Performance
OUR ROTARY HEAT EXCHANGER SENSIBLE HEAT PERFORMANCE TABLES
The sensible heat performance of all our manufactured Mylar Heat Wheels are given in the following three tables presented as pdf files for the three Series S426, S430 & S436 wheels. Each Series are available in 9 wheel sizes with rotor diameters 760mm to our largest 2750mm wheels.
All rotors have been constructed by spirally winding a 75 micron thin plastic Mylar (PET) smooth 10cm (4 in) wide plastic film. This ensures high heat transfer with a minimum air flow pressure drop. All rotors are "thin" 10cm deep to ensure exhaust air carryover is negligible resulting in true thermal efficiency not significantly affected by exhaust air mixing. This obviates the use of exhaust air purge, which is necessary with "fat" rotors.
The last two numbers in the Series denote the spacing between Mylar layers in thousands of an inch. Therefore S426 has more Mylar surface area in each rotor providing a higher thermal efficiency but with a correspondingly higher pressure drop.
This series is by far the most popular as in general the thermal savings benefits of the larger quantity of Mylar per wheel outweighs the smaller fan energy usage.
Each performance tables list the rotor diameter size at the top row with the first two columns giving thermal efficiency as a percentage and air flow pressure drop in Pa.
All thermal performance is based on balanced counterflow through the rotors i.e. flows are in opposite direction and equal for both sides of the wheel. Any unbalance in flow will dramatically affect performance. If unbalance performance is required please contact us.
RHE PERFORMANCE TABLE FOR SERIES S426 WHEELS
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RHE PERFORMANCE TABLE FOR SERIES S430 WHEELS
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RHE PERFORMANCE TABLE FOR SERIES S436 WHEELS
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HEAT EXCHANGER TECHNOLOGY EXPLAINED SIMPLY
PERFORMANCE CHARACTERISTICSThe basic theory of heat exchanger performance is applicable to all forms of heat exchangers such as for example plate, rotary, radiator coil or shell and tube heat exchangers.
Unlike fans heat exchangers are regarded by many consultants as a bit of a mystery. In this discussion my aim is to explain in simple terms what are preserved to be complex heat exchanger concepts. You don’t need to be an engineer to follow this.
Fan performance is generally characterised by flow and pressure. Fan characteristics are curves showing what pressure is produced by the fan for different air flows. This is most important to HVAC designers of air ducting systems and well understood throughout the industry.
Heat exchangers like fans also have performance characteristics vital to HVAC designers. The complexity comes from the fact that they have two air flows, one for the cold and one for the hot path. They also have a third characteristic parameter, efficiency. Each of these flows will have an associated pressure drop and temperature efficiency which is independent of temperature. The energy transfered is dependent on the temperature difference between the inlets but the proportion or efficiency is not.
For simplicity and clarity heat exchanger performance characteristics are given for balanced air flow i.e. equal flows through each side. This way there is no ambiguity and more transparency in understanding their performance. In this case the heat exchanger has a closer analogy with the fan characteristic. It now becomes a flow verses pressure drop and efficiency characteristic. The values are equivalent for either side of the heat exchanger.
UNBALANCED FLOW PERFORMANCEThis is where the analogy with fans breaks down. If you increase the flow in a fan you are making it work harder and the pressure it can deliver decreases as depicted in its flow-pressure characteristic.
However if we increase for example the hot flow rate of a heat exchanger we are giving it a much better chance of heating the smaller cold flow and thermal efficiency will actually increase. We have actually given the heat exchanger an unfair advantage and we must be mindful of what we are achieving and how we compare the performance of different heat exchangers.
Going back to our fan analogy, we can increase the flow of a fan if we ignore the fact that it will result in it developing a lower pressure duty. Both characteristic parameters are required when designing projects or comparing fans,
Increasing the size of the heat exchanger also gives it an efficiency advantage by providing more surface area for heat transfer but at a cost penalty.
WHAT IS EFFICIENCY?Thermal EfficiencyHeat exchanger efficiency may mean different things to different people. Ultimately we are talking about how much cooling or heating we can achieve.We can define thermal efficiency as the percentage proportion of energy transferred from the hot flow to the cold flow.
According to the law of energy conservation i.e. energy can not be created or destroyed, the heat lost by the hot flow must equate to that gained by the cold flow – ignoring any losses to the surroundings which will be insignificant. This is true for both balanced and unbalanced flow conditions. Therefore there is no difference in thermal efficiency between the two sides irrespective of air flow balance.
Temperature EfficiencyFor simplicity and to facilitate performance measurement thermal performance can be deduced from simply calculating temperature difference ratios.
We define the temperature efficiency or sometimes described as temperature effectivity, as the temperature difference achieved on one side as a ratio of the two inlet flow temperatures. There will be a difference in temperature efficiency on the hot and cold fluid sides if the flows are unbalanced. In this case the thermal efficiency equates to the larger of these two values. As this is a little more difficult to explain simply, I will not attempt it here.
For balanced flows it can simply be shown that the temperature efficiency equates to thermal efficiency. The advantage here is that, in a balanced flow situation, the thermal performance of a heat exchanger can simply be calculated by measuring three air temperatures – the two inlets and one outlet. The efficiency is simply the ration of the temperature difference of one flow divided by the difference of the two inlets.
FLOW ORIENTATIONThe most important factor affecting heat exchanger performance is the flow orientation. This is the direction of the hot and cold flows in relation to each other. For example coil heat exchanger design is critically dependent on inlet position and row orientation.
In the case of plate and rotary heat exchangers this critical air flow orientation are described as counterflow, cross-flow and parallel flow. This describes the path taken by adjacent flows which are exchanging heat with each other. Plate heat exchangers may also have two or more of these arrangements within the same heat exchanger due to the generally long path lengths required and impracticality of attaining counterflow throughout the whole heat exchanger.
In theoretical terms using a mathematical integration analysis, it can be shown that counterflow has a double efficiency advantage over parallel flow and cross-flow is a middle compromise. The mathematical derivation is generally given in most text books on this subject. This shows that you can reach close to 100% efficiency only with counterflow and 50% with parallel flow as you increase the size of the exchanger. For this reason it is most important that the RHE is oriented in couterflow as parallel flow may halve its performance. This has also been extensively shown in the literature by experimental verification in the laboratory for a large variety of heat exchanger designs. For example Kays & London 1984 “Compact Heat Exchangers” a classic text book for this subject.
In the several decades of my involvement in this industry I find it curious that it has taken such a long time for the industry to move from talking up the advantages of cross-flow to talking up counterflow. However it is impossible to achieve full counterflow over the full length of a plate exchanger due to the need to separate the flows between each alternative plate which need to be spaced very close together.
Total, Sensible, Latent Heat EfficiencyUp till now we are discussing heat exchangers where the only energy transfer taking place is that due to thermodynamic conduction and convection with no transfer of water or water vapour. We call this Sensible heat transfer or Sensible heat exchangers.
Heat exchangers can transfer sensible heat or sensible and latent heat meaning total heat. Sensible heat is where no moisture is transferred and so latent heat of water vapour is not involved. In this case the energy transfer can be simply calculated from the temperature differences and flow rates.
A total heat exchanger transfers moisture and the energy of moisture from one stream to the next by using a moisture sorption compound that adsorbs water in one stream and desorbs it into the next. The extra transfer of latent energy is added to the total transfer. Terms such as enthalpy, total and moisture effectivities can be used to define these efficiencies and they are similarly defined to temperature efficiencies. The drying of the incoming humid air in a tropical application is achieved by the dryer airconditioned exhaust air. Thus this wheel acts as a moisture barrier preventing the humidity to enter the building in the fresh air. In indoor pool environments, generally the humidity inside the pool hall is higher than the ambient environment, so it would be inappropriate to use a Total RHE when trying to reduce pool hall humidity with fresh air.
Total heat exchangers are designed for heat recovery in humid tropical climates where latent heat transfer is significant. They are not required in most of the populated areas of Australia where humidity is low. In fact their sensible heat performance is generally lower than for the equivalent sensible only heat exchanger.
Bill Ellul BE(Mech), MAIRAH, FAIEAustCEO of Rotary Heat Exchangers Pty Ltd manufacturer of Australian rotary heat exchangers since 1998 and CEO of Ecopower Pty Ltd energy sustainability engineering consultants since 1987.
PERFORMANCE CHARACTERISTICSThe basic theory of heat exchanger performance is applicable to all forms of heat exchangers such as for example plate, rotary, radiator coil or shell and tube heat exchangers.
Unlike fans heat exchangers are regarded by many consultants as a bit of a mystery. In this discussion my aim is to explain in simple terms what are preserved to be complex heat exchanger concepts. You don’t need to be an engineer to follow this.
Fan performance is generally characterised by flow and pressure. Fan characteristics are curves showing what pressure is produced by the fan for different air flows. This is most important to HVAC designers of air ducting systems and well understood throughout the industry.
Heat exchangers like fans also have performance characteristics vital to HVAC designers. The complexity comes from the fact that they have two air flows, one for the cold and one for the hot path. They also have a third characteristic parameter, efficiency. Each of these flows will have an associated pressure drop and temperature efficiency which is independent of temperature. The energy transfered is dependent on the temperature difference between the inlets but the proportion or efficiency is not.
For simplicity and clarity heat exchanger performance characteristics are given for balanced air flow i.e. equal flows through each side. This way there is no ambiguity and more transparency in understanding their performance. In this case the heat exchanger has a closer analogy with the fan characteristic. It now becomes a flow verses pressure drop and efficiency characteristic. The values are equivalent for either side of the heat exchanger.
UNBALANCED FLOW PERFORMANCEThis is where the analogy with fans breaks down. If you increase the flow in a fan you are making it work harder and the pressure it can deliver decreases as depicted in its flow-pressure characteristic.
However if we increase for example the hot flow rate of a heat exchanger we are giving it a much better chance of heating the smaller cold flow and thermal efficiency will actually increase. We have actually given the heat exchanger an unfair advantage and we must be mindful of what we are achieving and how we compare the performance of different heat exchangers.
Going back to our fan analogy, we can increase the flow of a fan if we ignore the fact that it will result in it developing a lower pressure duty. Both characteristic parameters are required when designing projects or comparing fans,
Increasing the size of the heat exchanger also gives it an efficiency advantage by providing more surface area for heat transfer but at a cost penalty.
WHAT IS EFFICIENCY?Thermal EfficiencyHeat exchanger efficiency may mean different things to different people. Ultimately we are talking about how much cooling or heating we can achieve.We can define thermal efficiency as the percentage proportion of energy transferred from the hot flow to the cold flow.
According to the law of energy conservation i.e. energy can not be created or destroyed, the heat lost by the hot flow must equate to that gained by the cold flow – ignoring any losses to the surroundings which will be insignificant. This is true for both balanced and unbalanced flow conditions. Therefore there is no difference in thermal efficiency between the two sides irrespective of air flow balance.
Temperature EfficiencyFor simplicity and to facilitate performance measurement thermal performance can be deduced from simply calculating temperature difference ratios.
We define the temperature efficiency or sometimes described as temperature effectivity, as the temperature difference achieved on one side as a ratio of the two inlet flow temperatures. There will be a difference in temperature efficiency on the hot and cold fluid sides if the flows are unbalanced. In this case the thermal efficiency equates to the larger of these two values. As this is a little more difficult to explain simply, I will not attempt it here.
For balanced flows it can simply be shown that the temperature efficiency equates to thermal efficiency. The advantage here is that, in a balanced flow situation, the thermal performance of a heat exchanger can simply be calculated by measuring three air temperatures – the two inlets and one outlet. The efficiency is simply the ration of the temperature difference of one flow divided by the difference of the two inlets.
FLOW ORIENTATIONThe most important factor affecting heat exchanger performance is the flow orientation. This is the direction of the hot and cold flows in relation to each other. For example coil heat exchanger design is critically dependent on inlet position and row orientation.
In the case of plate and rotary heat exchangers this critical air flow orientation are described as counterflow, cross-flow and parallel flow. This describes the path taken by adjacent flows which are exchanging heat with each other. Plate heat exchangers may also have two or more of these arrangements within the same heat exchanger due to the generally long path lengths required and impracticality of attaining counterflow throughout the whole heat exchanger.
In theoretical terms using a mathematical integration analysis, it can be shown that counterflow has a double efficiency advantage over parallel flow and cross-flow is a middle compromise. The mathematical derivation is generally given in most text books on this subject. This shows that you can reach close to 100% efficiency only with counterflow and 50% with parallel flow as you increase the size of the exchanger. For this reason it is most important that the RHE is oriented in couterflow as parallel flow may halve its performance. This has also been extensively shown in the literature by experimental verification in the laboratory for a large variety of heat exchanger designs. For example Kays & London 1984 “Compact Heat Exchangers” a classic text book for this subject.
In the several decades of my involvement in this industry I find it curious that it has taken such a long time for the industry to move from talking up the advantages of cross-flow to talking up counterflow. However it is impossible to achieve full counterflow over the full length of a plate exchanger due to the need to separate the flows between each alternative plate which need to be spaced very close together.
Total, Sensible, Latent Heat EfficiencyUp till now we are discussing heat exchangers where the only energy transfer taking place is that due to thermodynamic conduction and convection with no transfer of water or water vapour. We call this Sensible heat transfer or Sensible heat exchangers.
Heat exchangers can transfer sensible heat or sensible and latent heat meaning total heat. Sensible heat is where no moisture is transferred and so latent heat of water vapour is not involved. In this case the energy transfer can be simply calculated from the temperature differences and flow rates.
A total heat exchanger transfers moisture and the energy of moisture from one stream to the next by using a moisture sorption compound that adsorbs water in one stream and desorbs it into the next. The extra transfer of latent energy is added to the total transfer. Terms such as enthalpy, total and moisture effectivities can be used to define these efficiencies and they are similarly defined to temperature efficiencies. The drying of the incoming humid air in a tropical application is achieved by the dryer airconditioned exhaust air. Thus this wheel acts as a moisture barrier preventing the humidity to enter the building in the fresh air. In indoor pool environments, generally the humidity inside the pool hall is higher than the ambient environment, so it would be inappropriate to use a Total RHE when trying to reduce pool hall humidity with fresh air.
Total heat exchangers are designed for heat recovery in humid tropical climates where latent heat transfer is significant. They are not required in most of the populated areas of Australia where humidity is low. In fact their sensible heat performance is generally lower than for the equivalent sensible only heat exchanger.
Bill Ellul BE(Mech), MAIRAH, FAIEAustCEO of Rotary Heat Exchangers Pty Ltd manufacturer of Australian rotary heat exchangers since 1998 and CEO of Ecopower Pty Ltd energy sustainability engineering consultants since 1987.
SEE Bill's published article "Performance Characteristics explained in simple term" in Climate Control News July 2014, which can be downloaded from the Published Articles menu.
EFFECT OF FLOW ORIENTATION ON PERFORMANCE
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TECHNICAL PAPER ON THEORY AND DESIGN OF AUSTRALIAN MYLAR ROTARY HEAT EXCHANGERS
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WHY RHE'S IN ENERGY RECOVERY/RECYCLING VENTILATION MODE SHOULD NOT VARY ROTOR SPEED TO CONTROL TEMPERATURE
VSD ROTOR SPEED CONTROL STRATEGY
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RHE COMPARED WITH A HEAT PUMP (Download Article ECOLIBRIUM 2024 pages 36-37 from the "Publiched Articles" Section from the Menu)
There is a need to electrify the heating of indoor pools in an effort to reduce the dependence of the use of the fossil fuel natural gas. One obvious solution is the use of electrically driven heat pumps to extract heat energy from outside environments having regard to the advantages provided by the hopefully high coefficient of performance, COP of heat pumps.
Put simply, this relies on the fact that utilising a refrigerated heat pump with its outdoor fan coil component combined with its indoor fan coil counterpart, driven by a compressor which circulates a refrigerant continuously between the two, uses only 1/COP times the electrical energy that would be used to provide the equivalent resistive heating.
In purely economic terms ignoring any environmental concerns, as long as COP is high enough it can require less energy to use a heat pump to provide the heating than the gas alternative. The energy cost may also be cheaper depending on how the ratio of kWhe/MJgas cost compares with the COP of the heat pump.Now when we come to the practical problem of transferring the heat from the exhaust air to heat the incoming fresh air, we can either use a simple heat exchanger or a heat pump or both.
It is a simple fact that extracting heat from a hot air stream to heat a cold air stream using an efficient heat exchanger would utilise less electrical energy than with the use of a heat pump which needs the use of a compressor together with the fan pressure loss across both fresh and exhaust fan coil units. So, it makes sense to recover the “low hanging fruit” free heating with an efficient heat exchanger before introducing the heat pump for any other added heating benefit.
There is therefore still the requirement to use a highly efficient 90% free heating rotary heat exchanger in the design of a totally electrified heat pump heating system for indoor pools. If a lower efficient, higher pressure drop heat exchanger is used, this would result in a higher electrical power consumption system.
For example, we can compare the performance of a RHE and a Heat Pump for reclaiming heat from an exhaust air stream to heat fresh air. Here we have chosen the example of an RHE2750S426 heat wheel handling 8,000 l/s indoor pool fresh and exhaust air with 83% heat transfer efficiency and the indoor temperature is 27C and outdoor air 10C.
RHE HEAT TRANSFER 8 m3/h air flow with Dt of (27-10) = 17C and Eta of 83% q = 8 x 1.2 x 1.012 x 17 x 0.83 = 137 kW Total Power to drive 2 fans and 300W RHE drive e = 3.1 kW COP = q/e = 44 Heat transferred per elec input e/q = 1/COP = 0.023 kW heat per kW electric
FOR A TYPICAL HEAT PUMP COP = q/e = 3.00 Heat transferred per elec input e/q = 1/COP = 0.3333 kW heat per kW electric
BENEFIT OF RHE OVER HEAT PUMP FOR TRANSDERRING HEAT = 0.3333/0.023 = 1,449%
CONCLUSIONThere is no doubt that the heat pump will be required to provide the heating as an alternative to using natural gas, but the use of a highly efficient low energy use heat exchanger will still play a major role in heat reclaim.
There is a need to electrify the heating of indoor pools in an effort to reduce the dependence of the use of the fossil fuel natural gas. One obvious solution is the use of electrically driven heat pumps to extract heat energy from outside environments having regard to the advantages provided by the hopefully high coefficient of performance, COP of heat pumps.
Put simply, this relies on the fact that utilising a refrigerated heat pump with its outdoor fan coil component combined with its indoor fan coil counterpart, driven by a compressor which circulates a refrigerant continuously between the two, uses only 1/COP times the electrical energy that would be used to provide the equivalent resistive heating.
In purely economic terms ignoring any environmental concerns, as long as COP is high enough it can require less energy to use a heat pump to provide the heating than the gas alternative. The energy cost may also be cheaper depending on how the ratio of kWhe/MJgas cost compares with the COP of the heat pump.Now when we come to the practical problem of transferring the heat from the exhaust air to heat the incoming fresh air, we can either use a simple heat exchanger or a heat pump or both.
It is a simple fact that extracting heat from a hot air stream to heat a cold air stream using an efficient heat exchanger would utilise less electrical energy than with the use of a heat pump which needs the use of a compressor together with the fan pressure loss across both fresh and exhaust fan coil units. So, it makes sense to recover the “low hanging fruit” free heating with an efficient heat exchanger before introducing the heat pump for any other added heating benefit.
There is therefore still the requirement to use a highly efficient 90% free heating rotary heat exchanger in the design of a totally electrified heat pump heating system for indoor pools. If a lower efficient, higher pressure drop heat exchanger is used, this would result in a higher electrical power consumption system.
For example, we can compare the performance of a RHE and a Heat Pump for reclaiming heat from an exhaust air stream to heat fresh air. Here we have chosen the example of an RHE2750S426 heat wheel handling 8,000 l/s indoor pool fresh and exhaust air with 83% heat transfer efficiency and the indoor temperature is 27C and outdoor air 10C.
RHE HEAT TRANSFER 8 m3/h air flow with Dt of (27-10) = 17C and Eta of 83% q = 8 x 1.2 x 1.012 x 17 x 0.83 = 137 kW Total Power to drive 2 fans and 300W RHE drive e = 3.1 kW COP = q/e = 44 Heat transferred per elec input e/q = 1/COP = 0.023 kW heat per kW electric
FOR A TYPICAL HEAT PUMP COP = q/e = 3.00 Heat transferred per elec input e/q = 1/COP = 0.3333 kW heat per kW electric
BENEFIT OF RHE OVER HEAT PUMP FOR TRANSDERRING HEAT = 0.3333/0.023 = 1,449%
CONCLUSIONThere is no doubt that the heat pump will be required to provide the heating as an alternative to using natural gas, but the use of a highly efficient low energy use heat exchanger will still play a major role in heat reclaim.