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It’s quite a charge

Many new installations are adopting a secondary refrigeration system as a cost-effective and reliable alternative, says Rich Pedley

Recent years have seen an increase in the use of secondary refrigeration systems as contractors, consultants and end-users look to reduce environmental effects, comply with new regulations and work to stricter budgets and constraints.

While traditional direct expansion (DX) systems are still the most widely operated across a variety of industries, many new installations are adopting a secondary refrigeration system as a reliable alternative.

A centralised DX set-up is typical for refrigeration systems installed throughout Europe and usually uses between three to eight compressors, located in a plant room, which are connected to an external air-cooled condenser.

High-pressure refrigerant is fed directly from the plant room to the chiller units, via a liquid receiver, the refrigerant vapour is then returned to the compressors via a suction line.

The main issue experienced with DX solutions is the leakage rate of refrigerant, due to the large refrigerant charge required – a typical supermarket DX system will use between 1,360 kg – 2,270 kg of refrigerant.

The most common refrigerants used in this set-up are R-22, R-404A and R-507, with an average annual refrigerant leakage rate of up to 15 per cent occurring in these systems, which can rise to up to 30 per cent in older systems.

In contrast, secondary refrigeration systems use two separate circuits, with heat from the chiller units transferred to a heat exchanger by circulating a cooling medium, where the heat is absorbed by a primary refrigerant.

Typically, the cooling medium used is a water and either glycol or hycool mixture, with propylene glycol being the most popular, particularly in supermarket applications, due to its non-toxic properties.

Due to the primary circuit being confined to the plant room, up to 90 per cent less refrigerant is needed for operation of the secondary refrigeration system, significantly reducing the risk of leaks and completely eliminating the opportunity for leaks near food chillers as the refrigerant does not leave the plant room.

There is a common misconception that secondary refrigeration systems have a higher installed cost due to the additional expense of the pumps and heat exchanger, but these initial costs can be offset against other savings over the lifetime of the system.

The pipework requirements are less demanding within a secondary loop solution, helping to bring down the initial set-up costs.

The pipe network for a DX system needs to be designed to ensure oil returns to the compressor, which is eliminated within a secondary loop, as there is no need for an oil return.

Secondary refrigeration systems also boast a lower cost of operation due to the increased energy efficiency, resulting in average energy savings of 4.9 per cent compared with DX systems. Less maintenance requirements also add to the operational cost savings, as a simplified pipe network, reduced number of leaks and a more easily accessible primary circuit all add to a reduction in maintenance costs of up to 20 per cent.

Factory assembly

Due to the changes in pipework requirements, secondary loop systems can be delivered as a factory-assembled unit, which can be connected easily to the pipe system on site, offering further installation time and cost savings.

However, if this option is pursued, it is important to select a pipe network that can mirror these installation savings to help meet budget and project constraints.

A reliable pipework system is integral to transfer the cooling medium around the refrigeration circuit, with single thinwall steel the most widely installed for this application. But glycol and hycool solutions can be highly corrosive, and if a corrosion inhibitor is not added to the solution, they can corrode metal materials within two years.

Due to its corrosion resistant properties, ABS pipework, such as Durapipe’s SuperFLO ABS, is increasingly being used as a more effective and durable solution for secondary refrigeration systems. Its wide operational temperature range of -40 deg C to +70 deg C makes it ideal for a secondary refrigeration application, which typically operates at -15 deg C to -17 deg C.

In addition to the performance benefits, ABS can also aid the installation process. At one-sixth of the weight of steel, its lightweight nature makes it easier to handle on site, while its quick and simple solvent weld jointing technique significantly reduces installation time. Taking all these factors into consideration and when compared with single wall steel, ABS pipe offers an installed cost saving of 30 per cent, making it a more cost-effective option.

As the industry has been forced to look into alternative options for refrigeration systems because of recent legislation changes, more consultants and contractors are realising the benefits of secondary refrigeration solutions.

The increased performance –along with the installation, maintenance and cost savings associated with a secondary refrigeration system – simply cannot be overlooked.   

Rich Pedley is industrial brand manager at Durapipe

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