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Article Date: 10th November 2016

SPC - Using Heat Pipes to Enhance Dehumidification in the UK

Heat Pipes - Dehumidification Systems - Humidity Control - Chilled Beams

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Heat pipes have been widely used for dehumidification in hot and humid countries – particularly in the Middle East – for many years. According to SPC, despite the UK’s less extreme climate, the technology can be successfully used to enhance dehumidification in this country, and, as an installation at a UK hospital demonstrates, payback periods can be under three years. Richard Meskimmon, technical manager at SPC, explains more.

In its simplest form, a heat pipe is a sealed tube (usually made from copper), which is evacuated and charged with a working fluid; for HVAC purposes, this is usually the refrigerant R134A, although SPC offers a greener, more efficient, patented solution that uses water.

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 the absorbed heat with it. 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. It is the most effective method of passively transferring heat from one place to another.

Heat pipes can be used to significantly improve dehumidification, without any additional energy input, and for many years have been successfully used for this purpose in hot and humid environments, such as in the Middle East. And, despite the UK’s less extreme climate, the technology can enhance dehumidification in this country too.

While the UK’s climate means that high humidity is only a problem on a handful of days per year, and thus conventional air conditioning systems have often been considered a satisfactory solution, the drive towards increased energy efficiency has seen a move towards systems which rely on the control of space humidity, because they use less energy. With chilled beam air conditioning, for example, less air movement is required, so energy is saved, but systems rely on the close control of humidity for correct operation.

Chilled beams are sensible cooling terminal systems, which cannot be allowed to condense moisture because the cooling surface is generally exposed above the occupied space. Therefore the outside air needs to be efficiently conditioned in order for them to function properly. Building occupants benefit too; 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.

However, traditional dehumidification, which can be either indirect or direct, poses issues. With indirect methods, air is cooled below its dew point, usually achieved using a chilled water cooling coil, or perhaps DX. Typical SHR values for cooling coils are 80 per cent; if the process requires lower values then the dry bulb temperature of the air must be reduced below its supply temperature to remove moisture and then reheated, which imposes a significant energy penalty.

The direct method involves desiccants, with the process requiring continual regeneration. It is only used for very low humidity control, and only becomes economical for air conditioning applications if a source of thermal energy is readily available.

Heat pipes can be used to treat the outside air in a more energy efficient manner. Cooling coil enhancements are made to match the SHR of the process to the load SHR. Wraparound heat pipes allow passive transfer of heat around the cooling coil, and air is precooled upstream and reheated downstream.

The SHR of the heat pipe/cooling coil combination is lower than for a conventional cooling coil, and can therefore be considered as an enhanced cooling/dehumidifying coil.

Based on design conditions, the conventional cooling rate is 28kW for each m³/s of airflow and reheat rate is 6kW (figure 1). The addition of heat pipe reduces cooling rate to 22kW and reheat to zero (figure 2). 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.

Panasonic Heat Pump

While there will be limited operating hours – the heat pipe will only function when the cooling coil is operating, so the number of hours is limited by the UK climate – the use of heat pipes for dehumidification in this country can be justified via ROI/payback analysis. In fact we have a recent example where heat pipes were incorporated to improve dehumidification at a UK hospital with a chilled beam system, and the payback period was under three years.

At the hospital, chilled beams have been installed in critical areas, but the existing chiller plant was of insufficient capacity to provide the necessary dehumidification, with an 8.5°C dew point required in some areas. The inclusion of heat pipes in the design has enabled the existing chillers to achieve the required dehumidification, and an annual energy cost saving of £27,363 and a payback of 2.9 years has been achieved.

Panasonic Heat PumpIn 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.

In terms of payback and energy savings, calculations must be based on true installation costs and accurate annual weather data. The capital costs associated with the heat pipe are the cost of the heat pipe itself and its installation cost within the AHU. It is important to allow for the lower capital costs associated with reducing the size of cooling plant and ancillaries; the use of heat pipes results in lower chilled water rates, leading to smaller chiller plant, pipework and pumping equipment.

The reduction in installed chiller plant size is equal to the rate at which the heat pipe provides precooling at the maximum design condition. This is a capital cost saving irrespective of the number of operating hours. The reheat plant capital cost is unlikely to be affected, as it will be required for winter operation with or without heat pipes.

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.

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.

Panasonic Heat Pump

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