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Article Date: 7th November 2016

SPC - Improving Dehumidification with Heat Pipes

Heat Pipes - Cooling Systems - Space Cooling - Radiant Conditioning

SPC - Heat Pipe

Heat pipes have been commonplace within the aerospace, electronics and air conditioning industries since the 1960s. And, thanks to their ability to enhance dehumidification, they can be successfully used in modern cooling systems installed in the UK, says Richard Meskimmon, technical manager at SPC.

Heat pipes are the most effective passive method of transferring heat from one place to another. In their simplest form, a sealed tube (usually made from copper) is evacuated and charged with a working fluid. For HVAC purposes, the fluid is usually the refrigerant R134A, although SPC offers a patented solution where water is used as an alternative, providing a greener option and improving efficiency.

With a heat pipe, heat is absorbed from the incoming warm air stream in the evaporator section, boiling the working fluid. Due to its elevated vapour pressure, the vapour moves rapidly to the cooler condenser section of the heat pipe, carrying with it the absorbed heat.

As the vapour reaches the condensing area of the heat pipe, heat is released to the cooler air and the vapour condenses. The liquid returns by gravity to complete the cycle. The entire heat transfer process occurs with a very small temperature difference along the pipe.

One of the main benefits of heat pipes is that they can significantly improve dehumidification, without any additional energy input – and they have been successfully used in places with hot and humid conditions, such as the Middle East, for this purpose.

Traditionally, cooling in the UK has concentrated on maintaining a comfortable space temperature, rather than controlling humidity levels. Our climate means that high humidity is a concern on only a handful of days per year, and so conventional air conditioning systems have often been considered satisfactory in terms of humidity control.

However, the drive towards increased energy efficiency has seen a change from traditional methods of providing space cooling in favour of systems which rely on the control of space humidity, which use less energy. Chilled beam air conditioning and radiant cooling systems, for example, rely on the close control of humidity for proper operation, and, as less air movement is required, energy is saved.

As well as being more energy efficient, this approach also benefits building occupants. The humidity of indoor air has a profound effect on people’s quality of life; maintaining air quality within the optimum zone of 40-60 per cent relative humidity makes people more productive and results in fewer days lost through illness.

Chilled beam air conditioning and radiant cooling systems are sensible cooling terminal systems, which cannot be allowed to condense moisture because the cooling surface is generally exposed vertically above the occupied space.

Reduced air movement and increased chilled water temperatures associated with these systems allow significant energy benefits to be realised, but only if the outside air is efficiently conditioned in order for them to function properly.

It is therefore vital that the sensible and latent cooling is decoupled in order to optimise the benefits of these systems – and heat pipes can be used to treat the outside air in an energy efficient manner. The inclusion of heat pipes in the design means the supply air is generated at a sufficiently low dew point, allowing chillers to achieve the required dehumidification.

When used for dehumidification, heat pipes remove heat from the incoming stream of humid air and add it to the outgoing stream of dry air. Between these two stages, the air passes across the dehumidifier's cooling coil. The pre-cooling provided by the heat pipe brings the temperature of the humid air nearer to its dew point. The heat pipe extracts the heat energy associated with sensible cooling before the air reaches the cooling coil.

This makes the work of the cooling coil more efficient. Instead of working on sensible cooling, the cooling coil can extract the latent energy released through condensation at the dew point. Up to twice as much condensate can be produced, taking the humidity of the recycled air well below the level that could be achieved by the cooling coil alone. The final stage is a reheating of the over-cooled air by the heat pipe to give a warm and dry stream of exhaust air.

In terms of payback and energy savings, calculations must be based on true installation costs and accurate annual weather data. We have a recent example where heat pipes were incorporated to improve dehumidification at a hospital with a chilled beam system, and the payback period was under three years. In this instance copper tubes and fins and stainless steel casings were used in accordance with Health Technical Memorandum (HTM) guidance; a standard construction of aluminium fins and galvanised steel casings for the heat pipe would halve the cost (approximately) and hence the payback period.

It’s also important to consider that heat pipes have no moving parts to break or wear out, and they are virtually maintenance free - indeed they will probably outlast the HVAC equipment itself.

Heat pipes have been widely used for dehumidification in hot and humid countries - particularly in the Middle East - for many years. We believe the technology can be successfully used in the UK’s less extreme climate to enhance dehumidification, without unjustifiable capital expenditure.

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