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AIR CONDITIONING & HEAT PUMP SYSTEMS
A/C - HEAT PUMP CONTROLS & SWITCHES
AIR CONDITIONER COMPONENT PARTS
AIR CONDITIONER TYPES, ENERGY SOURCES
AIR FILTER EFFICIENCY
AIR FILTERS, FIBERGLASS PARTICLES
AIR FLOW MEASUREMENT CFM
APPLIANCE DIAGNOSIS & REPAIR
APPLIANCE EFFICIENCY RATINGS
BLOWER DOORS & AIR INFILTRATION
BLOWER FAN CONTINUOUS OPERATION
BLOWER FAN OPERATION & TESTING
BOOKSTORE - Air Conditioning "How To" Books
CAPACITORS for HARD STARTING MOTORS
CLEANING & Legionella BACTERIA
CHINESE DRYWALL HAZARDS
CONDENSATION or SWEATING PIPES, TANKS
DEFINITION of HEATING & COOLING TERMS
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
DIAGNOSTIC GUIDES A/C / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DUCTS - Asbestos
DUCT INSULATION, Asbestos Paper
DUCT INSULATION for SOUNDPROOFING
DUCT SYSTEM & DUCT DEFECTS
DUCT SYSTEM NOISES
DUCTS, Asbestos Transite Pipe
DUST, HVAC CONTAMINATION STUDY
ELECTRIC MOTOR OVERLOAD RESET SWITCH
EVAPORATIVE COOLING SYSTEMS
FAN LIMIT SWITCH
GAS EXPOSURE EFFECTS, TOXIC
GAS DETECTION INSTRUMENTS
HEAT LOSS (or GAIN) in buildings
HEAT LOSS (or GAIN) INDICATORS
HEAT LOSS R U & K VALUE CALCULATION
HEATING SMALL LOADS
INSPECTION CHECKLIST - OUTDOOR UNIT
INSPECTION LIMITATIONS, A/C SYSTEMS
LEED GREEN BUILDING CERTIFICATION
LOST COOLING CAPACITY
LOW VOLTAGE TRANSFORMER TEST
MOTOR OVERLOAD RESET SWITCH
MOLD in AIR HANDLERS & DUCT WORK
OPERATING COST, AIR CONDITIONER
OPERATING DEFECTS, AIR CONDITIONING
REPAIR GUIDES A/C / HEAT PUMP
REPAIR & DIAGNOSTIC FAQs for A/C
THERMOSTATS, HEATING / COOLING
THERMOSTATIC EXPANSION VALVES
WATER COOLED AIR CONDITIONERS
WINDOW / WALL AIR CONDITIONERS
WINDOW / WALL A/C SUPPORTS
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.
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Guide to Expansion Valves for Refrigerant Metering on Air Conditioners & Heat Pumps: Definitions of AEVs, TEVs, Manual Valves, Cap Tubes
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.
All types of refrigerant metering devices are discussed in this article.
Photograph of the thermostatic expansion valve at page top is courtesy of Alan Carson, Carson Dunlop Associates in Toronto.
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.)
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.
Other types of refrigerant expansion valves control refrigerant by various means other than sensing coil temperature. For completeness in this article we also define and explain other refrigerant metering devices including AEVs (Automatic Expansion Valves), "non-adustable" expansion valves (actually they can be set), and manually adjusted expansion valves, CAPILLARY TUBES and Low Side or High Side refrigerant float valves.
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.
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.
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.]
Reader Question: 7/22/2014 Tom said:
Very interesting and informative article. I've recently had "added" to my responsibilities a lot of A/C kit. Although an engineer I haven't any qualifications in A/C but would like to be aware of what's happening (or isn't) when these guys visit the site. So I bought books on the subject and read up on it.
Question: I've just been reading a book (printed 2009) where the author lists and describes different methods of charging an A/C system. One method he dscribes uses the compressor motor current to indicate when the unit is full of gas and none of his other methods mention subcooling anywhere. Comments please.
Indeed we've read & discussed measuring current draw which is an indirect measure of motor loading which in turn can describe what the motor is "seeing" in discussing evaluation of water wells. So in concept I agree that one could have an idea of the refrigerant charge by noting whether or not the compressor were running under load - as would happen when there is enough refrigerant present to lead to high pressures on the high side.
But frankly I'm dubious that one could have an accurate idea of the actual charge level with this approach.
It may be possible that with a specific compressor motor, TEV, a specific refrigerant gas, and other variables held constant and with some experience with having put in a measured charge and then measured motor current draw one could infer something about that specific set-up.
But as motor properties vary and refrigerant gas properties vary, and as normal operating pressures vary among HVAC systems by refrigerant gas itself, I'm skeptical about the claim that merely measuring current could be at all accurate as an indicator of the precise charge.
You could, in sum, determine if there were a significant undercharge (low current) or a stuck TEV (high current) ... maybe.
And many texts including my own pay too little attention to superheating or subcooling though those are important measurements, perhaps because deeper understanding of refrigeration principles is required. As you've probably read, subcooling and superheating are important conditions that can be measured in a properly-functioning HVACR system.
As long as you keep the scale consistent across various measurements, tables, standards, the temperature scale can be in Kelvin, Centigrade, or Farenheit - it doesn't matter.
[Click to enlarge any image]
Superheated high pressure refrigerant gas exits the compressor where it is condensed to a liquid in the compressor/condenser unit outdoor condensing coil.
The damage referred to is "liquid slugging" - a compressor's valves are designed to pass gas, not liquid. Slamming a piston into liquid destroys the compressor.
For discussion of the refrigerant pressure / temperature chart shown above see REFRIGERANT PRESSURE READINGS
A sub-cooled liquid refrigerant is at a temperature below (colder-than) the temperature at which the refrigerant would evaporate ("boil" or change from a liquid refrigerant to a gas - also called the refrigerant saturation temperature).
Subcooling is measured in degrees of temperature (on any scale) and can be defined as the difference in temperature degrees between the liquid refrigerant's saturation temperature and the current or actual liquid refrigerant temperature.
Higher subcooling temperatures of a liquid refrigerant mean a more efficient HVACR system operation beause more heat is being removed per unit volume (or unit weight) of refrigerant circulating in the system. Thus some HVACR technicians measure subcooling to take a look at the operating efficiency of the system and to compare it with the manufacturer's specifications. Higher subcooling numbers mean that the equipment will have to run less time to adequately cool the area being refrigerated or air conditioned.
Lower or too-low subcooling temperatures risk accidental conversion of liquid refrigerant to gas state within the piping system before it reaches the refrigerant metering device (capillary tube or thermostatic expansion valve (TEV)), reducing the efficiency of the system and possibly interfering with the proper operation of the TEV. - adapted from Emerson Climate Technologies (2005)
What's the difference between a TEV (Thermostatic Expansion Valve) and an AEV (Automatic Expansion Valve) ?
A Thermostatic Expansion Valve or TEV (circled in red) is similar to an Automatic Expansion Valve (AEV) but unlike the single-bellows internal design of the AEV, a TEV has a second bellows on the top of the valve along with a tube (purple) that attaches the TEV valve top bellows to a temperature sensing bulb mounted at the end of the cooling coil.
The sensing bulb of a TEV (purple rectangle above the "In" of "InspectAPedia) measures the superheat across the cooling coil (light blue) regardless of the actual refrigerant pressure inside the coil and uses that information to maintain the frost line at the end of the evaporator coil.
In contrast, the AEV regulates refrigerant just by pressure - it does not include the sensing bulb control.
Our TEV operation sketch (above left) illustrates the placement of the sensing bulb and its small diameter tube attached to the TEV top (purple) that controls the TEV release of refrigerant into the cooling or evaporator coil. You'll also see the temperature numbers written along the evaporator coil (blue coil at left side of sketch) showing the likely increase in temperature as we get close to the end of the evaporator coil.
Thermostatic expansion valves (TEVs) are designed to meter refrigerant into the cooling coil at the proper rate. This design can keep the proper dose of refrigerant entering the cooling coil for maximum air conditioning or heat pump system operating efficiency. TEVs are similar to automatic expansion valves (AEVs) discussed below, but incorporate the signal from a temperature sensor mounted at the end of the evaporator coil
Details: If you are diagnosing a problem with an air conditioner or heat pump and the TEV appears to be involved, check the TEV installation details against the information we list in detail at TEV INSTALL & REPAIR
Example Refrigeration Equipment Field Diagnosis & Repair: Thermostatic Expansion Valve Inspection, Testing, Experiments with TEV and Pressure Control Switch
How to Adjust the Thermostatic Expansion Valve
For most TEVs, adjusting the thermostatic expansion valve woks as follows:
Field Notes from TEV Adjustment
The following are from my [DF] notes from a refrigeration service call  VERY early in my [DF] refrigeration training:.
Case outline & initial observations: Commercial cooler running too warm - (WACOOP), hermetically sealed compressor, Kramer W14, unknown refrigerant (thought from a label maybe it should be R22 but someone may have charged with R12), cooler running too warm, need to diagnose cooling coil and fan operation and control settings on an old, used cooler just brought in. Very common on old equipment like this: no labels, no data tags, not much information at all.
Fanco refrigerant pressure switch: found set at 35# and 10# differential, connected improperly to the low side service port, cannot fully shut off the service port as a result - maybe leaking?
Compressor pump: running continuously.
Ambient temperature about 90 DegF; R12 in my service canister is at 100 psi static.
Low side refrigerant pressure: measured 40 psi. If there is R12 in the system I'd expect about 45 deg. temp at proper charge, and if R22 in the system I'd expect about 20 degF temp at proper charge and operation. But there were NO frost lines on the equipment, so I know that there is little or no liquid refrigerant and the system is operating at about 45 degF so must be filled with R12.
Actions and Tests:
Set the TEV 8 quarter turns more open - out and down, to see what happens.
The frost line moved at least to the se3nsor bulb and the pump (compressor) shut off. The low side pressure went up to 36 psi and then the pump restarted. This is telling me what the pressure control switch is doing.
Further actions and observations:
Opened the TEV 1/4 turn more to see the effect.
Kept a series of observations from 10:55 PM to 12:42 AM (service call made during hours the business was closed to avoid disruption)
5 then 9 more turns opening the TEV, low side up to 75# & can see gas in the sight glass in the refrigerant line - this is "wide open" TEV setting
9 turns closing down the TEV to almost shut - so there are about 10 turns from wide open to fully shut on this TEV. At 9 turns towards shut from wide open, the low side pressure falls FAST!
9 turns back open at the TEV confirms gas bubbles again in the sight glass and 70# pressure.
Closed the TEV completely (about 9+ turns to the right or "up" or "in"). Suction lines closed, no gas in the sight glass, rapid low side pressure drop to 26#, compressor turns off at 20#.
I am convinced the pressure control switch is working properly, that is it does what it's pressure settings say it should be doing.
Set the TEV to 12 1/4-turns (in other words 3 full turns) open from fully shut. Suction line very cold, low side goes to 30#. 2 more turns open, low side goes up to 34#.
Finally decide to run the system with the TEV open 3 quarter-turns (about 3/4 of one turn) from fully shut. The system stabilizes with the cooler (a refrigerator) in the mid to upper 40's, no more oscillating, no coil frosting.
If I set the cut-in pressure way down the cooling coil ices over and the compressor will run continuously without cooling anything. See frost moving down the low side line. So that's not the right "fix".
I could set the pressure switch to 12.5 psi, left the TEV alone, and got the cooler down to our target of 32 degF.
The HI event sets the defrost cycle by setting the cut-in. The low event sets the cutout and therefore the lowest temp we will reach. A bigger low event means a lower target temperature, but the risk is that if you set it too low the compressor will run continuously and ice up the coil without ever running a defrost cycle.
The TEV seemed to be sometimes sticking. The low side pressure would hang at 28# or rise only very slowly as if the TEV was not opening when I expected it to. Have to be sure the blower fan is also running when checking this performance.
Final resolution of the cooler operation troubles:
Ultimately I replaced the TEV with a Singer TXV223FA 1/2 with a TE value of 9 (heat delta), installed a filter dryer (#082 PN 2003), set the pressure control to 35# on and 20# off. adjusted the system to get NO frost on the suction line near the compressor. Final pressure switch settings were 33# and 18# hi and low. We were able to get the cooler, charged with R12, to hold a stable 34 degF at cutoff, rising to 39 degF at which point the compressor would cut back on.
Guide to Other Types of Refrigerant Metering Devices besides TEVs: AEVs, Cap Tubes, Float Switches, Manually Adjustable Refrigerant Metering Valves
Capillary Tubes - a simpler method for metering refrigerant
Separately at CAPILLARY TUBES. we explain how capillary tubes are used to meter refrigerant in air conditioners, dehumidifiers, refrigerators, & freezers. We include a description of the operating properties of cap tubes, we contrast their use and function with thermostatic expansion valves or similar devices, and we include cap tube problem diagnostic tips for air conditioning service and repair purposes.
What's the Difference Between a Refrigerant Capillary Tube or "Cap Tube" and a Refrigerant Expansion Valve or TEV / AEV?
As we detail at CAPILLARY TUBES, compared with a capillary tube, the TEV or AEV adds a level of control - the TEV / AEV or even float valves and manually and adjustable refrigerant metering valves can open or shut in response to an attached bulb or pressure sensor (AEVs) which actually monitors temperatures in the refrigerant tubing.
Capillary tubes are found on residential refrigerators, dehumidifiers, and many window air conditioners. TEVs are found on larger air conditioners and central air conditioning systems where more control is needed.
In our TEV sketch (left) the small diameter tube at the top of the thermostatic expansion valve is connected to a temperature sensing bulb (not shown) that is located at the outlet end of the cooling or evaporating coil in the air handler.
The tubing at the left and right permit liquid refrigerant to flow into the valve from the compressor/condenser and, metered by the TEV, onwards into the evaporator coil. The large nut on the bottom of this TEV covers an adjustment screw that can change the latent heat settings and thus the behavior of the valve once it is installed. (Normally you should leave the valve at its factory setting.)
Singer and other manufacturers point out that TEVs are adjusted at the factory before shipment. The factory setting of a thermostatic expansion valve is printed on a label found on the head of the valve and for most installations the factory superheat settings should be left alone.
How & When to set "Non-Adjustable" Thermostatic Expansion Valves
Non-adjustable TEVs (such as Singer TEV models 226, 326, 426) can actually be adjusted before the valve is installed, by turning an adjustment screw through the valve outlet opening. Once these valves have been installed, however, adjusting the valve would require removing it from the system, thus also requiring an evacuation and recharge of system refrigerant - not something to do casually.
How to Set Manually Adjustable Thermostatic Expansion Valves
Manually adjustable TEVs permit the device to be set to continuously maintain the proper refrigerant level entering the evaporator coil or cooling coil. Automatic expansion valves are discussed below.
Automatic expansion valves used to meter refrigerant into a cooling system are similar to Thermostatic Expansion Valves (discussed above) but AEVs do not use a temperature sensing device mounted on the cooling coil. Rather the automatic expansion valve is mechanically set by adjusting a screw that presses a spring that presses a diaphragm that regulates refrigerant release by constant pressure.
Automatic Expansion Valve (AEVs) (center of sketch at left)
In normal operation the adjusting screw (see our sketch) is set to maintain a given pressure on the low side of the refrigeration system. During system operation the Automatic Expansion Valve pulsates - it opens and closes constantly.
Diagnosing AEV trouble: AEVs can close up (stop working) if you add extra load to the refrigeration system - because of the extra heat load - the system will appear to run as if it were short-charged. So automatic valves are used on systems where the load is more consistent.
[Click any image to see an enlarged, detailed version]
Automatic Expansion Valves (AEVs) are repairable. These devices are most often used on constant-load refrigeration systems such as restaurant coolers.
Avoiding Trouble with Adjustable Expansion Valves
Watch out: on AEVs that have an accessible adjusting screw, it's important to keep the protective rubber cap on the device. The cap provides insulation to prevent the water produced during TEV defrost cycle from seeping into the bellows of the device where it can freeze and rupture the bellows.
Also review our advice on Adjusting the Thermostatic Expansion Valve - if you don't allow sufficient time for the system to stabilize after each turn of the TEV adjusting screw you can easily over adjust and lose control of the system.
On many commercial air conditioning and larger refrigeration systems you may see a large canister on top of the equipment (sketch at left) that acts as a refrigerant reservoir or receiver. Vapor exits at the top of the reservoir and liquid refrigerant enters lower in that canister, typically from a side port.
A float with a needle valve allows liquid refrigerant to enter the canister to replace refrigerant that has boiled off and exited out of the upper valve as a gas. Float type refrigerant metering systems are used on refrigeration systems in which the evaporator is the "flooded type" - always kept full of refrigerant down to the end of the coil.
Low-Side Float Refrigerant Metering Devices
Low-side floats are used mostly on So2 refrigerant systems.
And if the refrigeration system is indeed using SO2 experts recommend that you do not attempt to repair this device.
[Click any image to see an enlarged, detailed version]
High-Side Float Refrigerant Metering Devices
A high side float refrigerant metering device is illustrated in our sketch. As our drawing notes explain, the float is located on the HIGH side of the system.
A pressure-regulating valve prevents frosting at the receiver outlet (PRV in in the sketch at top center).
Details are at TEV INSTALL & REPAIR - separate article. In addition the following articles will be helpful:
Continue reading at TEV INSTALL & REPAIR or select a topic from the More Reading links shown below.
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