Research And Insight

Condensing-boiler

A condenser is used in a cooling system, or it can be part of a condensing boiler. In a cooling system, the condenser removes heat from cooling medium that flows through it. The cooling medium can be one of several coolants. Heat from the evaporator and compressor of the cooling system is transferred to the coolant before it reaches the condenser. When the coolant flows through the condenser it is cooled and condenses, i.e. it changes phases from gas to liquid form.

Heat is typically removed from the condenser using either air or water. Thus, condensers are generally either air-cooled or liquid-cooled. An air-cooled condenser is supplied with cooling air by a fan, whereas a water-cooled condenser is supplied with cooling water by a pump.

There are many different designs of condensers. An air-cooled condenser is typically equipped with copper or steel pipe, onto which aluminium fins are fitted. The cooling medium flows through the pipes, transferring the heat to the fins, which in turn are cooled by passing air. A water-cooled condenser often has an elongated tank with a built-in pipe system. The cooling medium flows around the pipes in the tank, while cold water flows inside the pipes, extracting the heat from the cooling medium.

A condensing boiler is a boiler provided with a condensing device designed to recover energy normally discharged to the atmosphere through the flue. Water vapour is produced by the burning of gas or oil in the boiler, where it condenses back into liquid water. The energy contained in the vapour would without a condenser normally be discharged to the atmosphere through the flue.

When a condensing boiler is working at peak efficiency the water vapour produced by the burning of gas or oil in the boiler condenses back into liquid water. The boiler uses a heat exchanger so that incoming air or water cools the exhaust, forcing the condensation of the water vapour it contains; this heats the incoming air (if an air-to-air heat exchanger is used) or pre-heats the water (if an air-to-water heat exchanger is used).

A small proportion of the extra efficiency of the condensing boiler is due to the cooling of the exhaust gases, but the majority of the energy recovered is from the condensation of the water vapour in the exhaust gases. This releases the latent heat of vaporisation of the water: 2260 kJ/kg (970btuh/pound) of condensate, i.e. the water vapour released whenever one burns fossil fuels.

The actual operating efficiency of a condensing boiler depends on the ambient air temperature and the relative humidity. If the incoming air is at 100 % relative humidity, the condensing boiler will operate at its maximum efficiency, as it can condense all the extra water vapour introduced by combustion. As the relative humidity falls, so will the actual efficiency of the condensing boiler, because less of the water vapour produced can be recovered from the exhaust.

Grundfos supplies circulators as condenser pumps and for condensing boilers.

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