Rotary Heat Exchangers Pty Ltd

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 (S4##). 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 peformance tables list the rotor diameter size at the top row with the first two collumns 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.


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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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NEW VARIABLE ROTOR POROSITY SERIES FROM RHE

We have introduced the new rotor variable porosity series into our range of heat wheels.

They continue to have the same design and overall dimensions of all our other series S426, S430 and S436. This new variable series is custom built to a selected porosity between the existing two extremes of S426 (low porosity, more Mylar, higher efficiency at higher pressure drop) to S436 (high porosity, less Mylar, lower efficiency lower pressure drop).

This is achieved by varying the small spacing between the Mylar layers to better match the specific design performance of efficiency and pressure drop of a particular project. This effectively increases or decreases the total length of Mylar film wound thus increasing the total surface area for heat transfer between hot and cold air flow streams per wheel diameter.

In order to promote and encourage the optimum sustainability solution our price is only dependant on rotor size irrespective of rotor series. This allows the designer or customer to make the optimum efficiency/pressure drop or energy vs running cost choice at no extra capital cost.

HEAT EXCHANGER TECHNOLOGY EXPLAINED SIMPLY

PERFORMANCE CHARACTERISTICS
The 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.

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 PERFORMANCE
This 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 Efficiency
Heat 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 Efficiency
For 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.

FOW ORRIENTATION
The most important factor affecting heat exchanger performance is flow orientation or arrangement of both hot and cold flows. 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. 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 change from talking up the advantages of cross-flow to talking up counterflow.

Total, Sensible, Latent Heat Efficiency
Up 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.

I will conclude by saying that total heat exchangers were designed for heat recovery in humid tropical climates and therefore are not required in most of the populated areas of Australia. In fact their sensible heat performance is generally lower than for the equivalent sensible only heat exchanger.

Bill Ellul BE(Mech), MAIRAH, FAIEAust
CEO of Rotary Heat Exchangers Pty Ltd manufacturer of Australian rotary heat exchangers since 1998 and CEO of Ecopower Pty Ltd energy sustainability engineering consultats 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 menue down the left hand side.
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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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Revised 30 Oct 2014 Copyright © 2014 Bill Ellul. All rights reserved