Designing an air-cooled Heat exchanger might sound like an insurmountably complex task, but when you break it down into smaller steps, it’s much more manageable. Knowing how to design and size these heat exchangers is crucial in a variety of industries like oil upgrading and refining, gas processing, petro-chemical, and LNG. Here, we walk you through the whole process from start to finish for designing an air-cooled heat exchanger.

Heat Exchanger Sizing and Geometry

Air-cooled heat exchanger geometry is a function of the process conditions, design constraints, and ambient air conditions.  Surface area will be calculated based on the required duty, log-mean (or average) temperature differential, and the overall heat-transfer coefficient (U-factor).  Higher ambient air temperatures will require more surface area and larger bays to meet the same duty.  Plot or transportation restrictions may limit the width or size of each bay, thereby increasing the total number of bays required.  

Gathering Data

Before diving into any design or calculations, gather all the necessary data. You’ll need process fluid information like flowrate, inlet and (desired) outlet temperatures, density, viscosity, thermal conductivity, specific heat, and fouling factor.  If the process is condensing (phase change), then heat-release and property curves will be needed.  Allowable pressure drop is a key factor in optimizing any heat exchanger design, so that should always be specified.  We’ll also want to know the operating and design pressures, design temperature, corrosion allowance, and metallurgy.  Site data like elevation (or atmospheric air pressure) and max summer ambient temperature are also required.  Finally, if the air-cooler will be run in cold climates and there’s a risk of freezing (or over-cooling) the process, then winterization via recirculation ducts should be specified.   ,

Calculating Heat Transfer

The heat transfer formula for an air-cooled heat exchanger is given by the equation:

Q = U * A * ΔT

Where:

Q is the rate of heat transfer (duty)

U is the overall Heat Transfer Coefficient.

A is the effective heat transfer area.

ΔT is the log mean temperature difference (LMDT), which can be calculated using the formula: ΔT = [(T1 – t2) – (T2 – t1)] / ln[(T1 – t2) / (T2 – t1)], where T1 and T2 are the inlet and outlet temperatures of the hot fluid and t1 and t2 are the inlet and outlet temperatures of the cold fluid.

Please note that this formula assumes that the heat exchanger is operating under steady-state conditions and that there is no phase change in any of the fluids involved.

Calculating The Overall Heat Transfer Coefficient (U-Value)

The U-value or overall heat transfer coefficient in an air-cooled heat exchanger is calculated by considering the heat transfer capabilities of both the air and the fluid in the tubes. It combines the heat transfer coefficients of the air side, tube side, and the tube material itself.

The formula for the U-value calculation is as follows:

1/U = 1/ha + t/k + 1/hi

Where:

U is the overall heat transfer coefficient

ha is the heat transfer coefficient of air

hi is the heat transfer coefficient of the fluid inside the tubes

t is the tube wall thickness

k is the thermal conductivity of the tube material

Each heat transfer coefficient (ha and hi) can be calculated using the appropriate correlations based on the Reynolds number, Prandtl number, and other relevant parameters. The tube wall thickness and thermal conductivity of the tube material are usually provided by the manufacturer or can be found in material property tables.

Finally, add the reciprocals of these coefficients together and take the reciprocal of the sum to find the U-value.

Optimizing Air-Cooler Size and Geometry

While designing air-cooled exchangers can be performed by hand, it is very time consuming with many iterations required to solve U-values for various geometries and configurations.  Air-cooled sizing is almost exclusively done by software now, with HTRI Xace being the industry standard.  Air-coolers must be sized with practical considerations in mind, such as fan size, bay width and length, commonly available tubing, bundle depth (number of rows), and dozens of other considerations.  A design that is fully optimized for thermal performance may not be practical in terms of cost and constructability.  In general, one larger bay is better than two smaller ones for cost.  6-rows tubing depth is a good starting point for designs, with 8 or 9 being the upper practical limit as performance greatly drops off due to ever increasing air temperatures through the bundle.  API 661 gives fin-tube selection based on operating temperatures.  Split headers may be required if the inlet and outlet process temperatures are greater than 200F (per API 661).  

Calculating Air Side Pressure Drop and Fan Power Requirement

You’ll also need to calculate the pressure drop on the air side. This is important for determining the fan power (motor horsepower) requirement. The fan must have enough power to overcome the pressure drop and maintain the desired air flow rate.  HTRI Xace can be used as a guideline, but typically the fan manufacturer has their own program.  

Other Factors To Consider In Air-Cooled Exchanger Design

In addition to the bundle design and fan and motor selection, the structure itself has many design considerations such as height under fan deck, recirculation over the side (or end), louvers, bug-screens, hail-guards, ladders, walk-ways, platforms, floors, electric or glycol ruffneck style heaters, steam coils, and painting or galvanizing, just to name a few.

Contact Altex Industries for Air-Cooled Heat Exchanger Design and Manufacturing

If you find all of this overwhelming, you don’t have to go it alone. Companies like Altex Industries specialize in designing and manufacturing air-cooled heat exchangers. We can guide you through the complexities and ensure you end up with an air-cooled heat exchanger that matches your exact requirements. Contact us to learn more about our heat exchanger design and manufacturing process.

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