The core of heat recovery is the heat exchanger. 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 into the house continuously and an equal volume of stale air simultaneously flows out of the home. These air flows 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. For more info, visit ZehnderAmerica.com

1. Understanding Heat Exchangers- Cross-flow, Counterflow (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

2. •
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? W ould 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. W e 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

3. 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

4. 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.

5. 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. 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.

7. 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.

8. 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 % .

9. 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

10. 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.

11. 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.

12. 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.

13. 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. W hile 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-counterflow and counterflow
Rotary

14. Efficiency achieved
70 %
Possibility of humidity exchange with suitable
95 %
95 %
with suitable
with suitable surface
membrane
small
tendency to freeze
membrane
big, can be reduced big, can be reduced by
by moisture
moisture exchange
exchange
Air tightness
good
good
Not good due to moving
wheel, esp. for small
quantities of air
Hygienic separation of supply
good
good
Can't be used everywhere
because air touches the
and return air (important for
same surface
residential buildings)
low
mid to high
low
low
Pressure loss, energy
high
high
consumption of fans
price
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.
Table: Air temperature and water in the form of water vapour
Air temperature
Water in the form of w ater v apour
0° C
4.4 g/m 3
20° C
18 g/m 3
30° C
33 g/m 3

15. 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 0.67kW h/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? W hen 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.

16. 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 .

17. 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 W ithin 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

18. 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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