Getting smart with warm air
Published: 08 January, 2015
When selecting a warm air heating solution for large spaces such as factories and warehouses it’s important to be aware of the options available and select the most appropriate technology. *Phil Brompton of Powrmatic discusses the issues with PWE.
Maintaining comfortable conditions in large ‘shed -like’ spaces without spending a fortune on energy consumption or related ‘carbon taxes’ poses a challenge that needs to be met through the of high efficiency heating systems.
Many such buildings now have a more airtight construction than would have been the case some decades ago. For this reason warm air space heating is increasingly the preferred option as it is both efficient and highly responsive to changing heating requirements within the space.
Traditionally in the UK warm air heaters have been distributed through the space, either suspended from the ceiling mounted on the floor, or with heat being distributed from a central heat source through ductwork. These approaches still have considerable merit, particularly with the development of condensing warm air heaters, but an alternative option now available is air rotation heating.
Air rotation heating
Air rotation heating technology is relatively new to the UK, though it was developed by Powrmatic's sister company in the USA some 40 years ago.
This was certainly the most suitable option for office furniture manufacturer Senator International, where a 500kW air rotation heater has been installed by specialist HVAC installers CK Services 1990 Ltd at the company’s warehouse in Lancashire.
CK’s Andy Smith explained: “We needed to give Senator a heating system that would maintain suitable environmental conditions for both staff and warehouse stock, while minimising energy consumption and carbon emissions. It was clear that a Powrmatic TE air rotation heater would meet these criteria more effectively than a distributed warm air or radiant system. Using a single, free standing air rotation heater also helped to reduce installation and ongoing lifecycle costs.”
Air rotation heating works by moving large volumes of air at relatively low temperatures and velocities through the space; with the air being recycled through the heater approximately every 30 minutes. High-efficiency axial fans are used to move the air with minimum energy consumption, while low temperature operation helps to maximise condensing for further energy savings.
A major benefit of the air rotation heating approach is that it is able to provide excellent temperature control (typically no more than +/-1.5°C of the set point) irrespective of the layout of the space. This also means that any re-configuration of the space does not impact on the heating or require the heating system to be redesigned, as might be the case with fixed systems.
Air rotation heating can also be adjusted to changing occupancy levels through the day, with the option to set back to a temperature of perhaps 8-10°C below the set point operating temperature that is used when the space is occupied.
Condensing warm air heaters
In other situations suspended or floor-standing warm air space heaters continue to be the most appropriate option. Most of the warm air heaters that are currently installed in the UK are non-condensing as these are still able to meet the Building Regulations requirement for a minimum efficiency of 91%. This situation is changing, however, and future legislation is set to require the use of condensing warm air heaters.
In fact, condensing warm air heaters have been available for several years and have proven themselves in a range of applications. Their operation is very similar to the more familiar non-condensing versions, the major difference being a requirement to dispose of condensate – as is the case with condensing boilers. Typical condensate production rates are 0.06 litres/kWh for natural gas and 0.03 litres/kWh for LPG.
In new installations a condensate removal system can be designed in from the start and will have a minimal impact on installation costs. In a retrofit situation, where non-condensing units are being replaced by condensing heaters, there will be a need to install additional pipework to carry the condensate away.
This will result in slightly higher capital and installation costs but these are outweighed by savings in energy costs so that a reasonably fast return on investment can still be achieved. For example, a 140kW output gas-fired condensing warm air space heater will deliver a payback in less than two years, compared to a non-condensing heater with the same output (based on 10 hours per day operation, 5.5 days per week during a typical heating season).
While buildings heated by air rotation heaters are less prone to stratification buildings heated by more conventional systems will experience some degree of thermal stratification, where warm air rises to the ceiling or roof space and displaces cooler air, pushing it downwards into the occupied space. The higher the space the greater the temperature difference will be between the roof and the floor.
For example, in a typical pitched roof building with an average height of 5-7m, the temperature difference between the working zone and the roof space will be around 5°C. This is a clear indication of significant energy wastage, as much of the energy consumed by the heating plant is being used to heat unoccupied areas of the building – namely the roof space.
The higher temperature air in the roof space also has implications for how quickly the building loses heat. For instance, if the set-point temperature for the working zone is 19°C and the outdoor temperature is -1°C the temperature difference between the working zone and the outside is obviously 20°C. However, as noted above, the air in the roof space will be at a temperature of around 24°C, creating a temperature difference between the roof space and the outside air of 25°C. This higher temperature gradient will increase the rate of heat loss through the roof membrane.
A well-designed destratification system addresses all of these issues by ensuring effective distribution of heat throughout the space so that stratification is minimised and there is only a very small temperature gradient between the working zone and the roof space.
In fact, the Carbon Trust estimates that the use of destratification fans in industrial buildings with high ceilings, operating a warm air heating system, can reduce energy consumption by 20%. A simple but highly effective solution is to use destratification fans working in harmony with a warm air heating system to return the warm air from the roof space back to the occupied space.
The ability of the destratification system to reduce the temperature of the air in the roof space also reduces the rate of heat loss through the roof. For example, if the roof space temperature were reduced from 24°C to 20.5°C this would result in around a 14% reduction in the heat loss through the roof membrane.
In many cases improving the distribution of warm air with destratification will make it possible to maintain the required temperature in the working zone with fewer warm air heaters, thereby reducing both capital and running costs. In industrial buildings, the destratification fans will also recirculate the heat from machinery, so that less energy is consumed by the heating system.
Furthermore, improved air distribution helps to eliminate hot and cold spots in the working zone to improve comfort levels. Destratification fans can also be used to provide cooling draughts of air during the summer.
As with so many building services projects, the key is to fully understand how the space is being used and is likely to be used in the future – and how the heating system needs to accommodate that usage. Then it is simply a matter of finding the best solution, perhaps by working with specialists that can steer you in the right direction.
*Phil Brompton is managing director of Powrmatic Ltd.
For further information please visit: www.powrmatic.co.uk