Stainless Steel for a Revolutionary Heat Exchanger

Posted 31st July 1993

Stainless steel plates are the core component of a revolutionary brazed plate heat exchanger manufactured by Melbourne firm Multistack International Limited.

The heat exchangers are designed to replace conventional gasketted-plate and shell-and-tube designs. After pioneering work was conducted in Europe in the late 1970's Multistack have achieved higher efficiencies and longer operational life after an Australian development program of several years and "many millions of dollars".

Type 316 stainless steel plates stacked on top of each other form the basis of the heat exchanger's design. Each plate is embossed with a channel formed under high pressure, to very fine tolerances and then enrobed with a thin sheet of copper. Plates are paired to form a complex lattice of channels and alternating sets of channels carry refrigerant and coolant. Multistack claims that in its heat exchangers turbulent flow at high pressure allows more efficient heat transfer than any other design.

Paired sets of plates are assembled on top of each other, tested and brazed at high temperature under vacuum conditions. Through capillary action the copper brazing medium forges the paired lattice system together forming a strong pressure vessel.

"Stainless steel has been specified for its ability to be cut, formed and heat treated reliably to form a pressure vessel which is durable and able to resist attack from aggressive refrigerants and coolants", according to Multistack Managing Director Roger Richmond-Smith.

Multistack heat exchangers have achieved exceptional reliability in service and are constructed to conform with the American Society of Mechanical Engineers pressure vessel code. Meticulous testing of stainless steel mate-rials is conducted before any pressing takes place.

The heat exchangers can be used for many fluid to fluid (including gas) heat transfers where it is not necessary to clean the heat exchanger's surfaces, but the largest proportion of Multistack's production is for use in the company's patented modular water chillers, for large central plant air conditioning systems.

The advantages of the compact heat exchangers are exploited in the chillers to create modules that can be transported in standard lifts (greatly reducing the costs of refitting existing buildings), use HCFC22 refrigerant (which is favoured under the Montreal Protocol for protection of the ozone layer) and optimises efficiency by operating only enough modules to meet required cooling load during the course of each day.

With these advantages it is not surprising that large numbers of Multistack chillers are in service globally in a wide variety of configurations. The company reports that its second largest installation (39 modules) is the World Trade Center, New York and its largest (42 modules) is Zhian Jin Shopping Centre, Henan Province in China.

The potential use for Multistack modular chillers is not restricted by the health of the building industry. Adherence to the Montreal Protocol will see the replacement or adaptation of about 4,000 existing water chillers in Australia well before the end of the century. The numbers in Europe and the United States are a staggering 45,000 and 80,000 chillers respectively, according to recent industry estimates.

To service international customers the bulk of Australian chiller production is exported and licensed chiller production plants are operating in the United States, China, Spain and South Africa.

This article featured in Australian Stainless magazine - Issue 1, July 1993.

Lissel Pilcher