Superheat HVAC: 7 Complete Quick Facts

by

CONTENT

SUPERHEAT DEFINITION HVAC | HVAC SUPERHEAT DEFINITION

Superheat in HVAC system is the heat that the refrigerant in the evaporator coils can handle whereby the liquid refrigerant boils to form a vapor. It is a known fact that water will vaporize into steam when the temperature is increased after a certain point. The same principle is used in a refrigeration system where the fluid will be a refrigerant and not just water.

Superheat HVAC
HVAC System (Credits: Wikipedia)

Suppose we left the water to boil beyond a certain limit, then it is obvious that the steam would get hotter and hotter. When the temperature of the fluid increases, the pressure is also expected to increase, and the water will evaporate like steam.

Similarly, the refrigerant in the evaporator will also start to boil with the additional heat that is added to it. The heat absorption process does not stop and continues. The heat absorbed by the refrigerant as it changes from liquid to vapor over a given temperature is referred to be superheated.

Superheating in physics is also defined as heating a fluid beyond the boiling temperature where the fluid is expected to be in a metastable state wherein the internal effects can result in boiling of the fluid at any time.

Superheat for an HVAC system is calculated while starting up a refrigeration unit or while resolving an issue with the operating system. Further, the system should be operating for more than 15 minutes to achieve a steady state to take an accurate reading. The reading that is taken is compared to the industry standards.

SUPERHEAT HVAC FORMULA

The Superheat for an HVAC system is calculated as the temperature difference between the saturation temperature of the fluid and the actual temperature of the gas. The refrigerants which are used in the HVAC system often boil at temperatures lower than that of water. Suppose a refrigerant’s boiling temperature is -200C and it is heated to -100C, then the refrigerant is superheated by 10 degrees although the temperature is in negative value.

Superheat = Current Temperature – Boiling Temperature

A lower superheat suggests that the refrigerant is more than there isn’t sufficient heat load which might result in liquid refrigerant entering the compressor coils resulting in their damage. While a high superheat suggests that there is a limited amount of refrigerant for the heat load which can result in overheating and the efficiency of the refrigeration system is compromised.

By calculating the superheat, an HVAC engineer can tell how much of the liquid is entering the evaporator coils or how far the refrigerant is moving through the coils.

HOW TO MEASURE SUPERHEAT IN HVAC?

To measure superheat in HVAC, the following steps need to be followed which are

  • It is essential to measure the pressure at the lower side of the system using a pressure gauge.
  • The measured pressure should be used for determining the temperature using an HVAC chart.
  • In the next step, it is essential to measure the temperature of the suction line leaving the condenser but should be 4 to 6 inches away from the compressor.
  • These measurements can help one in determining the superheat or achieving the target superheat. Suppose the measurement of temperature at the suction line gives a value of 55 degrees and the conversion of the suction pressure to respective temperature gives 40 degrees as the value then the difference between the two values will give the superheat which is 15 degrees in this example.

It is essential for an HVAC engineer to know how to calculate, measure, or find the target superheat for an HVAC system. It also makes life easy for an HVAC engineer to troubleshoot issues with the refrigeration system.

WHAT IS SUPERHEAT AND SUBCOOLING IN HVAC?

What is Superheating?

The refrigerant that enters the coils of an evaporator vaporizes completely before approaching the exit of the evaporator. The vapor becomes cold as it evaporated entirely. As the cold vapor again enters the coils of the evaporator, it starts absorbing heat from the surroundings and then becomes superheated. As the vapor becomes superheated, it absorbs only the sensible heat in the evaporator coils. This process increases the efficiency of the system

Effect of Superheating

Superheating occurs at invariable pressure and a temperature higher than the saturation temperature. When the vapor undergoes sensible heating, that is when the process is termed superheating. The efficiency of the refrigeration process increases with superheating but the vapor density decreases as it exits the evaporator and enters the compressor. Further, the amount of vapor that enters the compressor is subsequently reduced.

From this, we can conclude that the capacity of the refrigeration process increases with an increase in superheat and decreases with a decreased density of the superheated vapor. Hence the possible outcome from these opposite trends can be established based on the amount of superheat that is available.

What is Subcooling?

Subcooling is the process whereby the refrigerant is cooled to a temperature lower than the saturation temperature of the refrigerant at corresponding condenser pressure. The refrigerant that is being cooled will be in a liquid state. The refrigerant can be subcooled in two different ways which are

  • By bringing about modifications in the condenser such that the subcooling process can be attained
  • Upgrading the system with internal and external heat exchangers would enhance the subcooling process.

Effects of Subcooling

The capacity of the refrigeration process is enhanced when a refrigerant is subcooled using some source of the coolant. It is observed that the efficiency of the refrigeration system can be improved by 1% for every 2 degrees of subcooling. There are new condenser designs in the market that can enhance the subcooling process thereby increasing the efficiency of the refrigeration process.

Flash gas production is minimal during the expansion process and higher latitude can be attained which makes it easier to manage the piping and evaporator location.

Importance of Subcool, Superheat and Temperature difference

To ensure that there is proper refrigerant charge in an HVAC system, it is essential to calculate the superheat, subcooling and to know the temperature gradient across the coil. The importance or advantages of knowing the subcool, superheat and temperature difference are given below

1. It notifies an HVAC engineer to have appropriate refrigerant levels to achieve high refrigeration efficiency and capacity.

2. Helps in proper diagnosis and repair of the respective problem. i.e., avoids diagnosing and repairing the evaporator when the issue is with the compressor. This could turn out to be an expensive mistake.

3. If the superheat is observed below, the possible issue should be that there is too much refrigerant in the evaporator.

4. If the superheat is observed to be too high, this indicates that the amount of refrigerant is too low for the available heat load. The possible reasons for the high superheat could be due to plugged evaporator coils or defective metering unit.

An HVAC system is said to be running with high superheat or low subcool when there is a limited amount of refrigerant in both the evaporator coils and in the compressor. The possible reason for the high superheat and low subcool could be due

1. Restriction in the liquid line

2. Faulty metering system

3. Excessive airflow through the evaporator coils.

4. Plugged compressor coils

5.  Limited airflow through the evaporator coils

SUCTION SUPERHEAT IN HVAC

In an HVAC system, converting a refrigerant from liquid to vapor involves adding heat to the system at boiling temperature. Heat added above boiling temperature is referred to as superheat.

To find superheat in the suction line, it is essential to know the suction pressure and boiling temperature in the evaporator at any given pressure. This method of finding the superheat from the pressure and temperature is often referred to as temperature- pressure method for finding superheat.

As the evaporator coils more and more heat, the liquid refrigerant starts boiling and at some point, only vapor can be found in the coils. There might be some vapor left behind which is still cold.

The cold vapor passes through the evaporator coils and absorbs heat, after a point; all the available vapor will be heated to a temperature above the saturation temperature. After all the liquid boils off, the additional heat that is added to the vapor is referred to as the Suction Superheat.

Example: A refrigerant is saturated state enters the evaporator coils at 45F and this temperature is obtained from the suction pressure at 120 PSIG for R-410 A. The temperature probe that is placed at the suction line reads 55F. From the temperature reading at the suction line, it is evident that the refrigerant is superheated by 10 degrees.

After the state of the refrigerant has changed and the process has stopped, the cooling of the refrigerant ceases. The temperature of the cool vapor rises rapidly. The heating of the refrigerant vapor ensures that no liquid will enter the compressor coils and thereby reducing the chances of compressor damage.

HVAC SUPERHEAT CHARGING CHART

Often the manufacturers of HVAC systems provide pressure-temperature charts that make the technicians’ life easier. This chart helps a technician to charge an HVAC system with an appropriate amount of refrigerant. These charts are often provided near the condensing unit of the HVAC unit. The charge of refrigerant is based on factors such as ambient temperature and the load capability of the system.

Most of the condensers in HVAC systems are already charged with refrigerant. The refrigerant charge in the condenser and the line set up will depend upon the manufacturer. In this way, the installation process becomes much easier for an HVAC engineer. The charge adjustments can be made as per the length of the line set up.

This method of charging units with refrigerant works well with refrigeration systems that come as a pack wherein the loop requires repair while the charge must be recovered. The refrigerant must be charged as recommended by the manufacturer in terms of an ounce. There are means of charging an HVAC system without using an appropriate superheat or subcooling method.

When an HVAC engineer is charging an HVAC unit, the technician needs to get the exact temperature difference from where the fluid changed its state. If the superheat is high, the system will be undercharged and if the superheat is low, the system will be overcharged. This method of charging the system is called superheat method and is not used while charging a heat pump or an air conditioner.

But if an air conditioner was equipped with a thermostatic expansion valve, then the system needs to be charged using the superheat method or the subcooling method.

CONCLUSION

It is of great importance for an HVAC engineer to understand superheat and subcooling as it is closely tied to the diagnosis of an HVAC unit. For an apprentice or a fresher in the HVAC department, it is essential to know how to deduct the superheat capacity of an HVAC system. Further, one should also develop skills in reading the pressure-temperature charts that are provided by the manufacturer as these days most units are provided with these charts.

It is recommended to understand the basic laws associated with HVAC systems such as Boyles Law, Sensible Heat, etc which would make lives easy for an HVAC engineer. Also important concepts on High Superheat, Low Superheat, and Superheater would be beneficial for a mechanical engineer or a technician.

INTERVIEW QUESTIONS AND ANSWERS ON SUPERHEAT IN HVAC

1. What is superheat and subcooling in an HVAC system

An HVAC system is said to be running with high superheat or low subcool when there is a limited amount of refrigerant in both the evaporator coils and in the compressor.

2. What are the possible reasons for high superheat in a refrigeration unit?

The possible reason for the high superheat could be due to the following reasons

1. Restriction in the liquid line

2. Faulty metering system

3. Excessive airflow through the evaporator coils.

4. Plugged compressor coils

5.  Limited airflow through the evaporator coils

3. How to calculate the superheat for a refrigerant at a temperature of 58.500C?

Superheat is calculated as the difference between boiling temperature and current temperature

Boiling Temperature of refrigerant = 48.500C

Superheat = Current Temperature – Boiling Temperature

Superheat = 58.50 – 48.50

= 100C

To read about more topics on Mechanical or Thermal Engineering, please Click Here