Guide to Thermostatic Expansion Valves & Other Refrigerant Metering Devices

Guide to Thermostatic Expansion Valves & Other Refrigerant Metering Devices

HVACR Thermostatic Expansion Valves - TEVs: this air conditioning repair article series explains the function and installation of all types of refrigerant metering devices, beginning with the most-common thermostatic expansion valve or TEV (or thermal expansion valve) that controls release of refrigerant into the evaporator coil of an air conditioning or heat pump system.

We define and explain other refrigeration equipment metering devices including AEVs (Automatic Expansion Valves), manually adjusted expansion valves, capillary tubes and Low Side or High Side refrigerant float valves .

This article describes how TEVs work, where and how a thermostatic expansion valve is installed on an air conditioner or heat pump, and how the TEV may be adjusted.
We also list possible errors in TEV installation such as improper positioning of the TEV sensor bulb.
Most of our discussion below focuses on TEVs (Thermostatic Expansion Valves) as these refrigerant metering devices are most widely used on residential air conditioners and heat pump systems.

 

Definition of TEV - Thermostatic expansion valve: An air conditioner thermal expansion valve or "TEV" or just "expansion valve" (tan colored device in the page top photo) is a device located at the cooling coil and connected between the incoming liquid refrigerant line and the refrigerant inlet to the cooling coil in the air handler. (Schematic of a thermostatic expansion valve courtesy of Carson Dunlop Associates.)

A temperature sensor mounted at the end of the cooling coil controls the rate at which the TEV releases refrigerant into the coil - hence the term "thermostatic" expansion valve.

It's an "expansion" valve because by controlling the release of refrigerant into the coil the expansion valve releases high pressure refrigerant into the low pressure environment of the cooling coil, causing the refrigerant to expand and evaporate - cooling the coil. We explain this concept in more detail at REFRIGERATION BASICS.

Refrigerant expansion valves or metering components are used on both air conditioning systems and on heat pumps as well as on dehumidifiers; heat pumps are essentially the same in components as air conditioners except for additional control features to permit refrigerant to circulate in either direction in the system, moving heat outside (air conditioning) or moving heat inside (heat pumps).

On residential refrigerators and freezers, room and portable air conditioners, and dehumidifiers a simpler (and not adjustable) CAPILLARY TUBES may be used for refrigerant metering.

 Refrigeration Basics: Why we need a refrigerant metering device like a capillary tube or thermostatic expansion valve

Before we explain how refrigerant metering devices work in detail it's useful to get a most basic view: the refrigerant metering device provides a restriction in the flow of liquid refrigerant from the compressor/condenser into the evaporator coil.

It is this restriction that, by limiting the flow rate of refrigerant into the evaporator, allows the compressor (a pump) to raise the refrigerant pressure on the high side (condensing it into a liquid) and drop the refrigerant pressure on the low side (evaporating the liquid back into a gas in the cooling coil).

The state change (vaporization from liquid to gas in the cooling coil) is what cools the coil and thus cools indoor air air blown across the cooling coil. But that state change is not enough. To get our refrigerant gas back to a liquid state (to continue the cycle) we need to be able to raise the temperature at which the refrigerant gas will change back to a liquid. It is the high pressure provided by the compressor that accomplishes this step.

It is the flow restriction provided by a cap tube or by an expansion valve such as a TEV in the refrigerant piping system that allows the compressor pump to raise the system pressure and thus increase the temperature at which the coolant changes state. Raising the coolant temperature above outdoor ambient temperature causes heat to flow from the coolant into outdoor air.

So in sum the TEV or cap tube allows the compressor to reduce pressure on the LOW side of the metering device and raise pressure on the HIGH side of the metering device.

Expansion Valves & Thermostatic Expansion Valves for Refrigerant Metering Actually Work

All cooling and refrigeration systems and heat pumps using refrigerant gases make use of some type of expansion valve or refrigerant metering device, of varying complexity.

Even a simple window air conditioner or a refrigerator make use of an expansion valve [shown at left] or a small-diameter CAPILLARY TUBE or "cap tube" which meters refrigerant into the cooling coil. How do these refrigerant metering devices actually work?

Step 1: Expansion valve meters liquid refrigerant into the cooling coil: Inside of the thermostatic expansion valve (TEV) or other metering device the refrigerant passing through is mostly liquid.
The refrigerant metering device is the "doorway" between the refrigeration system high side (compressor output) and low side (cooling coil interior) as it releases liquid refrigerant into the cooling coil at a controlled rate.

Step 2: Liquid refrigerant boils to a gas in the cooling coil: Inside the cooling coil, the liquid refrigerant being metered in through the TEV (or equivalent metering device) converts increasingly to a gas (it "boils" and changes state from liquid to gas) as it enters and then flows down through the evaporator coil, until the refrigerant is totally in a low-pressure, low temperature gaseous state by the time it reaches the end of the evaporator coil.
The energy absorbed by the change in refrigerant from a liquid to a gas inside the cooling coil is what absorbs sensible heat and chills the indoor air handler cooling coil so that the coil cools air blown across it.

Step 3: Low pressure refrigerant gas is drawn back into the compressor/condenser unit: This low pressure, low temperature refrigerant gas present at the end of the cooling coil or "evaporator coil" is then drawn back into the compressor via the suction line connecting the evaporator coil outlet to the compressor inlet port.

Step 4: Low pressure refrigerant gas is compressed to high temperature/high pressure gas and then condensed back to liquid refrigerant out in the compressor/condenser unit (typically located outdoors). It's the high temperature of the gas entering the outdoor condensing coil that allows heat to ultimately be transferred into outdoor air (or into water if we're using a water-based air refrigeration system).
Our photo (above left shows another tan TEV. The very thin coil of copper tubing connects the TEV to its sensor bulb that appears to be taped to the refrigerant suction line just outside of this air conditioning air handler. The TEV shown in this photo is used on a heat pump system so it includes extra tubing so that it can permit the refrigerant to reverse its flow of direction when changing from cooling mode (move indoor heat to outdoors) to heating mode (collect and move outdoor heat to indoors).
The larger diameter copper tubing feeds liquid refrigerant into the TEV from the compressor/condenser unit (the left side of this valve) and the copper tubing on the right side of the valve loops up and into the air handler where inside the unit you'd see it entering the top of the cooling coil. In this photo you cannot see the adjustment point on the bottom of this valve. We say more about the proper position and location of this valve below.
The TEV valve maintains the pressure difference (high and low) at the entry point to the cooling coil, thus assuring that as the high-pressure refrigerant enters the low pressure space of the cooling coil, it can "evaporate" from a refrigerant liquid to a gaseous form, thus producing the temperature drop that cools the cooling coil itself.

The thermostatic expansion valve is a refrigerant metering control device, and it is not a control or switch which can be directly operated by the user when using an air conditioning system, but it is a critical control needed for metering refrigerant into the cooling coil.

Exactly how the Thermostatic Expansion Valve Works to Control Refrigerant Release into the Cooling Coil (Evaporator Coil):

When the pressure sensed by the TEV sensor bulb (P₂) is less than or equal to the pressure in the TEV bottom (P₁) then this condition allows the spring inside the TEV to close the valve.
When the pressure sensed by the TEV sensor bulb (P₂) [and transmitted to the TEV valve top by the sensor tubing] is greater than the pressure in the TEV bottom (P₁) then this condition allows the valve to open [the cooling coil temperature is up] thus allowing more refrigerant to enter the cooling coil, thus boiling more liquid refrigerant, thus dropping the cooling coil temperature back down.
Our sketch also show the two bellows on the typical TEV control (circled in red). The bottom bellows are adjusted by the TEV adjustment screw, and the top bellows are adjusted by the temperature sensing bulb and copper tubing (purple) that connect that bulb to the TEV top cap. The high pressure line is shown in red. A dryer is shown in green. [Click any image to see an enlarged, detailed version.]