Air Handling Unit
Elements
Heat Recovery
Department of Building Service and Process Engineering
Balázs Bokor, PhD student
Building ”D”, Room 124 a
Tel: 463-2535; e-mail: [email protected]
INTRODUCTION
Source: genvex.co.uk
Why to apply heat recovery units?
• They make mechanical
ventilation more cost
effective by reclaiming
energy from exhaust
airflows.
• They use heat
exchangers to heat or
cool incoming fresh air,
recapturing 40 to 85 %
of the conditioned
temperatures that
would otherwise be
lost.
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INTRODUCTION
How does heat recovery work?
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CLASSIFICATION
HEAT EXCHANGERS
RECUPERATOR
REGENERATOR
• The fluid streams are
separated by a thin wall,
which serves as heat
transfer surface.
• Heat from hot fluid is
intermittently stored in
thermal storage medium
before being transmitted
to cold fluid.
• The fluid streams enter
the recuperator at the
same time.
• The fluid streams enter
the regenerator one
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after the other.
CLASSIFICATION
HEAT EXCHANGERS
RECUPERATOR
REGENERATOR
• Fixed-plate cross-flow
heat exchanger
• Rotary heat exchanger
• Run around coil
technology
• Regenerative heat
storage package
• Heat pipe
• Heat pump
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CLASSIFICATION
HEAT EXCHANGERS
Air to air heat transfer
Use of heat transfer medium
• Fixed-plate cross-flow
heat exchanger
• Run around coil
technology
• Rotary heat exchanger
• Heat pipe
• Regenerative heat
storage package
• Heat pump
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RECUPERATORS
Fixed-Plate Cross-Flow Heat Exchangers
7
Source: hallenklimatisierung.com
RECUPERATORS
Fixed-Plate Cross-Flow Heat Exchangers
• Two air-streams are separated by several aluminium plates
• Air-flow through several parallel passages
• The distance between the plates is provided by spacers
8
Source: klingenburg.de
RECUPERATORS
Fixed-Plate Cross-flow Heat Exchangers
• Some applications require complete separation of supply and
extract air
• No cross-leakage is acceptable in the heat recovery unit
• Double-folded plate edges provide for air-tightness
9
Source: klingenburg.de
RECUPERATORS
Counterflow Plate Heat Exchangers
Advantages
• No moving parts
• High sensible effectiveness
>80%
• Little cross-leakage between
air streams
• Easy to clean
Disadvantages
• Requires adjacent air
streams
• Requires very careful
transportation
• High first cost in large
applications
• Special membrane is needed
for moisture recovery
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RECUPERATORS
Capacity Control of Counterflow Plate Heat Exchangers
• Face and bypass
dampers
• If face dampers closed
and bypass dampers
open → reduced airflow
through heat exchanger
• With the use of dampers
the overheating of the
supply air can be
avoided (when there’s
significant surplus heat
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in the building)
RECUPERATORS
Counter-flow Plate Heat Exchangers
• Additional effective heat transfer surface arranged properly
into counter-flow patterns can increase heat transfer
effectiveness.
12
Source: tps-imp.si, klingenburg.de
RECUPERATORS
Special Coatings
• Under circumstances where air have a highly corrosive effect
on materials, epoxy-coated heat exchangers are used
• Swimming pools, seaside locations, special industrial appl.
13
Source: klingenburg.de
RECUPERATORS
Frost
• Frost is most likely
to develop in the
cold corner
• In this corner
exhaust air is in
contact with the
coldest surface of
the heat exchanger
• Defrosting must be
made possible (only
warm exhaust air
through HRU) 14
RECUPERATORS
Heat Pipe Technology
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Source: enerton-inc.com
RECUPERATORS
The Heat Pipe’s Principle of Operation
• Hot air flowing over the evaporator end of the heat pipe
vaporises the working fluid.
• The vapour pressure gradient drives the vapour to the
condenser end of the heat pipe.
• Vapour condenses releasing the latent energy of
vaporisation.
• The condensed fluid flows back to the evaporator where it is
being re-vaporised, thus completing the cycle.
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RECUPERATORS
Heat Pipe Technology
• Passive energy recovery device
• Pipe heat exchanger divided into
evaporator and condenser by a
partition plate
• Changing the slope (tilt) of the
heat pipe controls the amount of
heat it transfers
• The heat pipe itself is a sensitive
device
• Effectiveness of heat recovery:
50-60 %
• Relatively rare in building service
systems
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RECUPERATORS
Heat Pipe Technology
Advantages
Disadvantages
• Little cross-leakage between
air streams
• Requires adjacent air
streams
• Relatively low maintenance
• Contains refrigerant
• Can be packaged inside air
handler
• Transfers only sensible heat
• Vacuum technology is
required for the production of
the heat pipe
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RECUPERATORS
Coil Loop Technology
• Two or more finned tube coils are
piped together in a closed loop
• A small pump circulates the working
fluid through the two coils
• Working fluid: a solution of inhibited
glycol and water
• Modulating capacity (three-way mixing
valve or a variable-speed drive on the
pump)
• The most flexible energy transfer
between air streams that are physically
separated by some distance
• Can recover energy from multiple
exhaust streams (using multiole
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exhaust-side coils)
Source: renewableenergyhub.co.uk
RECUPERATORS
Performance Control of Coil Loop Heat Recovery
• Sensible effectiveness 45-65%,
balanced airflow
• Airside static pressure loss of 75-250
Pa per coil
• Adding more rows and fins to the coils
– Increases the sensible effectiveness of the
coil loop
– The fan consumes more energy
• Net energy saved, but additional fan
and pump energy necessary
• Higher fluid flow rate
– Increases sens. Effectiveness of the coil
– Increases energy consumption of the pump
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Source: renewableenergyhub.co.uk
RECUPERATORS
Coil Loop Technology
Advantages
• Transfers energy between air
streams that are separated in
distance
• No cross-leakage between air
streamsFlexible design: coils can
be selected for the optimum
amount of energy transfer
Disadvantages
• Transfers only sensible heat
• May require an expansion tank to
accomodate expansion and
contraction of heat-transfer fluid
• Requires design and fields of
installation for piping, pump, exp.
tank, and mixing valve
• Easily turned off when energy
recovery in not beneficial
• Fits readily within the casing of a
packaged air handler
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REGENERATORS
Rotary Heat Exchangers
Revolving cylinder filled with airpermeable medium having large internal
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surface area.
Source: www.rgs-service.de
REGENERATORS
Rotary Heat Exchangers
• Adjacent supply and exhaust
airstreams each flow through onehalf the exchanger in counter-flow
pattern.
• Heat transfer medium (permeable
structure) gains and loses heat
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periodically
Source: www.lakasszelloztetes.hu
REGENERATORS
Rotary Heat Exchangers
Transfer of sensible heat
• The medium picks up and stores heat
from the warm air stream and releases
it to the cold one.
Transfer of latent heat
• The medium condenses moisture from
the airstream with higher absolute
humidity.
• The medium releases moisture through
evaporation into the air stream with
lower absolute humidity.
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REGENERATORS
Rotary Heat Exchangers: Cross-Contamination
Carryover: air entrained within the volume of the rotation medium is carried
into the other airstream.
Leakage: differential static pressure across two airstreams drives air from
higher to lower static pressure region.
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REGENERATORS
Cross-Contamination of Rotary Heat Exchangers: Purge Section
The purge section is installed to reduce cross-contamination. In this a small
fan „washes” through the rotor with fresh air to get rid of contaminations in
the rotor’s structure, so that these cannot end up in the supply airstream.
The purge section’s efficiency can be measured by a tracer gas experiment.
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REGENERATORS
Renovation of Rotary Heat Exchangers
Dirt
• Higher resistance, higher
pressure drop
Mechanical damage
• Leakage
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Source: www.lautner.eu
REGENERATORS
Regulation of Heat Recovery with Rotary Heat Exchangers
• Supply air bypass control
• Varying wheel rotational speed
• Exhaust air bypass control
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REGENERATORS
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Source: www.tab.de
Rotary Heat Exchangers for Large Scale Applications
REGENERATORS
Rotary Heat Exchangers
Advantages
• High effectiveness ~85%
• Total energy wheels transfer
both sensible heat and
moisture
Disadvantages
• Requires adjacent air
streams
• May permit leakage between
air streams
• Belt, motor and bearings
require periodic maintenance
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REGENERATORS
Rotary Heat Exchangers for Data Processing Centres
• Extract air (35°C) streams through one half of the rotor, where it cools down
to ~22°C before being returned to the computer room.
• Cold outside air streams through the other half of the rotor in counter-flow
pattern. It warms up and then it is led to the outside.
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Source: klingenburg.de
REGENERATORS
Rotary Heat Exchangers for Data Processing Centres
Advantages
• Only air is used for
the cooling
process
• No water needed
in the highly
sensitive computer
room
• Compared to fixedplate heat
exchanger lower
pressure loss and
higher efficiency
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Source: klingenburg.de
SOIL HEAT EXCHANGER
Air Preheating or Precooling Using Ground Energy
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Source: rehau.com
SOIL HEAT EXCHANGER
Monitored System: Office Building in Erlangen (Germany)
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Source: rehau.com
SOIL HEAT EXCHANGER
Monitored System: Erlangen (Germany) 1/1/2013 - 31/12/2013
TEMPERATURE
RELATIVE HUMIDITY
VOLUME FLOW
TRANSFERRED HEAT
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Source: emt.rehau.com
SOIL HEAT EXCHANGER
Laying Methods: Loop around the Building
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Source: heinze.de, rehau.com
SOIL HEAT EXCHANGER
Laying Methods: Tichelman-Switch
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Source: heinze.de
THANK YOU
FOR
YOUR ATTENTION
Department for Building Service and Process Engineering
Balázs Bokor, PhD student
Building ”D”, Room 124 a
Tel: 463-2535; e-mail: [email protected]
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