Fouling and corrosion tackled by new heat exchangers

Paul Boughton

Sean Ottewell reports on the latest high-pressure spiral heat exchangers, plate heat exchangers and brazed plate heat exchangers that are designed to reduce fouling and cut corrosion

A problem that causes continuous concern for many process companies is fouling of heat transfer equipment. Alfa Laval has risen to this challenge with its new high-pressure spiral heat exchangers (HPSHEs). These are high-pressure versions of the company's existing SHEs with a design pressure of 100 barg and what the company describes as a highly efficient, self-cleaning effect. True process heat exchangers, they can replace shell-and-tube units (S&Ts) in sensitive processes, eliminate fouling problems and offer a high level of heat recovery.

Fabrizio Palmeri, manager, market unit refinery, Alfa Laval, says: "An HPSHE continuously recovers maximum heat energy from bottom residues, while its single channel geometry gives it an ultra-efficient self-cleaning effect that prevents it from fouling. The HPSHE's capability to withstand pressures of up to 100 bar and high temperatures makes it an efficient process heat exchanger for visbreaking, fluid catalytic cracking, desalting, coking and other processes. Although compact, it offers two to three times higher heat transfer efficiency than an S&T unit."

An S&T unit normally requires cleaning several times a year. An HPSHE only requires servicing every four to five years and can be cleaned easily by backflushing, hydroblasting, or in-line chemical cleaning. HPSHEs interiors are accessed by means of an easily opened front cover. This gives them a small service footprint, unlike S&Ts where space must be allowed for long tube bundles to be withdrawn and cleaned manually (Fig.1).

"Since the heat recovery performance of an HPSHE will not be reduced by fouling, extra process heat will not be required from the furnace," says Palmeri. "This cuts fuel costs and reduces carbon dioxide emissions."

HPSHEs consume only a quarter of the pumping energy required by S&Ts. Also, as there should be no fouling to increase the pressure drop over the heat exchangers, no extra pumping energy will be needed either. This means substantial energy savings.

Cleaner wastewater

In Sweden, Örebro-based EON for many years has employed a policy of re-using purified wastewater. Wastewater is pumped from a pumping station in Skebäck up to two heating pumps at the Åbyverket CHP plant, where heat is recovered from the cleaned wastewater. Once that process is complete, the now cooled wastewater is pumped back via Skebäck and released into the river Svartån which flows into Lake Hjälmaren.

In 1998, EON also began supplying district cooling to Örebro University Hospital, Örebro County Council and international paper and packaging company Mondi. As by definition the wastewater is not clean, it has to be purified. This meant that EON had to invest significant time and resources on site to clean customers' heat exchangers. This was viewed as an unnecessary cost by both customers and the supplier.

When EON was awarded the contract to supply district cooling to central Örebro, the decision was taken to build a dedicated heat exchange station adjacent to the pumping station in Skebäck. Wastewater is now exchanged via a clean water system that is then distributed to the Åbyverket plant and sent on to district cooling customers.

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Cooling water

Tranter has supplied two GXD-205 gasketed plate heat exchangers to exchange between purified wastewater and district cooling water. Total output is 18MW, 9MW per heat exchanger. Tranter's plate heat exchangers are ideal for applications with close temperature approach.

For this particular application, the heat exchangers were designed with one bar pressure drop. The relative high pressure drop maintains channel velocity and shear stress at a high level, thus reducing maintenance requirements - which is important for the wastewater.

Tranter is involved in another heat exchange project at Gävle Hamn, which has a strategically important location just 170km from Stockholm and a catchment area that extends from Uppland and Dalarna northwards.

Aviation fuel

Among other things, Gävle Hamn has, for some time, been supplying Arlanda airport with all its aviation fuel directly by train. One train replaces just over 30 tank trucks, which previously transported the fuel through Stockholm up to Arlanda. Gävle Hamn has total storage capacity of 950,000m3 in around 140 tanks.

The large oil and chemical tanks in the port have previously been heated with fossil fuel in the form of oil. This relatively expensive method of heating is now being replaced with green district heating.

The district heating specialist Palmat System chose Tranter's plate heat exchangers for the order it received from Gävle Rörteam, which is installing the district heating in Gävle Hamn.

"Tranter represents cutting edge technical knowhow. Combined with their fast, reliable deliveries, this is a decisive competitive advantage in our customers' eyes," says Per-Arne Tönners of Palmat System in Stockholm.

In other heat exchanger news, SWEP has developed an SMO 254 steel version of its DP300 brazed plate heat exchanger (BPHE). The DP300 is already available in AISI 304 and AISI 304/316 steels, and the company says that this new high-molybdenum SMO 254 model gives customers even greater flexibility in their choice of materials for demanding applications. SMO 254 offers higher resistance against corrosion, for eample in cooling towers and ground source heat pumps where a high chloride content in the water can be an issue.

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Flexible chillers

The DP300 is an efficient and versatile true dual BPHE optimised for the environmentally preferred refrigerant R410A. The DP300 is a natural choice for flexible chillers and climate control applications, where its ability to offer full performance at both full- and half-load is a major advantage.

In 2010 the company won the Frost & Sullivan Europe Market Share Leadership Award in the BPHE market after capturing nearly 40 per cent of the market.

SWEP has continually focused on improving the heat transfer capability of its products. Enhanced material selection, such as the use of new types of stainless steel, has increased product resistance to corrosion.

The company's primary focus has been on three business segments: air conditioning, heating and refrigeration. It is also actively trying to convert the technology used in S&T exchangers - which are used widely in air conditioners - for use in BPHEs. It is looking to replace the traditional use of other PHEs in heating applications with appropriate brazed heat exchangers that are more effective, compact and reduce the overall size of the system.