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Flat Plate Crossflow

Purpose

The purpose of this document is to provide general information typically not found in product guides about air-to-air, cross-flow, flat-plate heat exchangers. Topics covered include condensation, fully developed air flows, freezing and frost control, leakage, pressure differentials, and dos and don’ts.

Introduction

In flat-plate heat exchangers, two airstreams cross by each other. In typical HVAC
applications:

  • One of these airstreams consists of air being removed from the ventilated space. The part of this airstream upstream of the exchanger is commonly called “return air.” When it emerges from the opposite end of the exchanger, is called “exhaust air” and is typically discharged outdoors.
  • The other airstream is the fresh outdoor air being supplied to the ventilated space. The air in this airstream upstream of the exchanger is called “outdoor air.” On the opposite end of the exchanger, the fresh air flowing out is called “supply air” because it is supplied to the ventilated space.

The flat plates can be made of various materials (e.g. aluminum, plastic, resin coated paper, etc.). In the exchanger, they form a stack, aligned parallel to each other and separated by a certain spacing. The two airstreams flow through the channels formed between the flat plates. Each airstream flows into alternating channels—i.e., supply air in one channel, return air in the next, supply in the one after that, etc.—so while they pass near each other, they never mix. Sensible energy from the warm air flowing through one side of the exchanger is transferred to the cold air flowing through the other. Some flat plate exchangers also transfer latent (moisture) energy from the high-vapour-pressure side to the low-pressure side of the plates.

In applications such as process cooling, the return air is used to pre-cool incoming fresh air. In non-HVAC applications such as equipment cooling, one air flow can operate in a closed-loop providing contaminate-free cooling, while the other airflow is simply scavenger air—outdoor air brought through the exchanger to cool the closed-loop air, then discarded outdoors.

Condensation in Heat Exchangers

Condensation occurs when an airstream containing water vapor is cooled down to the saturation point. Typically this occurs under winter conditions when heat from the return airstream is transferred to the supply airstream. At atmospheric pressure, the saturation point is determined by the temperature and water content of the air (relative humidity or absolute moisture content). As an example, the saturation point (i.e., condensation temperature) is given below for several
dry bulb and moisture conditions.

The first case above is a rather common value for the return air condition in HVAC applications. If the temperature of the outdoor air is 17.6 °F or below, an exchanger operating at 50% efficiency will cause the return air temperature to drop to the saturation point and water will condense in the exchanger. For more efficient exchangers (that transfer more heat), the outdoor temperature does not have to be as low for the return air to reach this point. From a heat transfer perspective, condensation increases the exchanger’s winter efficiency. When water condenses, it releases heat. When it condenses in an exchanger, some of this heat is transferred to the incoming outdoor air, providing more energy to warm the supply airstream. Although heavy condensation may cause an increase in pressure drop due to a slight narrowing of the air channels as water collects on the plate surfaces, this change is negligible. Typically, selection programs take into consideration in their calculations the amount of moisture in the return air and the increased efficiency caused by condensation.

Condensate Design Guidelines

When designing systems with flat plate exchangers, it is important to take condensation into account. The heat exchanger should be oriented so that the condensing water can easily flow downward into a drain pan and out of the unit. Also, to ensure that no condensate leaks into the supply airstream, the pressure in the dry airstream should be higher than in the condensing airstream.

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Return Air TemperatureRelative HumidityAbsolute Moisture Content (lbm water/lbm dry air)Condensation Temp
68 °F 40% .0058 42.8 °F
68 °F 20% .0029 25.5 °F
104 °F 20% .0092 55 °F
212 °F 10% .0690 115 °F

When the return airstream is configured to travel up through an exchanger, its velocity should be lower than 590 fpm. (At speeds below about 590 fpm, water will not be blown up through the exchanger by the velocity of the airstream.) In situations where there is a large volume of condensate, water can partially restrict the channels in an upward-flowing airstream causing the fans to pulsate. A downward return/exhaust airstream, working in the same direction as gravity, is the best way to ensure that water rapidly drains from the exchanger. In applications with large amounts of condensation, limestone and other minerals or contaminants may deposit on the plate surfaces. Over time, this will influence the performance of the exchanger. Exchangers should, therefore, be positioned to provide access for cleaning. Typically, return air containing corrosive vapors in moderate concentrations will not damage the heat exchanger surfaces unless condensation occurs. Even if, however, there is no condensation under normal operating conditions, it may occur during start-up or shut-down of the unit. It is therefore important to vent the unit thoroughly during start-up by sequencing the
blowers so that the return air blower always starts first and stops last. This way, as it is just getting started, the moist return airstream never faces the fully-developed heat sink of the cold supply airstream. Backdraft dampers can also help protect the unit during out-of-service periods.

Fully Developed Airflows


It is very important to be aware that the vast majority of performance calculations (for
efficiency, pressure drop, etc.) used by selection programs for air-to-air plate heat exchangers
assume the following:

  • The velocity profiles entering the heat exchanger are completely even, i.e. the airflow rate is uniform across the entire exchanger.
  • The temperature profiles of the airstreams as they enter the heat exchanger are also completely even. These are the only realistic conditions upon which a general calculation of air-to-air plate heat exchangers can be based. They also make it possible to compare the performance of different exchangers on an even basis. All deviations from these assumed conditions will reduce a heat exchanger’s efficiency. It is very important to take this into account when designing an air handling unit.

An even velocity distribution is best achieved by avoiding sharp airflow bends immediately before and after the heat exchanger and by positioning the blowers on the exit side of the heat exchanger so they operate in a draw-through manner, i.e. pulling the air through the exchanger. If the pressure drop that each airstream undergoes while passing through the heat exchanger is very low, the airflow through it may be uneven. In such cases, uniform air flow can be
achieved by placing a filter bank, or another restriction that creates a pressure drop, just before the exchanger.

Other things to take into account in the design of an air handling unit are:

  • Condensation—It is important that any condensate be able to leave the heat exchanger without restricting the air flow. Completely horizontal plates should be avoided.
  • Leakage—Air that is bypassing the heat exchanger or leakage between the two airstreams in the heat exchanger will reduce performance and may also result in cross contamination of particles, odors, and condensate between the two airstreams. Preventing leakage requires a good seal between the heat exchanger frame and the air handling unit. It is also important that the internal leakage in the heat exchanger be as low as possible (see also “Leakage and Sealing of Heat Exchangers,” below).

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