automatic expansion valves, electronically-governed expansion valves, balanced port TEVs, superheat and multi-circuited TEVs, and evaporators. The two basic functions of a metering device include:

  1. Feeding refrigerant to the evaporator
  2. Providing a pressure and temperature drop from the high side to low side

Thermostatic Expansion Valve (TEV)

So, what does an expansion valve do?

The functions of a TEV include measuring and adjusting superheat and providing a place for mounting the sensing bulb. TEVs are also used on multi-circuit evaporators. The sole role of a TEV is to control the flow rate of liquid refrigerant into the evaporator. It has no control over air temperature, humidity, capacity, suction pressure, or head pressure.

Inside the TEV, you’ll find:

  • Head and diaphragm
  • Pins
  • Needle and orifice
  • Inlet screen
  • Spring
  • Packing gland
  • Adjustment stem

The TEV is designed to respond to the temperature of refrigerant vapor as it leaves the evaporator, and to pressure in the evaporator itself (superheat).

The TEV opens with force from the bulb sensor and capillary tube (pressing on the head and diaphragm) and then closes again with pressure from the evaporator. The spring inside the TEV is usually adjustable so that a technician can modify its sensitivity to closing under evaporator pressure.

TEV Bulb Functionality

On a ⅞ “ suction line, the TEV bulb should be situated at 4 o’clock or 8 o’clock along the line. For lines that are smaller in diameter than ⅞”, it should be anywhere between 8 and 4, but should never be mounted at the bottom of the line. Bulbs that are mounted directly at the bottom of the line can’t measure temperature accurately. It’s important to remember that the suction line will have warmer vapor on top and cold oil along the bottom.

Your bulb should be mounted on a smooth section of the line (rather than a brazed joint), using stainless hose clamps. Tighten the clamps enough to ensure good thermal contact with the line, but not so tight that they deform and crush the bulb.

Ideally, the bulb should be mounted on a horizontal run of a pipe, but in a pinch it can be mounted vertically– just make sure the capillary line is pointed up and not down.

TEV Screens

The screen in a TEV is essentially a filter that traps any dirt and contaminants before they enter the valve. For TEVs that are sweat-mounted or flare-mounted, the screen is right at the inlet, allowing it to be easily unscrewed, inspected and cleaned.

Sporlan TEV Charts

TEVs need to be the right rating for the right application. Selection charts enable you to determine the right TEV according to:

  • Refrigerant
  • Tonnage
  • Application, with the code of “C” for medium temperature, “Z” for low temperature and “ZP40” for low temperature/outlet pressure limited to 40 psig

Here are the steps you’ll need to take to determine the right TEV according to Sporlan charts: 

  1. Determine the temperature of liquid refrigerant entering the valve
  2. Determine pressure drop across the valve
  3. Select the right valve from Sporlan chart
  4. Determine if external pressure equalizer is necessary
  5. Select body type according to style of connections (brazed, sweat, flare mount)
  6. Select Sporlan selective thermostatic charge

Valve Identification

In order to pick the right TEV for the right application, you have to be able to interpret the code that identifies the TEV. A typical valve code (printed on the valve head) might read:

R-22

SVE-2-GA

To break it down:

  • S: valve body style
  • V: type of refrigerant (R-22)
  • E: externally equalized
  • 2: two-ton capacity
  • GA: air conditioning application

Note that some TEVs can be used with more than one type of refrigerant, provided that evaporator temperatures and pressures are the same. Let’s look at another typical valve code:

FSE 1 ZP

  • F: valve body style
  • S: type of refrigerant (R404A, but in this case, this valve is also compatible with 502, 402A and 507)
  • E: externally equalized
  • 1: one-ton capacity
  • Z: low-temperature application (freeZer)
  • P: Pressure limiting

Understanding and Measuring Superheat

Superheat can be measured as the difference between suction line temperature and evaporator temperature.

A high superheat condition means that the evaporator is starving for refrigerant because it is boiling off too soon (before it can make it to the evaporator). Low superheat means the evaporator is flooded with liquid refrigerant, which may allow liquid refrigerant to make its way back down the line to the compressor and can lead to compressor failure.

You can calculate superheat by using a digital thermometer on the suction line, right around the TEV bulb, then taking a reading of suction pressure at the suction line’s fitting. From there, figure in the type of refrigerant used and refer to a table to determine the temperature for that refrigerant at that specific suction pressure. The difference between the two temps will be the superheat reading.

Excessive Superheat

If superheat is excessive, the technician can reduce spring pressure in the TEV, allowing more refrigerant to flow through into the evaporator. That will mean turning the valve stem counterclockwise, loosening the spring pressure. It’s advisable to make this adjustment about ¼ turn at a time, then wait about 15 minutes for the system pressure and temperature to level off again. For an R404A system, a full turn of the stem would mean an adjustment of 4 degrees of superheat.

How to Raise Superheat

The reverse is true for raising superheat. This involves increasing spring pressure to reduce the flow of refrigerant through the valve into the evaporator. Turning the stem clockwise tightens the spring. Again, make these adjustments ¼ turn at a time and wait 10-15 minutes before adjusting again.

It’s important to remember that these are general rules of thumb, as specs can vary from one design of TEV to another.

For more help building knowledge and understanding of metering devices and how they contribute to an HVAC system, check out SkillMill’s™ Meters, Switches, Loads, & Circuits course.

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