Chemical & Process Technology: How much heat removal by natural convection when Air Cooler fan off

Sunday, December 9, 2007

How much heat removal by natural convection when Air Cooler fan off

Natural Convection Phenomenon
An air-cooled heat exchanger fan off, heat transfer by natural convection may continuous where the air movement is resulted by the air density difference between cool ambient air and hot air (near tube with hot process fluid) within the tube bundle and duct. Natural air cooling may leads to some adverse and process problem. Two major examples are

1) waxy fluid crystallized & solidified and water freezing in air-cooled heat exchanger
2) Inability for vapor to condensed results vapor passing across air-cooled heat exchanger

Process design engineer shall address above scenarios as part of the design and understanding ability of natural air cooling becomes important.

Natural Convection Heat removal by Induced Draft & Forced Draft Fan

Works carried out by Berryman (1983) and Henry (1988) has shown that heat removal from induced draft and forced air cooler in the range of 20-40% and 5-15% (of normal heat duty) respectively. The percentage is increased with wind velocity. Refer to following image.

Higher heat removal is expected in induced draft compare to forced draft. This could be caused by the location of fan and fan blades. In forced draft (where fan is located below the tube bundle), fan blades will create disturbance of air flow and affect proper air distribution across the tube bundle and leads to lower heat removal. Disturbance of air flow may increases the possibility of vortex (air recirculation) formation between the fan blades and tube bundle. Air recirculation will reduce net cool air intake into tube bundle and lower heat removal.

Credit During Fan Off

Apart from process issue as discussed, natural air cooling by air-cooled heat exchanger during fan off and/or shutdown condition will help to reduce process fluid temperature. In the event of power failure which lead to fan off, residue hot gas may continue pass through air-cooled heat exchanger and results downstream system temperature continue to rise. Natural convection will partially cool the hot gas. Determining quantity of heat removal by natural convection for CAPEX optimization and maintaining plant integrity is getting critical in this context. API has provides some allowance in natural convection. Per API STD 521, section 5.6.4 Air-Cooler fan failure, partial condensing capacity of 20% - 30% of normal duty can be taken for natural convection of air-cooled heat exchanger. However, API STD 521 has not differentiated allowance for induced draft and forced draft fan.

Additional Thought
Considering Berryman and Henry studies, credit statement in API STD 521 and still air condition, the reasonable heat removal by natural convection during fan off for induced and forced draft may be 20% and 5% of normal duty respectively.

Besides, as the reasonable heat removal only 5% for forced draft, it has minimum incentive for further optimization and process designer can choose to ignore. However, 20% for induced draft is at it lower end and has reasonable incentive for further optimization, thus process designer may consider take the allowance but shall always confirm with final design during detailed design and fabrication.