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FEATURE HVAC SYSTEMS The case for plate heat exchangers


Paul Sands from Stokvis Energy Systems investigates the benefits of choosing PHE’s over traditional calorifiers and how this is offering not only a more efficient alternative but one that addresses the health and safety issues associated with conventional methods of providing hot water


he traditional way of providing domestic hot water was to utilise a boiler to heat the water via a coil in a storage tank, known as a calorifier and to store it until it is required. It’s a system that has worked satisfactorily for generations, but today – with more emphasis on energy efficiency, COST and health & safety – this system increasingly fails to meet expectations. While still widely used, hot water storage systems suffer one unavoidable disadvantage, and that is that they are designed to hold hot water at a certain temperature over an extended period. Domestic systems, producing hot water for a single household, are generally small affairs with storage tanks holding only a couple of hundred litres. Commercial calorifiers are huge by comparison, with capacities of 3,000 litres or more. Modern insulation methods can reduce heat-loss from calorifiers very effectively, but heat- loss is still unavoidable and lost heat means lost energy, which is wasteful and costly. Another problem is that calorifiers are not good at regulating temperature. A large body of hot water naturally develops a temperature gradient, with cooler water at the bottom and hotter water at the top. They sometimes rely on constant water movement, achieved by a circulation pump, to prevent stagnation and cold-spots developing in the system. For many years, there was no practical alternative to the established heating-and-storing of water but the development of modern plate heat exchangers (PHEs) has changed all that. A hot water system using a plate heat exchanger does not store hot water. The water is heated instantaneously and only when required. In the domestic context, this is seen in the guise of combi-boilers which fire up when you turn on the tap, while additionally producing hot water for space heating. A PHE consists of a pack of plates (usually stainless steel) to transfer heat from the primary fluid to the secondary fluid. The plates and gaskets are arranged to form alternating channels between the primary and secondary fluids and arranged in this way, the plates have an extremely large heat transfer surface area and therefore considerably high loads are achieved from a very compact plate heat exchanger.


T 12 WINTER 2015 | ENERGY MANAGEMENT


ideal as it is capable of producing hot water instantly and constantly at peak times. Calorifiers and direct-fired water heaters are less reliable and can empty of hot water should higher than normal demands occur.


EASY AND RELIABLE MAINTENANCE Plate heat exchangers are also easy to maintain. An engineer can usually strip, clean and re-assemble a typical unit within an hour, and they are easy to keep clean. Because they are non-storage units, PHEs present a far lower risk of contamination of the water with legionella microbes. The water is heated instantly on demand, and therefore the bacteria, which thrive in wet environments at temperatures between about 20 and 500


C,


Figure 1: The Econoplate from Stokvis


PLATE HEAT EXCHANGERS Even a powerful commercial plate heat exchanger capable of supplying a large hotel or apartment block typically measures little more than 1m high and 500mm in width. Having made the argument for


“The heat capacity of


the instantaneous generation of hot water,it is sometimes necessary to have a small buffer vessel working in conjunction with a plate heat exchanger, but this is generally only required if the boiler load is not available, but first one should consider if certain LTHW systems can be overridden during peak heating and hot water service periods, to provide HWS priority. For large commercial applications a PHE is


the PHE can be tailored to the exact requirements of the system, simply by varying the number of plates in the unit. Upgrades are easily achieved by adding more plates."


have no chance to breed. This offers peace of mind to the building occupants and also eliminates the need for annual insurance inspections and the disruption and downtime that results. Insurers usually require that hot water storage systems are opened and inspected at least once a year – unnecessary with a PHE. The heat capacity of the PHE can be tailored to the exact requirements of the system, simply by varying the number of plates in the unit. Upgrades are easily achieved by adding more plates. Even the problems posed by hard water are minimised with a plate heat exchanger. Less prolonged contact with the boiler water, even heat distribution and the high velocity of water through the PHE makes the accumulation of limescale less likely than in a conventional calorifier. There are compelling economic reasons for choosing PHEs over traditional calorifiers but convenience and health are also main attractions. It is always difficult to identify the exact savings that will be


made by replacing a storage calorifier or direct gas-fired water heater as every installation is


different. But what is not in doubt is that savings will be made.


Stokvis Energy Systems www.stokvisboilers.com 020 8783 3050


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