Heat Exchangers: Why Efficiency Matters

HEAT

EXCHANGERS: Why Efficiency Matters

888.778.6701

In this eBook we will

cover…

1.1 Heat recovery

1.2 The principle of heat exchange

1.3 Cross-flow heat exchanger

1.4 Counter-flow heat exchanger

1.5 Rotary heat exchanger

1.6 Cross-counter-flow heat exchanger

Zehnder America, Inc. · zehnderamerica.com · 888.778.6701

1.1 Heat Recovery Ventilation Heat Recovery Ventilation and Energy Recovery Ventilation reduce both the

heating/cooling costs and loads of the building.

In a leaky home, during a 0° C/32° F cold winter day, incoming air comes in the building

through cracks and openings at the same cold outside temperature. Therefore, an

inhabitant needs to operate heating equipment frequently to warm spaces to a

comfortable room temperature. The unconditioned, uncontrolled outside air can produce

uncomfortable drafts and variations in temperature throughout the building depending on

the sources of leaks/infiltration.

With a high-efficiency Heat Recovery Ventilator in an energy-efficient building, the fresh

filtered incoming outdoor air is pre-heated and mechanically ventilated throughout the

house within several degrees of the room temperature. Therefore, heating loads can be

greatly reduced and uncomfortable cold drafts eliminated.


1.2 The Principle of Heat Exchange

The core of heat recovery is the heat exchanger. Some heat exchangers are so efficient

that up to 95% of the heat from the outgoing stale air is recovered and used to temper the

incoming air from the outside. There are various types of heat exchangers available

including cross-flow, counter-flow (includes rotary/wheel) and cross-counter-flow.

Through a heat exchanger, fresh-filtered air flows in the house continuously and an equal

volume of stale air simultaneously flows out of the home. These airflows are allowed to

pass by each other- separated only by a thin membrane. The longer the two streams flow

past each other, the higher the efficiency.

Two distinct processes are at work here:

The air flows are moved against each other by a fan in the return air flow and a fan in the

supply air flow. The yellow return air becomes the brown expelled air and the green

outside air becomes the red supply air.

There is a temperature difference between the two air flows which pass by each other. We

can see this in our previous mentioned example. The air flow emerging from the house is

20° C/36° F warmer than the incoming air, which is at 0° C/32° F. Now the heat is

exchanged. Heat passes from the warm air flow to the membrane, and through the

membrane; the warm air flow cools little by little. The heat passes through the membrane

and into the colder air stream, and the cold air gets ever warmer.

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In the example above with a Zehnder HRV, it can clearly be seen that the temperature

difference between the incoming pre-heated air and the comfortable existing room

temperature air is only about 2°C/4° F. This eliminates cold unpleasant drafts in the

home. The incoming air temperature is warmed to about 18° C/64° F within 2° C/4° F of the

20° C/68° F existing room temperature.

This greatly reduces both the heating costs and loads. To reach 20° C/68° F there only

needs to be an energy source in the house to add the minimal extra warmth.


1.2.1 Efficiency

The efficiency can be defined as follows:

Using the general equation for energy transfer by convection

E = mass flow × specific heat capacity c × temperature difference (Tintake - Toutlet)

the following applies:

Because Vout A = Vret A and

the specific heat capacity for the two air flows is the same,

so, as long as the return air is dry:


1.3 Cross-flow heat exchanger

How a Cross-flow Heat Exchanger Works

Cross-flow heat exchangers can be constructed to be very compact. A significant drawback

though is the heat efficiency. The supply air does not reach as high a temperature as with

the counter-flow heat exchanger. In an optimal scenario it reaches about 70% efficiency.

The crossing of flows means that at two corners the air streams with the greatest

temperature difference meet each other.

It can be seen that the upper and lower air streams with larger temperature differences

encounter each other: that is the green and the yellow flows, and the brown and the red

ones.

At the sides however the situation is rather the counter flow principle (green flows past

brown and yellow flows past red).

The potential, to achieve a high temperature in the supply air using the heat from the

outlet air is lost in cross-flow heat exchange.

Going back to the original example, even at its optimal performance of 70% efficiency,

the make-up air temperature = 32°F + (68°F - 32°F)*(0.70) or 0°C + (20°C-0°C)*(0.70)

=14°C/57°F within 6° C/11° F of the 20° C/68° F existing room temperature. Therefore, one

will have to spend more on monthly heating to bring the rooms to a comfortable

temperature in comparison to a greater efficiency system.


1.4 Counter-flow heat exchanger

How a Counter-flow Heat Exchanger Works

These can be distinguished in that the counter-flow zone comprises the largest part of the

device. At the beginning and end of the device there are very small zones with crossed air

streams, and here too the problem of joining streams must be resolved. The efficiency in

the case of very long dimensions is entirely dependent on the available surface area and in

practice reaches 95%.


1.5 Rotary/Wheel heat exchanger

How a Rotary/Wheel Heat Exchanger Works

A rotary/wheel heat exchanger is considered a counter-flow heat exchanger. The

advantage of the rotary heat exchanger is that the air distribution is more straight-forward

than in some other counter-flow heat exchangers. In the latter device there is the issue of

the complex guiding of air from one side into the counter-flow zone and then on the other

leading out from the counter-flow zone. The counter-flow heat exchanger also involves

relatively complex membranes and complex problems of maintaining air tightness

between the membranes, as the two air flows are not to mix. However it's on exactly this

point that a drawback to the rotary/wheel exchanger arises. While the other devices

presented above take pains to ensure the air flows do not come in contact, the rotary

exchanger design accepts this will happen. The cell through which the return air flows will

have outside air flowing through it just a short time later. Therefore the airtightness is not

good due to the moving wheel and it's more susceptible to leakage between the fresh

air and stale airstream.

The air flows through a honeycomb matrix constructed in the form of a wheel. The wheel

turns and the outlet air blows through one half of the wheel and as it rotates the intake air

flows through.

The warm air (return air) heats up the honeycomb matrix. The wheel turns and moments

later the cold outside air passes from the opposite direction through the same section of

honeycomb. The air takes in heat from the warmed honeycomb walls.


1.6 Cross-counter-flow heat exchanger

(Zehnder HRV Exchanger)

How a Cross-counter-flow Heat Exchanger Works

This design has several

benefits:

a relatively small size can be achieved;

the crossing of the air streams at the

front and behind resolves the problem

of joining the multiple streams;

the counter-flow area in the middle

gives high efficiency

The thermally wasteful corners are omitted thanks to the counter-flow zone. The remaining

cross-flow zones do not play as crucial a role if the counter-flow zone has sufficient surface

area.

With this geometry too, an efficiency of up to 95% can be achieved. This is the best option

for both reducing heating/cooling loads and general comfort of the occupants.

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Zehnder America, Inc. 540 Portsmouth Ave.

Greenland, NH 03840

888.778.6701

Date post:	21-Dec-2014
Category:	Education
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Heat Exchangers: Why Efficiency Matters

Education