Energy-efficient cooling

Published:  22 August, 2006

bristol office
Evaporative cooling to reduce temperatures by around 8 K finds application in both office and industrial environments — as enjoyed by staff in the group head office of Garden & Leisure in Bristol.

Evaporative cooling can be very effective for cooling the indoor environment, particularly large spaces. Its capital and operating costs are also much lower than conventional air conditioning, as STEVE KIRKWOOD explains.

The aim of HVAC is to provide good air quality and a comfortable thermal environment that will ensure occupants’ satisfaction, health and high productivity. The challenge for engineers is to achieve this aim while keeping energy costs low.

Working in high temperatures affects people in both commercial and industrial surroundings. As outside temperatures increase, buildings with inadequate ventilation become unbearably hot and stuffy. This problem is compounded if sunlight penetrates into the building, causing it to behave like a greenhouse.

Not only do such conditions affect staff morale, but they can also lead to absenteeism, reduced productivity and accidents. According to studies carried out by the National Aeronautics and Space Administration (NASA) in 1968 productivity falls by 3.6% for every degree above 22°C and 4.7% over 30°C [ref: NASA report CR-1205 (1)]. In addition, moderate heat stress reduces mental performance by 2%.

How the human body handles heat

As environmental temperatures approach normal skin temperature, cooling of the body becomes more difficult. If air temperature is as warm as or warmer than the skin, blood brought to the body surface cannot lose its heat.

Under these conditions, the heart continues to pump blood to the body surface, the sweat glands release liquids containing electrolytes onto the surface of the skin and the evaporation of the sweat becomes the principal effective means of maintaining a constant body temperature.

Sweating does not cool the body unless the moisture is removed from the skin by evaporation. Under conditions of high humidity, often experienced in the UK, the evaporation of sweat from the skin is decreased and the body’s efforts to maintain an acceptable body temperature may be significantly impaired.

With so much blood going to the external surface of the body, relatively less goes to the active muscles, the brain and other internal organs. Strength declines, fatigue occurs and alertness and mental capacity may be affected. Workers who must perform delicate or detailed work may find their accuracy suffering, and others may find their comprehension and retention of information lowered.

Studies carried out by Langkilde, 1978, found that the optimum industrial work temperature to be 17 to 24°C, whilst the optimum temperatures for arm movements is 21°C. Other studies by the British Industrial Fatigue Board found that there was a lower output in hot/heavy industries in summer than winter and an increase in accidents when air temperature deviated from 20°C.

Office environments

Poor air quality in offices can have an enormous economic impact on costs due to lost productivity. Improvements to the workplace environment are a highly cost-effective way of enhancing employee satisfaction, productivity and the company bottom line. It has been shown that office task performance becomes slower as temperatures rise above 21°C. In a study carried out by Niemelã et al. (2001) it was found that the productivity of workers in call centres decreased by 1.8%/K when the temperature was above 25°C.

Whilst heat has a debilitating affect on a workforce, refrigerative air conditioning necessitates sealing a building — resulting in reduced ventilation and poor indoor air quality. The requirement to provide a healthy workplace environment under health-and-safety legislation is an ongoing problem for many employers. It is now known that all sorts of airborne contaminants are present in the workplace. Gases, vapours, dusts, fibres, pollens, insect fragments, bacteria and fungi and their spores, plus viruses all contribute to indoor pollution. Certainly many of the pollutants can adversely affect the health of occupants.

It is now thought that natural ventilation is the safest, most economic and healthiest way of introducing cooling into offices. Whilst opening windows on days when temperatures are below 20°C can produce a cooling effect, once outside temperatures exceed 23°C only warm air is being introduced.

The Workplace (Health, Safety & Welfare) Regulations 1992 state that workplaces should be adequately ventilated. For a workplace to be adequately ventilated, clean air should be drawn from outside and circulated around the building. The ventilation system should remove and dilute warm humid air and provide air movement — which should create a sense of freshness without causing draughts. Humidity and ventilation should be maintained at levels to prevent discomfort or problems of sore eyes, especially if display screen equipment is used.

Evaporative cooling accomplishes these requirements. First, it cools the air, producing a more comfortable thermal environment. Secondly, it adds water vapour to the air, increasing relative humidity and relieving symptoms such as dry skin, nasal and throat membranes. Third, it reduces the vapour-pressure deficit (VPD), which is the force that evaporates water from the environment.

Evaporative cooling reduces the temperature of the air by natural evaporation as it passes through a pad saturated with water. As water is evaporated, heat is drawn from the air to reduce its temperature. The major components of this system are water-saturated cooling pads, water supply and pump, water-distribution system, water reservoir, and supply-air fan.

Evaporative-cooling systems such as Breezair systems offer an automatic drain valve that keeps the system clean and is pre-programmed to automatically drain stale water from the reservoir and replace it with fresh. When the machine is switched off, the drain valve will automatically empty the water from the reservoir after a fixed predetermined time, leaving it clean, dry and healthy. At the same time a ‘weather-seal will close off the ducts to prevent dust entering the building and to minimise cold draughts.

environments

Providing cooling for workers in industrial environments to improve productivity and reduce absenteeism from heat stress has previously been perceived as financially prohibitive. Instead, companies have chosen to reduce heat and humidity by opening widows, using fans or creating airflow through exhaust ventilation or air blowers. Whilst in some cases this has gone some way to improve conditions, most workers report that such measures have not made a marked improvement on their working environment.

Now, however, many companies are turning to evaporative air-conditioning systems as a cost-effective solution to providing environmental cooling. For a fraction of the cost of air-conditioning to install and run, evaporative cooling can be utilised for spot cooling or to cool a complete building. By reducing temperature by about 8 K, evaporative cooling can have a marked effect on a company’s bottom line by increasing productivity, reduced operating temperatures, decreasing absenteeism and accidents.

It is not just during the summer that evaporative cooling is needed. High internal temperatures in factories, warehouses and office complexes effect workers and productivity throughout the year. Evaporative cooling provides all businesses with a cost-effective solution that helps increase production, decrease downtime and improve workers’ morale.

Seeley International (Europe) Ltd., Blythe Valley Innovation Centre, Central Boulevard, Blythe Valley Park, Solihull B90 8AJ.



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