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Air-to-Air Heat Exchangers for Air Handling Units: Fundamentals and Types

By AHUmag | on March 18, 2015 |

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Air-to-Air Heat Exchangers for Air Handling Units: Fundamentals and Types

Nowadays the Indoor Air Quality (IAQ) in buildings is a topic increasingly

taken into account in the design of HVAC systems.

With the need to provide to the buildings large amounts of outdoor air (to

comply with local legislations for air changes rates related with IAQ

requirements), became essential to recover energy form the exhaust air in

the Air Handling Units, that had previously been treated as fresh air

(heated/cooled) in order to reach the indoor comfort setpoint temperature.

Using heat recovery systems it is possible to reduce the energy

consumption used to heat or cool the fresh air.

The average payback time for this kind of components, depending of

factors like the design air conditions (outdoor and indoor), pressure drop

of the system (extra electrical consumption for the fans) and the

characteristic efficiency of each model and size of heat exchangers,

typically is between less than 1 year and 2 years.

Among all types of heat recovery systems the most used in Air Handling

Units are the air-to-air heat exchangers.

The air-to-air heat exchangers can be used for heat and/or moisture

recovery.

Fundamentals

The operating principle of air-to-air heat exchangers (or air-to-air heat

recovery systems) consists of exchange temperature and/or humidity

between the exhaust and supply airflows inside the Air Handling Units. This

way the thermal energy contained in the exhaust air is recovered for the

supply air and thus the load of energy required to

heat/cool/humidify/dehumidify the air is reduced, in more than 50%.

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Before the introduction of air-to-air heat exchangers in the AHUs (and the

HVAC systems) the exhaust conditioned air was simply discharged directly

to the atmosphere.

How to calculate the Sensible Heat Recovery Efficiency...

Sensible Effectiveness (ASHRAE 84) and Sensible Temperature Ratio

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Types

There are several types of air-to-air heat exchangers with mean recovery

efficiencies from 50% up to 90%, depending the type of heat exchanger

and the outdoor and indoor air conditions (temperatures and humidity).

From all types of air-to-air heat exchangers the most used in Air Handling

Units applications are:

Rotary Heat Exchangers

The rotary heat exchangers is one of the most efficient air-to-air heat

exchangers and are able to handle large airflows.

The core of this heat exchanger is the rotor, or wheel, made from

aluminium foils. Half of the rotor is in contact with the exhaust airflow

while the other half contacts the supply airflow. The wheel is driven by

means of a motor and a belt, rotating the wheel and transport the heat

recovered in the half wheel from the exhaust airflow to the supply airflow.

If the right conditions are achieved, outdoor and indoor temperatures and

humidity, the efficiency claimed by manufacturers for a rotary heat

exchanger can reach up to 90%.

The size of the rotors can go from small diameters size (domestic

applications) to huge diameters for industrial applications (5 meters).

There are several types of rotors, for different applications, that are able to

recover temperature and/or humidity. Depending the coating applied to

the wheel they can be used also to drying and humidification. Wheels used

to dry the air are also called as desiccant wheels.

Usually this kind of heat exchanger is not used in applications that

demands zero contamination between the exhaust airflow and the supply

airflow.

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Air-to-Air Rotary Heat Exchanger for Air Handling Units. Click to

zoom.

Plate Heat Exchangers

The plate heat exchangers are the most used solution amoung the ait-to-air

heat exchangers. This kind of solution offers high leak tightness which

means that the percentage of leakage from the exhaust airflow to the

supply airflow is near zero.

There are two types of plate heat exchangers: Counter Flow Plate Heat

Exchangers and Cross Flow Plate Heat Exchangers.

Air-to-Air Counter Flow Heat Exchanger for Air Handling Units. Click

to zoom.



Air-to-Air Cross Flow Heat Exchanger for Air Handling Units. Click to

zoom.

As the name itself suggests the design of the cross flow plate heat

exchanger forces the exhaust airflow and the supply airflow to crosses

between them. If comparing with the counter flow heat exchanger, where

the exhaust airflow and the supply airflow have a opposite path (counter),

the cross flow heat exchanger for the same conditions have lower

efficiency. Opposite airflows can maximize the heat energy exchange.

In the case of counter flow heat exchangers with the right conditions,

indoor and outdoor, manufacturers claim 85%-90% efficiency. This value

can reach to around 70% for the cross flow heat exchangers.

This lower efficiency, of the cross flow heat exchanger, is compensate with

the possibility to work with a wider range of airflows (due to his structural

design can be larger in size, due to modular construction).

Moreover the cross flow heat exchanger offer more options of spacing

between the plates than the counter flow heat exchanger, so for industrial

applications the pressure loss and the heavy contamination of the air will

play an important role to choose the cross flow heat exchanger as the

preferred option.

The standard material used to produce the plates is aluminium, but they

can also be manufactured in plastic and aluminium with epoxy coating for

special application, for example processes with high temperatures or

corrosive atmospheres like swimming pools.

Run Around Coils

Such as the heat recovery systems detailed in the previously points, the

Run Around Coils recover the heat from the exhaust airflow and transfer it

to the supply airflow.

Typically this system is composed by two water coils, connected each other

by pipework and a pump that circulates the water from one coil to the

other, among all the other accessories as expansion vessel, controls, etc.

The water, most of the times, have a percentage of glycol for anti-freeze

protection.

The heat recovered in the exhaust coil is transferred by the circulating

water to the supply coil that will then transfer that heat to the supply

airflow.

This system usually is used in installations where the exhaust airflow and

the supply airflow are split (one supply AHU and one exhaust AHU) or if

there is a demand of 0,0% leak between the two airflows, due to air



contamination issues.

In this kind of system the usual heat recovery efficiencies claimed by

manufacturers can go up to 60% (up to 75% in high efficiency option).

The run around coils only recover sensible temperature.

Air-to-Air Run Around Coils Heat Exchanger for Air Handling Units.

Click to zoom.

Heat Pipes

Typically this system is composed by one large coil, commun to the exhaust

and the supply airflows with a division between the two airflows, as two

coils conected by the pipework. The tubes of the coil are sealed and

charged with a refrigerant (one of the most used is the R134a). In this

system there is no need of any energy input, as well as in the plate heat

exchangers.

The exhaust warm air makes the gas inside the tubes to evaporate, and

that vapor moves to the cooler section of the heat pipes system. The vapor

when reach the part in the cold airflow of the tubes will condensate and

return to the warm airstram part again by gravity.

The average efficiencies claimed by manufacturers for this systems are up

to 75%.

When instaled in the vertical configuration (double-deck) this system only

work perfectly in one of the seasons (Winter or Summer), depending if the

exhaust airflow will be located in the upper deck or in the bottom deck.



Air-to-Air Heat Pipes Heat Exchanger for Air Handling Units. Click to

zoom.

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