Cross Flow Heat Recovery Exchangers Design Principles

Definition

They work according to the cross flow principle, with the help of plates, the cold and hot air are separated from each other and they provide heat transfer from the hot side to the cold side. Some plate type heat recovery exchangers also transmit moisture.

Applications

It is used for room and industrial ventilation, winter/summer heat recovery, separation of fresh and exhaust air, and utilization of exhaust air heat.

Design of Cross Flow Heat Recovery Exchangers

Mechanical Design

Plates
Aluminium, pre-coated aluminum (for corrosion resistance); plastic; resinous paper; materials such as stainless (for high temperature use) are used. There are different surface forms such as embossed, corrugated, etc.

Frame Covers
It is galvanized, aluminum or aluzink.

Edge Profiles
Chamfered, 900 cornered, protruding etc. to reduce the diagonal. In various forms such as aluminum extrusion profile or twisted alu sheet. Usually the profile is connected to the cover with screws.

Plate Edge Sealing
It is in the form of multi-layered edge clamping (sometimes with adhesive between the clamps).

Product Edge Sealing
Silicone-free adhesive etc. at standard temperatures (up to 900 -100°C). like; Fins and corner profiles are adhered at high temperatures (200-250°C) with adhesives such as high temperature silicone.

One Piece Designs
It ranges from 200 mm to 700 to 1200 mm.

Modular Designs
It consists of multiple uses of one-piece designs.

Choosing the appropriate edge length
According to the air flow, the product edge size can be selected from Figure 2. If the fresh air and exhaust air flow rates are not equal and the difference is large, parallel feeding can be applied.

Pressure Drop
For economic reasons, the pressure loss in the heat exchanger is taken between 150 and 250 Pa. The distance between the plates (pitch) varies between 2 and 6 mm for small products and between 4 and 12 mm for large products.

Velocity
The heat exchanger butt speed is selected according to the acceptable pressure loss level. Generally, manufacturers give forehead speed. The channel velocity within the plates is about 2 times the face velocity.

Heat Exchanger Length
It means that the length of the heat exchanger whose edge length and speed are determined is also determined. The maximum heat exchanger length is determined by the manufacturers based on the edge length. The curves of efficiency, pressure loss and capacity at different speeds and for different products are given below. In the curves, the product edge length (cm), heat exchanger length (cm), pitch (mm) and the ratio of the mass flow rate of expelled air to the mass flow rate of fresh air (if different from 1) are indicated in the product definition.

Efficiency increases when speed decreases, pitch decreases, size increases, mass ratio increases. The capacity increases when the speed increases, the pitch decreases, the product gets larger, and the mass ratio increases.

Efficiency
Efficiency in heating is around 50-65%. The type and thickness of the material has little effect on productivity. For this reason, plate heat exchangers are produced in many different materials. The effect of the surface form is important. The total efficiency of more than one plate heat exchanger connected in series can be calculated with the manufacturer’s software. Condensation on the hot air side in the heat exchanger causes an increase in the efficiency of the heat exchanger in winter due to the latent heat gained.

Dry yield (without condensation) is calculated as:
E2 = m2 (t22-t21)/ (mmin (t11-t21)
E: Efficiency
m2 : cold side mass flow rate (kg/h)
mmin: flow rate of the side with less mass flow (kg/h)
t: temperature °C 1: hot air 2: cold air

indices :

11: hot air inlet ; 12: hot air outlet
21: cold air inlet ; 22: cold air outlet

If there is condensation, wet bulb temperatures are taken. For higher efficiency, 2 or more plate heat exchangers can be arranged one after the other (series) as follows. Thus, for example, while the efficiency of the single heat exchanger is 50%, the total efficiency of the 2 heat exchangers connected in series is approximately 65%.

Other Specifications

Pressure Drop
Internal Pressure Drop
It is the difference between fresh and exhaust air. Different pressure differences occur depending on the position of the fans. Generally, the limit is 2000Pa -2500 Pa. In high temperature type this limit can be lowered. In the design of the system, the pressure difference should be checked at various points of the heat exchanger. If the internal pressure difference is high, the heat exchanger pressure drop will be more than the calculated one.

External Pressure Drop
It is the difference between the inside and outside of the heat exchanger. It is generally at the level of 2000-2500 Pa.

Leakage
It is of two types as external leakage and internal leakage between fresh and exhaust air. The fresh air side pressure is selected high because of the leakage between the return air and the fresh air.

Corrosion Resistance
The materials of the recovery exchanger such as fin, cover, edge profile and adhesive must be resistant to the conditions of the environment (atmosphere and fluid) used. Manufacturer’s recommendations should be taken into account in selecting the appropriate material.

Options
Standard type, hygienic, corrosion protected and high temperature types are available.

Accessories
Bypass, bypass damper, drip arrester for performance control

Temperature Limitations
The temperature limits of the sealants (adhesives), paint and pre-coated plate coating used should be taken into account when determining the temperature limits.

In standard practice: -40/10°C; -40/20°C in high temperature application

Mounting Position
Plates should generally not be mounted horizontally. Especially condensed water and dirt create a problem of accumulation on the plates. If it is desired to mount the plates in a horizontal position, the manufacturer must be notified in the order.

Moisture Conveying
In general, moisture transfer is not possible. This is sometimes an advantage in processes such as the drying process, swimming pool.

Condensation
The heat exchanger should be positioned so that the condensed water can be easily discharged. Return air intake is made from the top. If the air velocity in the heat exchanger is more than 2.5 m/s and there is too much moisture in the air, a drop trap is recommended to prevent the droplets from being transmitted to the duct.

Freezing Prevention, Control
The moisture condensed on the warm air side can be reduced to freezing temperature by the cold air. The area where the risk of freezing starts is called the “cold corner”. Freezing begins in the “cold corner”. The risk of freezing in the cold corner can be controlled with the manufacturer’s software.

To prevent freezing, full or partial fresh air bypass should be done (the amount of fresh air passing through the heat exchanger should be reduced) or defrost should be added. In addition, melting defrost water should be allowed to flow. Or, fresh air flow from the cold corner can be prevented to prevent freezing, the fresh air can be completely bypassed temporarily, or the fresh air is heated so that “cold corner” does not occur.

The heat exchanger is not damaged by freezing, but its performance is affected.

Performance Control

Fresh Air Bypass
Some of the fresh air is bypassed without passing through the heat exchanger. Thus, freezing in the heat exchanger is prevented due to the cooling of the expelled air.

Return Air Bypass
Some of the return air is bypassed without passing through the heat exchanger. It is applied by completely bypassing it, especially in cases where the exhausted air is polluted. In hygienic applications, the mixing of return air with fresh air should be minimized.

Noise Reduction
Plate heat exchangers often have sound reduction effects, depending on the plate size and pitch.

TRANSPORT

The heat exchangers should be lifted and transported as recommended by the manufacturer, in such a way that the plates and corner profiles do not deform, preventing the adhesives from opening.

ASSEMBLY

The system should be made as recommended by the heat exchanger manufacturer.

Drain Connection
Fresh air and exhaust air should be considered separately.

Temperature Sensor
The plates must not be damaged during the connection.

Onsite Assembly
If more than one heat exchanger is to be used as a package, suitable carriers should be used and impermeability should be ensured.

MAINTENANCE

Over time, especially when there is a high amount of dirt and condensation, and deposits can build up on the fins. Since heat affects the transfer, it should be cleaned regularly. The heat exchanger can be cleaned with a brush or vacuum, washed with hot water or cleaning agents approved by the manufacturer, with cleaning detergents suitable for aluminum.