Strong Acid System - Acid Cooling - Air Cooled Heat Exchanger
March 6, 2003

Introduction

Sulphuric acid air cooled heat exchangers are suitable for acid cooling duties when plants are located in regions where water resouces are scarce or the cost of fresh water is prohibitively high to operate a conventional closed-loop water system.  Other factors that favour the use of air coolers is when local regulations limit the moisture plumes from evaporative water cooling towers.

Air coolers have been used for many years in Europe to cool strong sulphuric acid in acid plants for some of the reasons noted above.  Most unit are constructed of stainless steel operating at temperatures less than 85°C (185°F).   The low temperatures are required to avoid excessive corrosion of the stainless steel tubes.  The relatively low acid temperatures meant high acid circulation rates and low LMTD for the operation of the air coolers.  The low LMTD results in extensive banks of air coolers to meet the required cooling duties during operation when ambient air temperatures are high.

Air coolers are not a really a necessity in North America because of the abundance of water resouses.  As well, plants were traditionally designed with higher acid operating temperatures (100°C (212°F)) which was too high a temperature for unprotected stainless steel.

To overcome the shortcomings of the then available air coolers, Chemetics and CIL undertook in 1970 a joint two-year development program to study the feasibility of applying anodic protection to air coolers.  Laboratory and field test were done to demonstrate the concept. In 1972 a full scale air cooler was built and commissioned at the No. 2 Copper Cliff acid plant.  The plant was a 450 T/D plant and the air cooler was installed on absorber circuit.  The entire cooling duty was handled by two bays of air coolers.

The design of the anodically protected air cooler was done after careful consideration of many different factors to ensure that the first unit would be succesfull.  Acid velocities were limited to avoid high turbulence which could cause erosion.  The tube to tube sheet welds were made using high fabrication standards to ensure there would be no leaks at the tube-to-tubesheet joint.  The fins on the outside of the tube are fabricated from aluminum and tension wound in a helical fashion onto the tube.  The botton rows of tubes were coated with a transparent resin to protect th fns against corrosion.

The anodically protected air coolers operated with 98.5% H2SO4 entering at 115°C (239°F) and exiting at 80°C (176°F).  The higher operating temperatures results in a significantly fewer banks of air coolers, lower fan power consumption, less noise and reduced capital and operating costs.  For a 900 t/d plant fan horsepower could be reduced from 800 Hp to 300 Hp and the area required could be reduced from 9600 ft² to 2450 ft².

At least three sites utilized the anodically protected air coolers.  Two of the sites are now shutdown and the status of the third installation is unknown.

From the time the first anodically protected air coolers were built and operated to present, there have been many advances in technology that could lead to air coolers being considered for more applications.  Materials such as Sandvik SX, SARAMET® and ZeCor™ are now available that are capable of handling hot acid without the need for anodic protection.  It is relatively simple to take tubes made of these materials and apply fins to the outside of the tube.  The header boxes and tubesheet would be made of the same material.

An air cooler as described above could be the next advancement in acid cooling equipment.