Understanding Heat Exchangers, Cross Flow Counter Flow and Cross Counter Flow Heat Exchangers

Understanding Heat Exchangers- Cross-flow, Counter-flow (Rotary/Wheel) and Cross-counter-flow Heat

Exchangers

6.1 Heat recovery

Now we will look at an example of how physical laws operate in practice.

We will assume an idealised typical situation and pose the question:

Idealised typical situation: A house with people living in it has a mechanical supply and return

ventilation system. Each fan produces a supply flow and a return flow. At any moment both flows are

the same size.

The air outside and thus the intake air is at a temperature of 0° C . The return air, and thus the

expelled air, is at a temperature of 20° C .

In our house the air is heated by various means.

For example by:

• space heating,

• heat given off by household appliances or

• sun rays coming through the window.

The intake air at 0° C is too cold and needs to be warmed.

Now the question we pose: Is it not a waste that air at 20° C is simply released outside? Can any

benefit be got from the warm return air at 20° C? Would it be possible to warm the fresh outside air

coming in?

There is in fact a fantastic solution here. The key concept is that of heat recovery . You can say

that the ventilation here has a thermal aspect too.

The core of heat recovery is the heat exchanger , a device which puts this exact idea into practice.

The heat exchanger recovers a significant amount of the heat which the air had acquired, by various

means, inside the house. Let's take a closer look at the principle of the heat exchanger.

6.2 The principle of heat exchange

Air flows into the house 24 hours a day, and simultaneously air flows out 24 hours a day. These air

flows are allowed pass by each other - separated only by a thin membrane.

Two distinct processes are at work here: 1. 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.

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

this in our example house above. The air flow emerging from the house is 20° C warmer than the

incoming air, which is at 0° C. 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.

It can be clearly seen that the temperature difference in our example is only 2° C.

The potential of this principle is enormous, because the temperatures are not simply "mixed" (which

would yield 10° C), but instead the temperature can reach to within 2° C of 20° C that is to say warmed

to 18° C.

To reach 20° C there must be an energy source in the house to add the minimal extra warmth.

6.2.1 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 - :

6.3 Counter flow-heat exchanger

When the principle is applied in a device, the longer the two streams flow past each other, the higher

the efficiency.

There are limits to these advantages however: 1. Such equipment can take up lots of space, and when longer than a certain length will involve

significant pressure drops. So that the air flows will continue to flow past each other, bigger fans must

be employed. This can lead to higher energy consumption and greater noise.

To improve performance very many plates (for example 100 ) have to be used.

This increases efficiency, but creates a new problem.

1. It becomes very difficult to guide the many parallel air streams, for example into pipes.

So how can all the air streams from the "red" plates be brought together, when there is always a

"yellow" plate alongside? All "red" air streams have to flow into a single pipe. And all yellow streams

flow out of a single pipe and into the heat exchanger, but have to be distributed among the "yellow"

plates.

The question can also be posed about the other side of the device, with the green and brown plates,

which symbolise the air flows of the outside air and the expelled air respectively.

How can the multiple air streams be laced together?

The next page shows a possible solution.

6.4 Cross-flow heat exchanger

To resolve this issue of lacing the streams together, the idea of crossing , the flows, instead of

having them flow against each other, was developed. In practice the air flows are guided by staggered

plates. Now there is separate access to all "red" flows, all "yellow" flows and so on.

This gives two initial advantages : 1. as already explained, only one type of air flow impinges on each of the four sides Large tubes can be

attached here, the problem of joining the streams is solved.

1. the manufacture of such devices is simpler and they require less space.

In practice there will again be a large number of plates (perhaps 100), laid out one behind the other.

But with this solution too there is a significant drawback . The crossing of flows means that at two

corners the air streams with the greatest temperature difference meet each other.

We moved the arrow in the diagram somewhat closer to the corner. It can now more readily 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.

6.5 Cross-counter-flow heat exchanger

Figuratively speaking, the cross-flow heat exchanger is divided in the middle and pulled apart.

The principle of air streams passing each other (counter-flow) is applied to the space created. The

result is the cross-counter-flow heat exchanger.

This design has several benefits : 1. a relatively small size can be achieved;

1. the crossing of the air streams at the front and behind resolves the problem of joining the multiple

streams;

1. the counter-flow area in the middle gives high efficiency

6.6 Set-up

We will briefly present again the three types of device with regard to the different zones , that is,

the counter-flow zone and the cross-flow zone.

1. Counter-flow heat exchangers

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. Cross-flow heat exchanger

The supply air does not reach quite as high a temperature as with the counter-flow heat exchanger,

as at two corners temperatures with high differences between them encounter each other.

The efficiency is therefore less, even when a very large exchange surface area is provided. In an

optimal scenario it reaches about 70% .

The large temperature difference at one part of the surface means this form of heat exchanger

reaches its maximum even with small surface areas. The devices can be constructed to be very

compact. This is in contrast to the counter-flow heat exchanger, which is more effective the longer it

is.

1. Cross-counter heat exchanger

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. W ith this

geometry too, an efficiency of up to 95 % can be achieved.

Plates or ducts The various zones are also important in the construction of the heat exchanger.

For the cross-flow zones the plates must be employed, as otherwise the stream-joining problem

would once again arise.

For the counter-flow zones it is possible - to increase efficiency - to subdivide the plates and make

use of ducts . This makes more surface area available for heat exchange.

Plates are chosen if the membrane material cannot easily be bent out of shape, ducts are generally

manufactured from thermoformable sheets.

Example Since the duct height is very small at 2.5 to 4 mm a very large exchange area can be packed

into a small volume, for example about 30m2 for the ComfoAir 350 of dimensions approx. 370 x 370 x

370 mm.

6.7 Rotary heat exchanger

To solve the problem of joining the multiple streams of air, there is another approach: the rotary heat

exchanger.It is also a form of counter-flow exchanger.

The problem of threading together all the streams of air is very easily solved by setting the

whole counter-flow zone in motion.

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.

Let's take a closer look at what's happening inside an individual cell of the honeycomb.

The honeycomb is made of ducts similar to those in the counter-flow zone in the duct heat exchanger.

The walls of the cells however are no longer paper-thin plastic membranes, but have a certain weight

to them. The need a little time to warm up.

The cell walls slowly warm up from the side of the return air to the side of the expelled air. Closer to

the return air side the heating effect is stronger, and is less close to the expelled air side.

The wheel turns and now the outside air flows through, becoming supply air.

The air warms up, and the area of temperature rise shifts toward the end of the duct.

Before the end of the duct falls below the intake air temperature however, the wheel turns on and the

warm return air flows through it again.

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

in the counter-flow heat exchanger. 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 exchanger arises. While the other

devices presented above took 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.

6.8 Table of comparison

Here is a summary of a few important results.

Table: Comparison of various heat exchangers

Heat exchanger type

Cross-flow Cross-counter-

flow and counter-

flow

Rotary

Table: Air temperature and water in the form of water vapour

Air temperature

Water in the form of water v apour

0° C

4.4 g/m3

20° C

18 g/m3

30° C

33 g/m3

Efficiency achieved

70 %

95 %

95 %

Possibility of humidity exchange with suitable

membrane

with suitable

membrane

with suitable surface

tendency to freeze

small big, can be reduced

by moisture

exchange

big, can be reduced by

moisture exchange

Air tightness

good

good Not good due to moving

wheel, esp. for small

quantities of air

Hygienic separation of supply

and return air (important for

residential buildings)

good

good Can't be used everywhere

because air touches the

same surface

Pressure loss, energy

consumption of fans

low

mid to high

low

price

low

high

high

Note Ventilation equipment which recover heat from the return air by means of a heat pump can

admittedly use all the return air heat, however it requires a lot more electricity than devices with heat

exchangers.

These types of devices are treated in Module 2 of the course Comfort Ventilation.

6.9 Damp air

Air has the capacity to take on water vapour. The warmer the air is, the more water it can hold without

condensation forming.

If warm moist air is cooled, the water condenses out of it. This phenomenon is familiar from the wet

surface of a bottle of chilled soft drink in summer.

The change from liquid water to water vapour requires a lot of energy, which is termed heat of

vaporisation . For water this is 0.67kWh/kg. 1. This type of energy is called latent (Latin "hidden") because in the transformation from liquid to

vapour or vice versa, there is no change in temperature in the mixture. The heat (energy) is absorbed

or given off sole to change the state, from vapour to liquid and vice versa.

1. By contrast, heat used to cool dry air is termed perceptible sensible heat .

If moist air is cooled in a cooling system, then the latent heat (heat of condensation) adds to to

"sensible heat" of the dry air given off. These quantities of heat together are termed enthalpy .

What does this imply about ventilation? When it is warm outside (more than about 5° C) many

dwellings tend to have a problem with too much moisture rather than too little. In section 2.4 we

pointed out that another role of ventilation is to remove moisture. Otherwise problems such as mould

and mildew can arise in a room with excessive moistur (mould). The problem of dehumidification

exists almost the whole year round. Only when it is cold (colder than 0° C), the opposite problem may

arise: a lack of humidity.

6.10 Heat exchangers with humid air

If the humid return air from a dwelling is cooled by the incoming cold outside air in the heat exchanger

of a Comfort Ventilation system, then condensation from the return air will form toward the cooler end

of the heat exchanger.

Because the heat of condensation (latent heat) is large in comparison to the sensible heat

involved in cooling the dry air, the return air in a heat exchanger is not cooled by the same

temperature difference as the dry air is warmed. Humid air holds in more heat energy, and does not

cool as readily. This is due to the heat of condensation, which must be added to the heat due to the

change in temperature.

And that also happens:

Move the mouse over the red cirlces for more information.

In an occupied house with significant humidity, the effect described here increases the efficiency

of the heat exchanger . For the same temperature of return air, a higher supply air temperature can

be achieved.

Plate heat exchangers use membranes which do not allow moisture to penetrate, for example made

from polystyrol or aluminium. The condensation flows away. This is why heat exchangers need

a condensate drain .

6.11 Enthalpy exchanger

If however a water-permeable membrane is used for the plates, then the moisture transfers from the

warm-moist return air to the cold-dry outside air - this is called an enthalpy exchanger .

In an enthalpy exchanger, because the moisture no longer condenses from the return air, but is

transferred to the outside air, the temperature drop in the return air and temperature rise in the

outside air are equal.

This was shown on page 6.2 and is depicted in the small diagram here. There are also significant

benefits to the transfer of moisture.

Further below on this page we will go into more detail on the topic Applications of enthalpy

exchangers.

Paper and synthetic material is used for the water permeable plate material, for example materials

such as those marketed under the brandname Gore-Tex (familiar in apparel) or Tyvel as used in

building insulation.

Barrier for odours and microbes An additional requirement on enthalpy membranes is that they also

function as a barrier for odours and microbes, and be resistant to growth of moulds. And also the

material must be significantly cheaper than that used for clothing.

Rotary exchanger as enthalpy exchanger The plate enthalpy exchangers are distinct from the rotary

exchangers by the hygienic separation of return air and supply air. This was discussed above when

the various types of heat exchangers were introduced. Using special materials for the duct walls,

rotary exchangers can absorb moisture and release it again, and so also function as enthalpy

exchangers. The issue of the lack of hygienic separation however remains.

Applications of enthalpy exchangers

In winter, buildings are frequently too dry, because the cold outside air contains little water vapour and

too little water vapour is produced in the rooms inside. This particularly applies to office buildings and

apartment buildings with a low occupancy or where the occupants are often out. The enthalpy

exchanger can raise the humidity of air in the apartment through the recovery of part of the moisture

contained in the return air, without expending any extra energy. If the apartment is actively humidified -

with a humidifier - the enthalpy exchanger will reduce the energy demand, because less water vapour

will be needed to achieve the target humidity.

Danger of frost Within the enthalpy exchanger, the expelled air gives off vapour to the outside air

coming in and is thus dried, it will only freeze at comparatively lower outside temperatures. This

measures against freezing are redundant until the temperature reaches about minus 10° C, depending

on the humidity of the return air.

Usage in hot-moist climates In hot-moist climates the enthalpy exchanger is used in reverse: Inside

the building, the supply air does not need intensive drying because the enthalpy exchanger will have

transferred a part of the excessive moisture of the outside air to the return air coming from the

dwelling. The supply air is thus partially pre-dried with no extra energy involved.

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