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Advanced Heat Exchangers: Lighter, Smaller, and Higher Performance

Mezzo Technologies, Baton Rouge, Louisiana

Mezzo’s micro channel heat exchangers outperform conventional heat exchangers due to a combination of favorable scaling laws and novel manufacturing processes. Very simple equations predicting the heat transfer and pressure drop show the reason that the Mezzo heat exchanger, with smaller flow passages, is able to obtain higher heat transfer/unit volume without a pressure drop penalty. In general, the heat transfer per unit volume (Q/V) scales with the inverse of the square of the hydraulic diameter (D).

Decreasing the passage width results in increased heat transfer per unit volume. This increase in heat transfer per unit volume does not come with the associated pressure drop penalties that are usually associated with micro channels since the flow goes through numerous passages. The graphs below compare Mezzo’s micro channel heat exchanger technology to aluminum fin heat exchangers.

The below graphs can be used to design an air-liquid cross flow unmixed-unmixed heat exchanger, where the air side thermal resistance is dominant. The air pressure must be ambient in the above curves. For other cases, Mezzo can provide designs and performance.

Assumed givens/knowns:

Required UA (kW/K).
Air face velocity (m/sec).
Allowable air side pressure drop.

Note:

UA is calculated from specific heat ratio, mass flow rates, and desired heat exchanger effectiveness.
Air face velocity is calculated from known mass flow rate of air, density of air, and face area of heat exchanger.

Design procedure to maximize performance (maximize UA):

For a particular model (i. e. Model #1 or Model #2), determine the air side pressure gradient (Pa/m) from Figure 1 for the known air face velocity. Then calculate the flow length of the air through the heat exchanger core by dividing the allowable pressure drop of the air by the air pressure gradient.
Determine the UA/volume (kW/K-m3) from Figure 2 for the known air face velocity.
Calculate the UA for the heat exchanger from by multiplying the UA/volume obtained from Figure 2 by the core volume (which is the product of the face area and the air flow length).

A UA greater than desired will provide more performance (higher effectiveness) than specified. To quantify the actual effectiveness, equations/tables from texts can be utilized, or call Mezzo!

Design procedure to minimize air side pressure drop and provide required UA:

Determine for a particular model (i.e. Model #1 or Model #2) the UA/volume (kW/K-m3) for a given air velocity from Figure 2
Calculate the core volume by dividing the required UA by the UA/volume value obtained from Figure 2.
Calculate the air flow length by dividing the core volume by face area.
From Figure 1, determine the air side pressure gradient (kPa/m) for the air face velocity.
Calculate the air pressure drop across the core by multiplying the air flow length by the air side pressure gradient.

Note: Figure 3 provides the ratio of (UA/volume)/(Pa/m) as a function of air face velocity. The designs with the highest (UA/volume)/(Pa/m) ratio will provide the best overall thermal performance, but will have larger volume and, possibly, weight.