Explore the essentials of low superheat in HVAC systems, including causes, implications, and adjustment techniques for optimal performance.
DEFINITION OF LOW SUPERHEAT
When there is an excess amount of refrigerant in the coils of the evaporator in comparison to the heat load. This condition is termed as low superheat. The reason for low superheat could be due to insufficient heat load or due to excessive amounts of refrigerant entering the evaporator.
There may be some amount of liquid refrigerant in the suction line which might enter the compressor and cause compressor damage. The reasons for low superheat are explained below:
1. Excess amount of Refrigerant
When there is an excess amount of refrigerant that is flowing through the evaporator coils, enough heat will not be absorbed by the evaporator to vaporize the liquid refrigerant. As a result, we have a low superheat and as the refrigerant can absorb enough heat in the suction line; there is a high possibility that it might enter the compressor and damage that unit.
2. Overfeeding in the metering unit
A metering unit that allows more than the needed amount of refrigerant to the evaporator coils will cause flooding. In case the sensing bulb of the thermal expansion valve is not insulated properly then there is a high possibility of the valve being flooded or overfed. When the device overfeeds, there are high chances for both the suction pressure and the discharge pressure to increase.
3. Reduced airflow through the evaporator
One of the most common reasons for low superheat is due to reduced airflow. With reduced airflow, there isn’t enough warm air to vaporize the refrigerant. As a result, there will be a reduced amount of refrigerant vapor and there is a high possibility for the liquid refrigerant to enter the compressor and cause damage to the unit. In this case, both suction and discharge pressures will be lower than usual levels.
It is recommended to clean dirty filters, coil, and motors to allow more air to enter through the evaporator.
4. Reduced airflow through the condenser
When the amount of air entering the condenser is low, there is a high possibility for higher pressure and temperature in the condenser and the condenser coils, the refrigerant is available to the metering device at higher pressure.
With an increased pressure drop across the metering device, more refrigerant enters the flow. As more refrigerant enters the flow, the suction and discharge pressure increase; also results in subcooling. The main reason for low airflow through the condenser is due to poor motor bearings or obstructions in the unit.
5. Large Sized Equipment
When the system or equipment is too large, but the load is not enough that is enough heat is not available to vaporize the liquid refrigerant into vapor, then it will result in low superheat. With oversized equipment, the indoor relative humidity is expected higher than usual.
LOW SUPERHEAT LOW SUBCOOLING
When there is an excess amount of refrigerant but a limited amount of heat load that is available in the evaporator, the condition is referred to as low superheat. This could be caused due to low airflow or due to plugged coils in an evaporator. When there is a limited amount of refrigerant entering the condenser, this could be the result of poor compression, an oversized metering device, or overfeeding.
This condition is referred to as low subcooling. When there is limited heat load in the evaporator and limited refrigerant in the condenser, this condition is referred to as low superheat low subcooling. The superheat will help in identifying if the low suction is a result of limited heat entering the evaporator coils.
LOW SUPERHEAT NORMAL SUBCOOLING
Low superheat normal subcooling can indicate that the refrigerant charging is high either due to plugged evaporator coils or due to plugged air filters. The reason for the normal subcooling despite the low superheat is because the refrigeration system is installed with a liquid line receiver. The temperature drop across the liquid line filter or dryer gives a clear indication of the possible cause is due to plugging.
HOW TO RAISE OR LOWER SUPERHEAT?
To raise superheat, there should be more heat load that is available for the evaporator coils to handle. While to lower superheat, more refrigerant should be added so that the heat load can be handled by the coils of the evaporator. It is recommended to add refrigerant to lower superheat and recover refrigerant to increase superheat. It should be noted that additional superheat should not be added if the superheat is found to be 5F already.
LOW DISCHARGE SUPERHEAT ALARM
A low discharge superheat alarm indicates that the compressor is flooding with the refrigerant. This is mostly because the expansion valve is overfeeding to the evaporator or due to a faulty actuator.
A LOW EVAPORATOR SUPERHEAT INDICATES
A low evaporator superheat is a condition wherein the refrigerant hasn’t been capable of carrying enough heat load to the compressor coils. This will limit the refrigerant from vaporizing, because of which liquid refrigerant will enter the compressor which will cause slugging that damages the compressor units and other components of the refrigeration system.
HIGH SUCTION PRESSURE LOW SUPERHEAT
A suction pressure low superheat condition occurs when the capacity regulator is large because of which it feeds in more refrigerant into the coils of the evaporator as the heat load is not enough for the available refrigerant. Another possible reason for this condition could be the high capacity of the thermal expansion valve.
To maintain the total capacity of the system, it is essential to have an appropriate refrigerant charge in the system so that suction pressure and superheat are kept to the right levels that would help in the proper functioning of the refrigeration system.
LOW SUCTION SUPERHEAT CARRIER | LOW SUCTION SUPERHEAT
A low suction superheat carrier is referred to when there isn’t enough air that flows through the evaporator coils. This limits the heat from being carried to the coils of the evaporator which results in low suction superheat. The possible reasons for low suction superheat could be the dirty of plugged evaporator coil that restricts air from flowing through the coils. It is recommended to add refrigerant to lower the suction superheat and add refrigerant to increase the suction superheat.
LOW-TEMPERATURE SUPERHEATER
In a low-temperature superheater, the steam entering the turbine has a high moisture content which increases the rate of erosion. Further, a decrease in the superheat temperature also causes quenching of the metal surfaces of the equipment it passes through.
There is the possibility of stresses on the surface of superheaters, steam pipes, stop valves, and turbine inlets. A severe vibration is reported in case of sudden chilling of the turbine rotor.
LOW SUCTION PRESSURE LOW SUPERHEAT
A low suction pressure low superheat is encountered when there is low heat load which could be because of dirty air filters, an insufficient amount of air flowing through the system, or because of the air being too cold. Other possible causes of low suction pressure low superheat are the non-uniform distribution of the refrigerant and could be the result of oil clogged evaporators.
LOW SUPERHEAT LOW SUBCOOLING TXV
Low superheat indicates that there is an excess amount of refrigerant in the evaporator, or the heat load is not sufficient to vaporize the liquid refrigerant to vapor before it moves to the compressor resulting in compressor damage. Plugging of the evaporator coils can also result in low superheat.
On the other hand, low subcooling indicates that there is an excess amount of refrigerant in the condenser. For refrigeration systems that using a thermostatic expansion valve, it is recommended to be maintained between 100F to 180 F.
Therefore, a low superheat low subcooling TXV is one where the refrigerant is in excess in the evaporator and is limited in the condenser resulting in variations in the subcooling below 100F
0 DEGREE SUPERHEAT ON LOW TEMP REFRIGERATION UNIT
0 Degree superheat or low superheat on a low-temperature refrigeration system could indicate that the refrigerant is not carrying enough heat through the coils of the evaporator to vaporize the refrigerant before entering the compressor coils. Even in a low-temperature refrigeration system, it is essential to collect enough heat that is equivalent to the refrigerant charge in the system.
HEAT PUMP LOW SUPERHEAT
A heat pump that is operating at low superheat does not have enough heat load for the excess amount of refrigerant that is available in the coils of the evaporator resulting in liquid refrigerant entering the compressor valves and causing damage to the compressor and other mechanical components of the refrigeration system.
It is therefore suggested to maintain the superheat of the refrigeration system within certain limits such that the damages to the parts of the refrigeration system are minimized. Further, it is recommended to carrying out timely cleaning of the evaporator coils and the compressor valves to avoid plugging that would reduce the flow of air which could also limit the efficiency of the system.
FAQs
1. What does a low superheat indicate?
It indicates that there isn’t enough heat load for refrigerant that is available in the evaporator coils which could result in flooding of the compressor. The compressor is designed to only work with vapors or gases and the entry of liquid will damage the compressor coils and their other components.
A low superheat could also be the result of plugged evaporator coils which is stopping the entry of the heat load. Limited airflow through the system could also result in low superheat because sufficient airflow is required for carrying the heat to vaporize the refrigerant. A faulty metering device or overfeeding of refrigerant can also result in low superheat.
2. If in recovery boiler feed water temp is low What effect of low temp will be in superheated steam or final steam?
The boiler operates with a layer of heat transfer surface which is hot, and water passes over this surface. As the water passes over the hot surface, steam is produced which enters the steam system. The pressure at the heat transfer surface is higher than at the water system because of the heat of the water.
The steam bubbles leaving the heat transfer surface will either be superheated or cooled to the saturation temperature as it rises through the water. The latter can happen. When water is fed to the boiler, it passes in between the heat transfer surface and the boiling water.
Water that is fed into the boiler is usually preheated but is always cooler than the water in the boiler. As the steam rises from the heat transfer surface to this cold-water layer, the steam bubbles condense resulting in two major issues.
The steam bubbles will have some tiny water droplets in them. As a large amount of feedwater enters, the quality of steam is reduced as the boiler reaches isothermal conditions. Secondly, the addition of cool water reduces steam production.
The issues mentioned above can be reduced by using a continuous steam boiler because, in such a boiler, water will be added at low rates because of which the boiler water will be at the isothermal condition and there will be no clouds or mist that will be formed.
3. How to increase low-pressure superheated steam to high pressure?
It is possible to increase the pressure of air using a vapor compressor, but it is not the same when it comes to the increasing pressure of steam as it contains condensate which can damage the compressor. Further, the increasing temperature cannot guarantee an increase in pressure of the superheat instead, the steam might get more superheated without any increase in pressure.
It is possible to increase low pressure superheat to high pressure superheat by combining a low-pressure steam flow with high-pressure steam. But this will result in the backflow of high-pressure steam into a low-pressure pipe. To prevent this backflow, an ejector needs to be installed.
In an ejector, the higher-pressure steam is used as means of pulling the low-pressure steam whereby the high-pressure steam does not backflow into the low-pressure line. This helps in maintaining the high pressure of the superheated steam in the outlet.
PROBLEM STATEMENT I
Superheated steam at a temperature of 3000C and absolute pressure of 1.013 bar enters a pipe. What is the additional amount of heat that the superheated steam carries in comparison to saturated steam passing the same pipe at the same pressure?
Enthalpy of saturated steam at 1.013 bar is 2676 kJ/kg (retrieved from the steam table)
Enthalpy of superheated steam at 3000C and 1.013 bar is 3075 kJ/kg (retrieved from the steam table)
Enthalpy of the superheat = Enthalpy of superheated steam – Enthalpy of saturated steam
3075 kJ/kg – 2676 kJ/kg = 399 kJ/kg
The specific heat capacity of the superheat can be determined by dividing enthalpy in the superheat by the difference between the saturation and superheat temperatures
Specific Heat Capacity = (Enthaply in Superheat)/(Superheat Temperature-Saturation Temperature)
= (399 kJ/kg)/(300-100)
= 1.995 kJ/kg 0C
I am Veena Parthan, completed my master’s in Thermal Engineering. I am 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 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.
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