Hoval A 60, A 370, A 250, A 750 Handbook For Design, Installation And Operation

Rotary heat exchangers

for Heat Recovery in Ventilation Systems

Handbook for Design, Installation and Operation

Drive motor

The 3-phase gear motor with belt pulley
and v-belt is installed on a rocker in
the corner of the casing. The speed

of rotation is innitely adjustable.

Peripheral slide seal

Constant-force springs permanently press
the abrasion-resistant ring seal against the
casing. The patented system permanently
minimises leakage and allows the unit

to be sized for smaller air ow rates.

Adjustable purge sector

The size of the purge sector can be adjusted

to suit requirements. The device (patent
pending) prevents contamination of the
supply air by the extract air and at the same
time minimises purge and energy loss.

Storage mass

Hoval supplies the storage mass in three types of
material: for condensation, enthalpy and sorption
wheels. The sorption coating guarantees a consist-

ently high degree of humidity efciency, even under

summer conditions.

1

1 Principle and Operation __________ 2

1.1 Heat transmission

1.2 Humidity transmission

1.3 Leakage of rotary heat exchangers

1.4 Frost limit

1.5 Temperature efciency

1.6 Pressure drop

1.7 Pressure difference

1.8 Hygiene

1.9 Reliable data

2 Performance control ____________ 7

3 Structure _____________________ 8

3.1 Wheel

3.2 Casing

3.3 Peripheral slide seal

3.4 Transverse seal

3.5 Drive

4 Options _____________________ 11

4.1 Drive

4.2 Control unit

4.3 Operating unit

4.4 Rotational speed monitoring

4.5 Inspection cover

4.6 Purge sector

4.7 Duct design

4.8 Coated casing

4.9 Offset wheel position

5 Dimensions of the exchangers ___ 15

6 Unit type reference ____________ 16

7 System design ________________ 18

7.1 Hoval CASER design program

7.2 Design data

7.3 Local conditions, installation position

7.4 Wheel type

7.5 Performance control

7.6 Using and setting the purge sector

7.7 Mixing of the air streams

7.8 Supply air humidication

7.9 Corrosion

7.10 Application limits

7.11 Danger or contamination

7.12 Condensation in the warm air stream

8 Transport and installation _______ 21

8.1 Transport

8.2 Mechanical installation

8.3 Installation of sensors

8.4 Electrical installation

8.5  Assembly of segmented rotary heat exchangers

8.6 Storage

9 Commissioning and maintenance _ 22

9.1 Commissioning

9.2 Maintenance

10 Specication texts ____________ 23

10.1 Condensation wheel

10.2 Enthalpy wheel

10.3 Sorption wheel

Content

2

1 Principle and Operation

Hoval rotary heat exchangers are regenerators with rotating
heat accumulators (category 3) in accordance with the guide-

lines for heat recovery (e.g. VDI 2071).
The heat-dissipating and heat-absorbing air ows heat

or cool the rotating, air-permeable storage accumulator.

Depending on the air conditions and the surface of the

accumulator material, humidity may also be transferred in the

process. Supply and exhaust air must therefore be brought
together and ow through the heat exchanger.

The storage mass consists of triangular, axially arranged
small ducts made of thin metal foil. The depth of the storage

mass (viewed in the direction air ow) is generally 200 mm;

the airway height is normally 1.4 – 1.9 mm, depending on the

application. With these dimensions the storage mass gener­ates a laminar ow in the wheel ducts.

Fresh air

t

21

x

21

Supply air

t

22

x

22

Exhaust air

t

12

x

12

Extract air

t

11

x

11

Fig. 1: Function diagram and air conditions

Denition of key data according to Eurovent

Temperature efciency

t

22

 -  t

21

  ηt  =  

t

11

 - t

21

Humidity efciency

x

22

 - x

21

  ηx  =  

x

11

 - x

21

Legend: t = Temperature [K; °C]

x = Absolute humidity [g/kg]

Index: …

11

Extract air

…

21

Fresh air

…

12

Exhaust air

…

22

Supply air

1.1 Heat transmission

The wheel with its axially arranged, smooth ducts acts as a
storage mass, half of which is heated by the warm air and

the other half of which is cooled by the counter-ow of cold

air. The temperature of the storage mass therefore depends
on the axis coordinates (wheel depth) and the angle of
rotation.
The function is easy to understand by following the status

of a wheel duct through one revolution (see Fig. 3). The

following can be recognised with reference to the heat
transfer from this process:

■ The air temperature after the exchanger varies; it depends

on the location on the wheel.

■ The heat recovery efciency can be varied by varying the

speed.

■ The heat recovery efciency can be changed with the

storage mass. This can be done with different cross-sec­tions of the wheel ducts, different thickness of the storage
material or by changing the wheel depth. However, in all
cases this varies the pressure drop.

■ The specic heat output depends on the temperature

difference between warm air and cold air. The rotary heat
exchanger is therefore suitable for heat and cool recovery,
i.e. for winter and summer operation.

Fig. 2: Geometry of
storage mass

Fig. 3: States depending on the turning angle

Principle and Operation

3

Fig. 3: States depending on the turning angle

Warm air entry

The rotation of the storage mass has
moved the wheel duct from the cold air
to the warm air. The storage material is
cooled almost to the temperature of the
cold air. This applies particularly to the
entry side of the cold air (= exit side of the

warm air). The warm air now ows through

the duct with reference to the temperature

in the counter-ow and is cooled greatly.

The storage mass is therefore heated. The

local temperature efciency, i.e. directly
at the inlet to the warm air, is very high.

Condensation can also occur very easily.

Mid warm air

The wheel duct now has passed half of
its time in the warm air. The storage mass

has been heated by the owing warm air;
therefore, the warm air is not cooled down

as much as in entry inlet zone. The wall
temperature at the entry and exit is approxi­mately the same. Condensation occurs only
with large humidity differences.

 Warm air exit

The wheel duct is now shortly before entry
to the cold air. It has virtually reached the
temperature of the extract air at the entry
side. The transferred performance is still
only low.
The dwell time in the warm air and in the
cold air, i.e. the speed of rotation, is deci­sive for the performance of the rotary heat
exchanger. It depends on the storage mass

(thickness, geometry), the heat transfer and

the air velocity.

Cold air exit

The wheel duct has passed through the
cold-air section. The storage mass has

greatly cooled, almost down to the cold-air

temperature in the entry section. After

crossover to the warm air side, the cycle

starts anew.

Mid cold air

Half of the dwell time in the cold air is past.
The storage mass has already cooled

signicantly. The temperatures at the entry

and exit are approximately equal.

 Cold air entry

After the transition from the warm air to the

cold air, the wheel duct now has cold air
owing through in the opposite direction

(referring to the temperature). With the
high temperature difference the transferred

performance is very high, i.e. the cold air is
very strongly heated; in reverse the storage

mass is strongly cooled. Any conden­sate formed on the exchanger surface is
(partially) absorbed by the heated cold air.

W

A

R

M

A

I

R

C

O

L

D

A

I

R

Principle and Operation

4

1.2 Humidity transmission

In addition to heat, humidity can also be transported with

rotary heat exchangers. The decisive factor here is the mate-

rial and/or the surface of the storage mass. Characteristic

features for different designs have been developed with
detailed measurements of wheels from different manufac-

turers by the building technology test centre of the University
of Lucerne. The reference factor for the humidity efciency
is the condensation potential; that is the humidity difference

between warm-air humidity and the saturation humidity of the

cold air (see Fig. 4).

Fig. 4: Denition of condensation potential κ

Sorption wheel

Enthalpy wheel

Condensation wheel

Warm air entry

Cold air entry

Saturated cold air

Condensation potential of

warm air κ

Humidity efciency η

x

0

0.2

0.4

0.6

0.8

0.9

1.0

0.7

0.5

0.3

0.1

-4 -2 0 2 4 6 8 10

Condensation potential κ [g/kg]

The following must be noted:

■ The greater the condensation potential the greater the

volume of condensate that can be expected at the warm
air side.

■ If the condensation potential is zero or negative, no

condensation can take place. Humidity transmission is
therefore only possible by sorption.

■ The derived characteristics reect typical values of 1 : 1

for the mass-ow ratio and the pressure drop of approx.
130 Pa at an airway height of 1.9 mm.

■ The area of application of reference magnitude κ, i.e. the

condensation potential, is restricted to the standard condi-

tions of ventilation technology. The temperature efciency

must be at least 70 %. The humidity transmission must
not be restricted by the saturation curve (e.g. with very
low outside temperatures).

Fig. 5: Typical course of humidity efciencies of various
wheels depending on the condensation potential

Temperature

WaterRelative humidity

Principle and Operation

5

There are 3 different designs:

Condensation wheel

The storage mass consists of smooth, untreated aluminium,
which only transmits humidity if condensation occurs on the
warm-air side and it is picked up by the cold air (partially).

Humidity efciency rates greater than 80 % can be reached if

the temperature difference is high.
The use of condensation wheels for heat and humidity trans­mission is recommended primarily for ventilation systems
without mechanical cooling, i.e. for winter operation.

Enthalpy wheel (hygroscopic wheel)

The metallic storage mass has been treated to form a capil­lary surface structure. The humidity is transmitted by sorption
and condensation, with the sorption component being very

low. Humidity transmission in summer operation (κ < 0) is

also very low.

Sorption wheel

The storage mass in this case has a surface that transmits
humidity by pure sorption (i.e. without condensation). The

humidity efciency is therefore virtually independent of the

condensation potential. The low decrease can be explained
with the simultaneous reduction of the temperature differ­ence.

Sorption wheels are recommended particularly in systems
with mechanical cooling. The high humidity efciency, even

under summer conditions, dries the fresh air. This requires
less cooling capacity and reduces energy costs for cooling
up to 50%.

1.3 Leakage of rotary heat exchangers

Rotary heat exchangers transfer heat and humidity via a

rotating storage mass that alternates between the exhaust

air and supply air ows. This functional principle delivers
extremely efcient energy recovery, but it does also entail a
certain leakage: the exhaust air and supply air ows cannot

be completely separated from one another. The seals are
not able to withstand the existing differential pressure with
100 percent effectiveness. The rotating storage mass trans-

fers a small quantity of air from one air ow to the other on

every rotation (carryover).
The effects of the leakage must be taken into account during

planning and conguration of air handling systems. The draft
standard EN 13779:2014 consequently denes the calcula­tion method for the leakage. It describes the following two

values:

■ Exhaust air transfer ratio EATR

This is the quantity of exhaust air that enters the supply
air due to carryover and seal leakage.

■ Outdoor air correction factor OACF

This is the ratio between the quantity of the fresh air and

supply air ows.

These two values are calculated using the design program

for a differential pressure to be specied between the supply
air and extract air (Δp

22 -11

). From April 2015, this calcula-

tion will be mandatory for Eurovent-certied rotary heat

exchangers.

Based on the calculated leakage values, it is possible to take
suitable measures according to the application. The following
must be noted:

■ The transfer from exhaust air to supply air can be signif-

icantly reduced or even completely eliminated by taking
the following measures:

– Using a purge sector
– Suitable arrangement of fans (supply air pushes,

exhaust air sucks)

■ The OACF value is decisive for setting the dimensions of

the fans:

– An OACF value greater than 1 means that fresh air

gets to the exhaust air side (due to seal leakage and/

or purge air). The size of the supply air fan will have to
be increased accordingly to ensure that the required
air volume is supplied to the building. This means more
energy is required for pumping the air.

– An OACF value less than 1 means air is moving in the

opposite direction, i.e. there is a proportion of recircu­lated air in the supply air.

Denition of leakage according to EN 13779:2014 (draft)

Exhaust air transfer ratio:

a

22

– a

21

EATR =

a

11

(Exhaust Air Transfer Ratio)

a

22

....... Concentration in supply air

a

21

....... Concentration in fresh air

a

11

....... Concentration in extract air

Outdoor air correction factor:

q

m 21

OACF =

q

m 22

(Outdoor Air Correction Factor)

q

m 21

..... Mass ow of fresh air

q

m 22

..... Mass ow of supply air

Principle and Operation

6

1.4 Frost limit

If the warm extract air stream is very strongly cooled conden­sate can be formed and it may even freeze. The fresh air
temperature at which this starts is referred to as the frost
limit.

■ Condensation wheel, enthalpy wheel: The condensate

generated by cooling the extract air may freeze at low
outside temperatures. There is a frost hazard at equiva-

lent mass ows for exhaust air and fresh air if the average
inlet temperature of the two air streams is less than 5 °C.

t

m

 = 

t11 + t

21

2

 < 5 °C

■ Sorption wheel: The gaseous humidity transmission by

sorption generally prevents condensation; the frost hazard

is reduced.

1.5 Temperature efficiency

Appropriate design and serial layout allows virtually any
temperature efciency to be reached. The 'correct' temper­ature efciency depends on the applicable regulations and

the economy calculations, i.e. the operating data such as
energy price, service life, operation time, temperatures,

maintenance requirements, interest etc. Even minor changes
(a few percent lower temperature efciency, a few pascals
more pressure drop) can mean signicantly poorer results for

capital value and amortisation period.

1.6 Pressure drop

Heat recovery units cause pressure drop on the extract

and supply air sides and as a result operating costs. With

current general conditions the economical values for wheels

are between 80 Pa and 130 Pa. However, to reduce costs,

more and more heat recovery units whose pressure drops
are above these economically reasonable values are being
installed. This affects the feasibility of the system.

1.7 Pressure difference

A distinction is made between internal pressure difference

(between exhaust air and supply air) and external pressure
difference (between the exchanger and the environment).

Internal pressure difference:

The internal leakage between the two air streams depends
greatly on the pressure difference. Hoval rotary heat
exchangers with high tightness seal compared with other
designs are certainly very leak-proof, but the following infor­mation should be taken into account in the design:

■ The pressure difference in the rotary heat exchanger

should be as low as possible.

■ In applications that involve the danger of odours the pres-

sure gradients and therefore possible leakage from the
fresh air to the exhaust air must be considered.

However, the internal pressure difference may also cause

deformation of the casing; a pressure difference of more than
2000 Pa is not permitted.

Notice

The pressure difference depends on the layout of the

fans. Overpressure on one side and underpressure

on the other side add up.

External pressure difference:

This is a major factor for the external leakage of the heat
exchanger. If a duct system is correctly and carefully

installed, this effect can be ignored.

1.8 Hygiene

Hoval rotary heat exchangers with high tightness seal have
been tested for conformity with hygiene requirements at the

Institute for Air Hygiene in Berlin. The test criteria were the

requirements relevant to hygiene for applications in general

building ventilation and in hospital applications. All hygiene

requirements were met.

Notice

Hoval rotary heat exchangers are tested and certi-

ed for operation in hospitals in accordance with
DIN 1946-4. Install rotary heat exchangers with the
'coated casing' option for such applications.

Fig. 6: Certicate of
hygiene conformity test

(valid for Hoval rotary

heat exchangers with high

tightness seal)

Principle and Operation

7

1.9 Reliable data

Hoval rotary heat exchangers are always tested by inde­pendent test organisations (e.g. at the building technology

testing laboratory of the University of Lucerne). All technical

data are based on these measurements. This means that
they are reliable data for planners, installers and operators.

Relative humidity efciency

0

5

10 15 20 25

0 %

20 %

40 %

60 %

80 %

100 %

Speed of rotation [rpm]

Fig. 8: Dependency of the humidity efciency on the rotational speed

Relative temperature efciency

0

5

10 15 20 25

0 %

20 %

40 %

60 %

80 %

100 %

Speed of rotation [rpm]

Fig. 7: Dependency of the temperature efciency on the rotational speed

2 Performance control

The Hoval rotary heat exchanger always operates as a

temperature rectier between the two air streams. The
ow direction of the heat is irrelevant in this context, i.e.

depending on the temperature gradients between extract
air and fresh air either heat or cold is harvested. Therefore,
regulation of the output of the Hoval rotary heat exchanger
is not necessary if the extract air temperature is identical to

the setpoint temperature. In this case, the fresh air is always

either heated or cooled in the direction of the set temperature
by the heat exchanger.
However, in most cases there are heat sources in the
ventilated rooms (people, machines, lighting, solar radiation,
processing systems) that increase the room temperature,
i.e. the extract air temperature is higher than the setpoint

temperature. In this case, check the outside temperature

from which the system is heated at full performance of the
rotary heat exchanger and – if this cannot be tolerated – the
performance of the heat exchanger must be controlled.

It is very simple and economical to reduce the performance

of the rotary heat exchanger for heating and also for humidity

transmission by reducing the speed of rotation. All Hoval

rotary heat exchangers can therefore be supplied with
speed-controlled drives.
There is also the option of diverting one or both air streams
past the wheel by a bypass. The method – used primarily in

process technology and at various air ow rates – must be

installed by the customer.

Performance control