Reliability
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Air Cooler Design

Heat Transfer Basics
Air cooled heat exchangers rely on thermodynamic properties of heat transfer. Specifically, heat transfer is energy released over time. Two standard formulas used to calculate heat transfer are as follows:
  • Fluid Mass Flow * Cp * Delta T
  • Rate * Area * LMTD  
    • Rate is the inverse of sum resistance to heat transfer (fouling, material conductivity)
    • Area is the cooler’s total finned heat transfer area
    • LMTD is the Log Mean Temperature Difference, or the driving force of heat transfer  
             ACHE LMTD Relationship
Given the above graph, GEA Rainey recommends an absolute minimum of 10°F Delta T for most applications based on economies of scale. Of course smaller Delta T’s, such as 5°F, have been designed.  Keep in mind as the ambient increases, the LMTD goes down reducing cooling ability.
 
Flow Pattern & LMTD Effects
There are three main types of flow patterns used in air cooled heat exchangers; counter-current flow, co-current flow and cross current flow.
 

Counter-Current Flow – By far the most common in the process industry, counter-current flow cools the hottest fluid with the warmest air, and the coldest fluid with the coldest air. In other words, the process fluid enters the heat exchanger and passes through the finned tubes at the top of the bundle. These top tubes are exposed to air warmed by the lower tube rows. As the process fluid cools and passes through the lower tube rows, the air temperature is lower as it has been exposed to less and less tube rows. 

FlowPattern-CounterCurrent

Co-Current Flow – This flow pattern is typically used in processes with critical pour points as it provides the highest outlet process temperature control since it has the lowest efficiency. In this pattern the ambient air cools the hottest fluid, and the hottest air attempts to cool the coldest fluid. The shaded arrows to the right illustrate this flow pattern.

FlowPattern-CoCurrent

Cross-Current Flow – Most common in the gas compression industry, the cross-current flow pattern exposes each pass of the process fluid to the same air stream. Therefore the pass plates inside the headers are vertical, rather than horizontal, to allow the fluid to pass perpendicular to the air stream.

FlowPattern-CrossCurrent
Minimizing Air Cooler Costs
GEA Rainey Solutions understands the customer’s needs to size and design air cooled heat exchangers using commercially available software programs. These programs, while not offering a thermal guarantee, can offer an advantage to customers when trying to compare air coolers from different manufacturers. To assist with this requirement, GEA has developed a short list of points to help you create the most economical design. Please note that while these points will assist with an initial design, most of the time GEA can provide more economical designs as our proprietary rating program has a built-in estimating program thus allowing quick selection between multiple designs.
  • Maximize tube length while maintaining >=40% fan coverage
  • Design aircooler with a 1 to 3 ratio. For example, if your cooler is 30’ long it should typically be around 10’ wide. This helps reduce the header size, the most expensive portion of an aircooler, while still maintaining proper fan coverage.
  • Minimize tube rows to increase heat transfer effectiveness of area, minimize header thickness. Typically between four to six tube rows
  • Try and maintain 1” tube diameters, depending on service. Even high viscosity services that appear to benefit from larger diameter tubes can typically be designed cheaper with more 1” diameter tubes.
  • Use a counter-current flow where possible as it reduces surface and potentially minimize header plate thickness.
  • Increase your allowable pressure drop. This allows more passes in the bundle reducing the cooler size.
  • Know who offers you a thermal guarantee!  Commercially available software does not offer such guarantee so please allow GEA to offer you the most economical design while still guaranteeing thermal performance!

 
Air Cooled Heat Exchanger Brochure
GEA Rainey Solutions
5202 West Channel Road
Catoosa , OK 74015
Phone:
(918) 266-3060
Fax: (918) 266-2464
Inquiries:  aircoolers
Service:    serviceparts