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The Venice Lagoon at San Croce (C) DanieL FriedmanDefinition of Superheat

Define superheat, measurement, uses, typical values

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Superheat measurement & use in HVACR systems:

Here is a tutorial on superheat and how that measurement is used.

We discuss: What is the definition of superheat? How and where is superheat measured in HVAC systems? What are typical superheat numbers?

This article series defines terms used in air conditioning, heating systems, refrigeration systems and other physics applications.

Page top photo: taking advantage of a trip to Italy we use the Venice lagoon temperatures as an example while we explain saturation temperature and subcooling temperature and similar heating, cooling, temperature and pressure terms in this article series.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

- Daniel Friedman, Publisher/Editor/Author - See WHO ARE WE?

Definition of Superheat or the latent heat of vaporization, condensation, solidification

Schematic explaining latent heat (C) Carson Dunlop Associates

Definition of Superheat

Superheat is the amount (degrees of temperature) by which a liquid or gas under pressure has been heated above its boiling point.

Superheating of a liquid occurs when the liquid heated in a closed system or container (such as a water heater tank) to a temperature above its boiling point.

Se can also superheat a gas such as a refrigerant gas or steam (water vapor).

Superheating of a gas (such as refrigerant or steam) occurs when the gas has been heated to a temperature above its temperature of saturation.

Definition of saturation temperature

The saturation temperature is simply the boiling point of the liquid form of the substance - the temperature at which the liquid will boil into a vapor. HVACR experts and physicists use the term saturation temperature to describe a condition at which a liquid (or gas) has been saturated with thermal energy - that is, it has absorbed as much energy as it can without changing state (boiling to a vapor or gas).

The "trick" of superheating, or getting a substance to a temperature above its boiling point is the use of pressure - that is, heating the substance inside of a closed container.

At higher pressures we can heat a substance to increased temperatures.

Refrigerant pressure temperature chart - InspectApediaYou'll see this expressed in charts showing the change in refrigerant saturation temperature as a function of change in refrigerant pressure.

As long as you keep the scale consistent across various measurements, tables, standards, the temperature scale can be in Kelvin, Centigrade, or Fahrenheit - it doesn't matter.

[Click to enlarge any image]

Definition of superheated

Any liquid (such as liquid refrigerant or ordinary water) that has been heated above its boiling point without having converted to a gas or vapor has been super-heated.

Relationship between Degrees of Superheat and Degrees of Temperature Above Boiling

1 degree of superheat = 1 degree of temperature above the substance's boiling point (in °F or in °C).

Where is Superheat Measured?

A superheat measurement as used in air conditioning and heat pump systems, is the number of temperature degrees to which a refrigerant (in liquid OR in gas form) is above its boiling point.

Danfoss (2005) TEV fitters notes provides details about where and how to measure superheat on HVACR equipment, from which we quote this excerpt:

Superheat is measured at the point where the bulb is located on the suction line and is the difference between the temperature at the bulb and the evaporating pressure/evaporating temperature at the same point. Superheat is measured in Kelvin (K) or °C and is used as a signal to regulate liquid injection through the expansion valve. - Danfoss (2005) 

Also see THERMOSTATIC EXPANSION VALVES

Steam superheat example

The boiling point of water is 212 °F (100 °C) - the point at which it will form steam. At that point both the water and the steam are both at a temperature of 212 °F (100 °C).

If we keep boiling the water (at sea level) in an open pot, we will not raise the temperature over the boiling point. Water has changed state from liquid to gas (water to steam) at 212 °F (100 °C) .

If, however, we boil water in a closed container (perhaps a steam heating system) we can continue to add heat to the container, raising the temperature of its contents (both water and steam) to above the boiling point.

Every degree of temperature that we raise our steam above 212 °F (100 °C) is a degree of superheat.

So if we heat our steam up to 242 °F (117 °C) we have superheated our steam by 30 °F.

Refrigerant Superheat Example

If our liquid refrigerant X boils at 40 °F (4.4 °C), as X passes through the evaporator coil across which warm building air is moving, the liquid refrigerant boils, that is converts to a refrigerant gas

In a properly adjusted system, by the time the last bit of liquid refrigerant gets to about the end of the cooling coil, it has all boiled into a refrigerant gas.

If we measure the temperature of the refrigerant at a foot past the end of the cooling coil (aka the evaporating coil) and we find that the refrigerant is at 45 °F (7.2 °C) then we have added 5 degrees of superheat to the refrigerant (45 - 40).

In a refrigeration system the refrigerant is in a closed network of tubing and perhaps a receiver and an area in the compressor motor. In a closed refrigeration system, pressure increases raise the boiling point while pressure decreases lower the boiling point of the refrigerant. For this reason a service technician cannot simply measure temperature, she also has to know the pressures in the system to make sense of the numbers obtained.

That's also why if you look at any table of pressures for a refrigerant you will see that the table specifies both the temperature and the pressure of the refrigerant together.

At REFRIGERANT PRESSURE READINGS & CHARTS you'll see some typical refrigerant pressures at specific temperatures.

You'll see that R410 at 80 °F produces a pressure of 236 psi while at 101.1 °F the system pressure would be expected to be 322 psi.

This is why I disappoint readers who often write to ask "what's the normal pressure of R410?".

It's sort of like saying what's the size of a cardboard box. The answer is, as Mark Cramer says, "it depends."

Rules of Thumb for A/C Superheat Target & Charging

Superheat charging rule of thumb 5 degF delta needs adjustment adapted from Trane & ACHR News by InspectApedia.com 2018Indoor Evaporator Coil Superheat Target

A common rule of thumb for superheat in an air conditioning system: set the thermostatic expansion valve to 8 - 12°F of superheat. - ( ACHR News 2001)

[Click to enlarge any image] Chart adapted from Trane & ACHR News 2000 cited below.

Outdoor Compressor/Condenser Superheat Target

When taking outdoor temperature account (Dry Bulb Temp. or DBT) by making use of Trane's Chart of Superheat vs. Outdoor Temperature,

If the superheat at the compressor (measured at the suction line close to its point of entry into the compressor) is more than 5°F above the chart reading you need to add refrigerant gas by low-side charging.

Watch out: Abnormallhy-low temperature can indicate low refrigerant and probably a leak that should be found and fixed.

If the superheat at the compressor is more than 5°F below the chart reading, the system is overcharged and you need to remove refrigerant by recovering it in an approved manner. - ( ACHR News 2000)

Watch out: be sure the system is allowed to run long enough to reach stable temperatures before and after adjusting a refrigerant charge and thus when making follow-up superheat measurements.

How to Measure Superheat at an Air Conditioner or Heat Pump

Superheated high pressure refrigerant gas exits the compressor where it is condensed to a liquid in the compressor/condenser unit outdoor condensing coil.

Superheat is determined by taking the low side pressure gauge reading, converting that pressure to temperature using a PT [pressure - temperature] chart [for the specific refrigerant involved], and then subtracting that temperature from the actual temperature measured (using an accurate thermometer or thermocouple) at the same point the pressure was taken.

Superheat gives an indication if the amount of refrigerant flowing into the evaporator is appropriate for the load. If the superheat is too high, then not enough refrigerant is being fed resulting in poor refrigeration and excess energy use.

If the superheat is too low, then too much refrigerant is being fed possibly resulting in liquid getting back to the compressor and causing compressor damage. Emerson Climate Technologies (2005)

Superheat Calculation Examples, Charts, Rules of Thumb, Tables

While I'm uncertain about the use of amps or current measurements to estimate superheat settings in an HVACR system discussed above with Tim, there are some useful examples and some rules of thumb that help understand the relationship between superheat and other air conditioner, heat pump, or refrigeration data.

Superheat Temperatures & Pressures for Refrigerants - Sporlan Parker Hannilfin Corp as adapted at InspectApedia.com

[Click to enlarge any image]

Watch out: there are at least three types of refrigerant pressure/temperature chart, each of which may be used slightly differently. The Sporlan chart offers tables that use at least two of these methods. See Lavelle 2006 cited below.

 

Superheat References

These notes and speculations on superheat are also indebted to the following:


...

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Citations & References

In addition to any citations in the article above, a full list is available on request.

  • Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com 11/06
  • Roger Hankey is the retired principal of Hankey and Brown Inspections formerly of Eden Prairie, MN. Mr. Hankey is a past chairman of the ASHI Standards Committee and served in other ASHI chapter and national leadership roles. Mr. Hankey was a National Radon Proficiency Program certified measurement professional and is a Level II infrared thermographer and a consultant on cold climate housing and moisture intrusion.  Contact Roger Hankey at: 970-393-6604 - rogerhankey47@gmail.comMr. Hankey is a frequent contributor to InspectAPedia.com.
  • ASHRAE resources on building insulation, dew point and wall condensation - see the ASHRAE Fundamentals Handbook, available in many libraries.
  • 2005 ASHRAE Handbook : Fundamentals: Inch-Pound Edition (2005 ASHRAE HANDBOOK : Fundamentals : I-P Edition) (Hardcover), Thomas H. Kuehn (Contributor), R. J. Couvillion (Contributor), John W. Coleman (Contributor), Narasipur Suryanarayana (Contributor), Zahid Ayub (Contributor), Robert Parsons (Author), ISBN-10: 1931862702 or ISBN-13: 978-1931862707
  • 2004 ASHRAE Handbook : Heating, Ventilating, and Air-Conditioning: Systems and Equipment : Inch-Pound Edition (2004 ASHRAE Handbook : HVAC Systems and Equipment : I-P Edition) (Hardcover)
    by American Society of Heating, ISBN-10: 1931862478 or ISBN-13: 978-1931862479
    "2004 ASHRAE Handbook - HVAC Systems and Equipment The 2004 ASHRAE HandbookHVAC Systems and Equipment discusses various common systems and the equipment (components or assemblies) that comprise them, and describes features and differences. This information helps system designers and operators in selecting and using equipment. Major sections include Air-Conditioning and Heating Systems (chapters on system analysis and selection, air distribution, in-room terminal systems, centralized and decentralized systems, heat pumps, panel heating and cooling, cogeneration and engine-driven systems, heat recovery, steam and hydronic systems, district systems, small forced-air systems, infrared radiant heating, and water heating); Air-Handling Equipment (chapters on duct construction, air distribution, fans, coils, evaporative air-coolers, humidifiers, mechanical and desiccant dehumidification, air cleaners, industrial gas cleaning and air pollution control); Heating Equipment (chapters on automatic fuel-burning equipment, boilers, furnaces, in-space heaters, chimneys and flue vent systems, unit heaters, makeup air units, radiators, and solar equipment); General Components (chapters on compressors, condensers, cooling towers, liquid coolers, liquid-chilling systems, centrifugal pumps, motors and drives, pipes and fittings, valves, heat exchangers, and energy recovery equipment); and Unitary Equipment (chapters on air conditioners and heat pumps, room air conditioners and packaged terminal equipment, and a new chapter on mechanical dehumidifiers and heat pipes)."
  • 1996 Ashrae Handbook Heating, Ventilating, and Air-Conditioning Systems and Equipment: Inch-Pound Edition (Hardcover), ISBN-10: 1883413346 or ISBN-13: 978-1883413347 ,
    "The 1996 HVAC Systems and Equipment Handbook is the result of ASHRAE's continuing effort to update, expand and reorganize the Handbook Series. Over a third of the book has been revised and augmented with new chapters on hydronic heating and cooling systems design; fans; unit ventilator; unit heaters; and makeup air units. Extensive changes have been added to chapters on panel heating and cooling; cogeneration systems and engine and turbine drives; applied heat pump and heat recovery systems; humidifiers; desiccant dehumidification and pressure drying equipment, air-heating coils; chimney, gas vent, fireplace systems; cooling towers; centrifugal pumps; and air-to-air energy recovery. Separate I-P and SI editions."
  • Principles of Heating, Ventilating, And Air Conditioning: A textbook with Design Data Based on 2005 ASHRAE Handbook - Fundamentals (Hardcover), Harry J., Jr. Sauer (Author), Ronald H. Howell, ISBN-10: 1931862923 or ISBN-13: 978-1931862929
  • 1993 ASHRAE Handbook Fundamentals (Hardcover), ISBN-10: 0910110964 or ISBN-13: 978-091011096
  • In addition to citations & references found in this article, see the research citations given at the end of the related articles found at our suggested

    CONTINUE READING or RECOMMENDED ARTICLES.

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