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conditions themselves — the moisture content of the air that you require — determines what method you need to add or remove moisture . In addition , there are always heat recovery methods to employ , says John Andersen , director of Specialised Climate Engineering .
“ The choice of system really depends on the space and what you require in that space : if it ' s people , there ' s specific criteria ; if it ' s materials , there are different criteria , because most materials have moisture isotherms . Then depending on the type of material , whether it be for example a museum , archive , chemical store or goods store , one has to appreciate that one has to not only control humidity for people but also the materials stored there ,” explains Andersen .
He notes that most material has an equilibrium ( isotherm ) on the basis of temperature and humidity . “ Raise the temperature and humidity and the material will re-establish an equilibrium at a higher moisture content . Examples are that nylon contains an enormous amount of water statically , while for polyethylene bottles you have to remove as much of the moisture as possible , or there can be quality problems .” Decay of products is associated with high moisture while with too low moisture , materials can become brittle , while static electricity issues also prevail .
Andersen explains that regarding human occupation of buildings , there are both SANS standards and ASHRAE standards . “ There is a climate ‘ window ’ associated with comfort : the right temperature and the right humidity . There can be low temperature with high humidity , or alternatively higher temperature with lower humidity . For humans it ' s all about how humidity evaporates from our skin , called vapour pressure deficit or difference , which gives us our comfort . Having the right temperature is important from an energy perspective , but having the right humidity is even more so ,” says Andersen .
“ However , controlling moisture is expensive , for instance requiring the adding of water in winter . The phase change energy when adding water to air using adiabatic methods is still the cheapest way of adding water to air , for example ,
“ Humidity conditions can vary sharply according to location - not only inland or coastal factors but differing regional weather cycles — thereby challenging the engineer to implement the correct affordable solution .” in a large occupied building space . From a fresh air makeup perspective , the fresh air rate that gets pressured into an occupied building should also be extracted with heat recovery — we don ' t do this in South Africa . What is ideal is to install what is called an air-to-air heat exchanger so that the air going in absorbs 80 % of the energy value of the stale air going out of the building .
“ Say , for example , our dry highveld climate is typically at four degrees and 90 % humidity , for roughly about five grams of moisture content per kg of air . But occupants want about 20 degrees at about 55 %, which is around nine and a half grams of moisture . There ' s a four and a half gram difference of moisture content . If one simply brings that air in from outside to heat it up , it then dries people ’ s eyes or sinuses making the occupants prone to infections . Therefore , moisture needs to be added and this can be expensive if using steam as an example .”
Andersen notes that adiabatic methods ( or free energy ) are much cheaper in terms of energy consumption . Also useful is where there ’ s a refrigeration plant , heat pump or solar energy whereby excess energy like hot water can be brought in as pre heat , with the pre heat allowed into any evaporative method . It gets even better if you use an air to air heat exchanger , where your in-air and out-air is in the same AHU physically transferring the higher energy value to the lower energy value . In winter it would be a balance between the air leaving for exhaust versus the air coming in which is fresh . “ That ' s a wonderful clean building philosophy in which at the same time you ' re recovering 80 % of the energy used .”
DEHUMIDIFICATION : SOME PHILOSOPHIES SURROUNDING REMOVING MOISTURE Dehumidification is not only a winter issue in wet climates like the Cape , but ironically a summer dryness one too .
In winter , with wet conditions and cold climates as well as hot very humid conditions , the typical experience at home , or in the commercial business space is normally corrosion , mould growth and decay — which can be expensive from a health and maintenance perspective . The cost of dehumidification is significant from a Capex perspective , but the benefits are significant , making the introduction a worthwhile Opex saving .
In summer , in very humid environments , dehumidification internally using the air-conditioning method within the building space leaves the occupants with some extreme humidity relief . However , it is often necessary to supplement this dehumidification with a dedicated drying system that brings the moisture level into the comfort window , ensuring health and a corrosion or decay free environment . Fresh air make-up systems in these humid environments require
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RACA Journal I March 2023 www . refrigerationandaircon . co . za