HIGH SUPERHEAT | Important Terms and 7 FAQs

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DEFINITION OF HIGH SUPERHEAT

In a refrigeration system, high superheat is a condition when the evaporator coil is not provided with enough refrigerant for the heat load that is present. In short, it means that an insufficient amount of refrigerant is reaching the evaporator coil, or the heat load is too much for the evaporator coil to work on.

Image Attribution: “Refrigeration system” (CC BY-SA 2.0) by rfc1036

HIGH SUPERHEAT CAUSES

 The possible reasons for high superheat are as follows:

1. Low Refrigerant in the system

If the amount of refrigerant is lower than what is required; it will evaporate soon after a few passes through the coil. Soon after the refrigerant evaporates, the vapor will continue the cycle by carrying away heat from the load while passing through the evaporator coil.

This heat picked up by the vapor will increase the temperature of the vapor to a higher value i.e., the vapor reaches superheat temperatures. When there exists less amount of refrigerant in the system, the pressure at both suction and discharge ends of the cycle is lower than usual.

2. Restriction in the liquid line

When the liquid line of the system is restricted, there will be an inadequate flow of the refrigerant to the evaporator coil. The pressure at the suction and the discharge ends of the cycle would be lower than normal pressure. The symptoms observed due to restriction in the liquid line are like those noted in a refrigeration system with low refrigerant.

There is an observed decrease in temperature at the location of restriction. There are also chances for the moisture in the system to freeze and cause the restriction.

3. Airflow through the evaporator is too high

When there is an excess flow of air through the evaporator coil, the capability of the system to remove moisture is reduced. The vapor picks up more than usual heat which causes the suction pressure to be higher than normal pressure and has a higher superheat.

4. Excessive heat load

With higher loads, there will more than the usual heat content that is passing over the evaporator coil which will be absorbed by the vapor. This increases its superheat. When the ambient temperature inside a room is higher than usual or when there are too many people in a room, there are higher chances for an increase in the superheat.

5. Faulty Metering Unit

There is a possibility of recording a higher superheat when the metering device is not installed correctly or due to faulty in the unit.

HIGH SUPERHEAT LOW SUBCOOLING

Superheat means the amount of refrigerant that is present in the evaporator. High superheat indicates that the amount of refrigerant in the evaporator is low or not sufficient. Subcooling indicates the amount of refrigerant that is available in the condenser. Low subcooling means that there is an insufficient amount of refrigerant in the condenser.

A refrigeration system is said to be running a high superheat and low subcooling condition when there exist insufficient amounts of refrigerant in the evaporator as well as the condenser.

HIGH SUPERHEAT HIGH SUBCOOLING

Superheat means the amount of refrigerant that is present in the evaporator. High superheat indicates that the amount of refrigerant in the evaporator is low or not sufficient. Subcooling indicates the amount of refrigerant that is available in the condenser. High subcooling means that there is an excessive amount of refrigerant in the condenser.

A refrigeration system is said to be running a high superheat and high subcooling condition when there exist insufficient amounts of refrigerant in the evaporator and excessive amounts of refrigerant in the condenser. The possible reasons for high subcooling are a faulty metering device, underfeeding, fault in the head pressure control system, especially during low ambient conditions.

High subcooling will reduce the performance of the refrigeration system and ultimately damage the compressor valves. Hence it is recommended to troubleshoot this issue at the earliest as possible.

HIGH SUPERHEAT NORMAL SUBCOOLING

When the amount of refrigerant in the evaporator is insufficient for the heat load, then the superheat condition is referred to as high superheat. The state of having an insufficient amount of refrigerant in the evaporator and enough refrigerant in the condenser is termed as High Superheat Normal Subcooling. It is rare for this condition to exist because usually when there is high superheat there should be either low subcooling or high subcooling.

HIGH SUBCOOLING NORMAL SUPERHEAT

As mentioned earlier, when the refrigerant in the condenser is in excess, that condition is referred to as high subcooling. When there is an adequate amount of refrigerant in the evaporator for the heat load, it is referred to as normal superheat. Therefore, a refrigeration system that operates with an adequate amount of refrigerant in the evaporator and with an excess amount of the refrigerant in the condenser is termed as High Subcooling Normal Superheat.

WHAT DOES HIGH SUPERHEAT INDICATE?

High superheat in a refrigeration system occurs when there is a limited amount of refrigerant in the evaporator for the heat load that is present. High superheat indicates that

1. Low levels of refrigerant

2. restriction in the liquid line

3. Airflow through the evaporator is too high

4. Excessive heat load

5. Faulty Metering Unit

HIGH SUPERHEAT HIGH SUCTION PRESSURE

A refrigeration system is expected to have a high suction pressure when there is leakage of refrigerant through the discharge valve. Further, the compressor is not capable of providing the evaporator coil with the required refrigerant to handle the heat load. This condition is termed as High superheat High suction pressure or High head pressure High superheat. The possible reasons for high suction pressure are

1. High heat load

2. Having a high expansion valve capacity

3. Leakage of compressor disc or discs

HIGH DISCHARGE SUPERHEAT

High discharge pressure superheat is a condition whereby there is air present in the system. When the refrigeration system is exposed to this condition, the best solution is to charge the system with refrigerant. Sometimes, even a clogged condenser can cause high discharge pressure. In such cases, it is advised to clean the condenser. In some cases, a closed discharge valve can also cause high discharge pressure and can be reduced by opening the discharge valve.

NO SUPERHEAT HIGH SUBCOOLING

No superheat or low superheat is an indication the refrigerant hasn’t picked up enough heat because of which the liquid will not completely boil into vapor. This liquid refrigerant which will be transferred into the compressor will damage the compressor. Along with this if there exist excess amount of refrigerant in the condenser. This condition is referred to as no super heat high subcooling.

HIGH SUPERHEAT LOW SUCTION PRESSURE

When the refrigerant is low in the system, there are high chances for low suction pressure. When the refrigeration system is running with high superheat and low subcooling, the refrigeration charge is usually low. In such a condition, the system is expected to be at high superheat and low suction pressure. Another possible reason for low suction pressure high superheat is the insufficient amount of heat entering the evaporator which could be because of limited airflow or due to a dirty/plugged evaporator.

ACCUMULATOR HIGH SUPERHEAT

An accumulator is a vessel that stores refrigerant in a saturated state and stops the liquid refrigerant from entering the compressor. It is used as a protection tank. Bigger accumulators are installed to contain larger volumes of liquid to protect the compressor while the increase in capacity of evaporators is not observed. When the amount of refrigerant is limited with an accumulator installed in the system. It is referred to as an accumulator high superheat

CAN LOW AIRFLOW CAUSE HIGH SUPERHEAT?

A dirty or plugged evaporator coil will limit the air flowing through the evaporator thereby reducing the amount of heat that enters the evaporator which results in high superheat. It is also a concern if there an excessive flow of air through the evaporator as the system’s capability to remove moisture is limited.

HEAT PUMP HIGH SUPERHEAT

A heat pump acts as a refrigeration system in the cooling mode. The indoor unit functions as an evaporator and the outdoor system function as a condenser. As the refrigerant charge in the evaporator is low, the heat pump will not be able to handle the heat load and this state is referred to as Heat pump High superheat

HIGH DELTA T AND LOW SUPERHEAT

A high delta T which is above 210F could be a result of limited airflow indoors. If the air movement in the environment around i.e., indoors is limited, the system is not capable to move enough heat from the surroundings to the evaporator of the system. Further, there will a decreased supply air temperature on the system which will, in turn, result in a higher delta T.  Hence this condition is termed as High delta T low superheat.

For systems with low delta T, the compressor of the refrigeration system will be a danger as the saturated liquid refrigerant will enter the compressor.

FAQS

1. Is High superheat bad?

Yes, high superheat is bad as it indicates that there isn’t sufficient refrigerant to handle the heat load from surroundings or environment that needs to be cooled. A high superheat could also indicate a restriction in the liquid line which is the reason for the limited flow of refrigerant into the evaporator coil. Further excessive airflow could also result in high superheat as the air will carry an excessive amount of heat which the evaporator coil is not ready to handle causing a high superheat. An incorrect metering unit or feeding device also results in high superheat which should be rectified.

2. How can I reduce superheat?

The superheat in a refrigeration system can be reduced based on the cause. If the cause is due to the limited refrigerant, then recharging of refrigerant in the condenser is the right step. In case the superheat is due to excessive airflow, then a sir release valve should be installed thereby maintaining the amount of superheat that can be handled by the evaporator. Troubleshooting the metering device is also a method of reducing the superheat.

3. What causes high discharge superheat? 

The possible reason for high discharge superheat could be leakage of refrigerant. Other possible reasons for high discharge superheat are restriction in liquid line or restriction in the filter. Further, a restriction in the actuator feeding to the evaporator could also result in high discharge superheat. There are cases where the system might face high discharge superheat due to restriction of the airflow to the condenser. In this case, it would be recommended to clean the condenser as it is clogged due to dirt.

4. What is a good superheat for 410a?

A good superheat for 410a would be approximately 10F around the evaporator. The suction pressure and suction temperature are measured. The temperature corresponding to the gauge pressure is taken and the difference between the two temperatures should be 10F for a good superheat. The charging and discharging of the refrigeration system will be based on this value.

5. Why do we have suction accumulator installed?

A suction accumulator is installed to avoid the refrigerant in liquid state from flooding the compressor. An accumulator is usually seen in a heat pump or on any device where liquid refrigerant is a concern.

6. What is meant by subcooling? Is subcooling desirable?

Subcooling can be defined as the condition whereby the liquid refrigerant is at a temperature lower than the saturation temperature. Subcooling is the difference between the liquid refrigerant temperature and the saturation temperature of the refrigerant.

It is desirable to have subcooling as it helps it enhancing the efficiency of the refrigeration system as the amount of heat removed per pound of refrigerant is higher. It also ensures that the liquid refrigerant reaches the expansion valve.

7. Is it necessary to know the superheat of a system. If yes, why?

Yes, it is essential to know the superheat of a system as it gives an indication if the level of refrigerant is too less or too much in the evaporator. If the superheat is high, then the amount of refrigerant is limited thereby reducing the efficiency of the system as more energy is required to operate the system. On the other hand, if the superheat is too low, then there are chances for the liquid to enter the compressor resulting in compressor damage.

For more posts on mechanical engineering, please visit Mechanical Engineering Page

About Veena Parthan

HIGH SUPERHEAT | Important Terms and 7 FAQsI am Veena Parthan, working as a Solar Operation and Maintenance Engineer for the UK Solar sector. I have more than 5 years’ experience in the field of Energy and Utilities. I have completed my Bachelor’s in Chemical engineering and Masters in Thermal Engineering. I have a profound interest in renewable energy and their optimization. I have published an article in AIP conference proceedings which is based on Cummins Genset and its flow optimization.
During my free hours, I engage in freelance technical writing and would love to offer my expertise on LambdaGeeks platform. Apart from that, I spend my free hours reading, engaging in some sport activities and trying to evolve into a better person.
Looking forward to connect you through LinkedIn - https://www.linkedin.com/in/veena-parthan-07981790/

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