Dry coolers: do they provide free cooling?

Dry Coolers are often the heat rejection of choice, since on the face of things they appear to provide free cooling, with little or no running costs. Primarily used to reject heat from air conditioning plants, they are often chosen in preference to wet systems, as they negate the risk of bacterial growth (e.g. Legionella) and require a minimal level of maintenance.
While Dry Coolers may appear to provide free cooling, are they always the most energy efficient option – after all, there’s no such thing as a free lunch!
In my view, the only free cooling available is natural ventilation, through a window, but even that comes at a price as you have to buy the window! It may seem that compared with an external chiller, complete with lumpy compressor, dry cooling is always a better option. In some instances however, a Dry Cooler is not the most effective system and the following guidelines are aimed to help those responsible to make informed choices.
A Dry Cooler may not be the best option for providing heat rejection to the vapour compression cycle because energy is required to blow air across the Dry Cooler heat exchange surface, as well as to pump water through it – so it’s not free. In addition the resultant off-coil water temperature will always be significantly higher than the ambient, causing an increase in the heat of rejection temperature. Consequently, the internal compressor will have to work harder than if it were outdoors and in direct contact with the air.
For instance a 5ºC rise in water temperature over the ambient air will mean vapour compression devices have to work 8-10% harder which means that they are less efficient!
Heating may also be required, in which case either a separate heat source is necessary (e.g. a boiler, costing additional capital costs), otherwise a further significant drop in efficiency will occur. To enable thermal exchange, the heating medium will again be 5°C or more below ambient if a heat pump is used.
Dry Coolers come into their own when used for high temperature fluids/functions, or as a night-time heat dump when the ambient air temperature is much lower, creating greater temperature differences and providing a better heat exchange – applicable in high thermal mass type systems.
As a rule of thumb, the means by which heat may be rejected to vapour compression systems, using instantaneous, not dynamic modelling (i.e. thermal mass), are listed below, in descending order of efficiency:

  1. Open Loop Ground Source (typically 10-12ºC)
  2. Cooling Towers: using wet bulb temperatures (typically 20-25ºC)
  3. Closed Loop Ground Source cooling (if water temperatures are kept below 30ºC)
  4. Wetted surface adiabatic closed circuit coolers (i.e. wet Dry Coolers)
  5. Direct Expansion at the outdoor unit (providing distribution losses have been taken into account, so the air temperature is 30ºC and condensing temperature 45ºC)
  6. Dry Heat rejection (where water temperatures are typically at 35ºC +).

Like everything in life, it is important to choose the right tools for the job. At Colt we advocate approaching each HVAC situation in a bespoke manner, helping consultants and building designers create the best possible solutions for the individual project.

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