3 reasons for using heaters to cool halls - How and under what conditions this is possible?

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More than half of all devices today are already equipped with cooling coils. Here, the idea crops up of taking the recirculating air heaters often used for large halls and using them for cooling. How and under what conditions this is possible?

With the rising demands for comfort, the proportion of ventilation systems with cooling has also risen: more than half of all devices today are already equipped with cooling coils. This trend towards comfort ventilation also continues in large halls (trade fair centres, multipurpose halls, sports arenas, production halls and warehouses). Here, the idea crops up of taking the recirculating air heaters often used for such large rooms and using them for cooling. How and under what conditions this is possible is described below.

Why cooling?

There are three reasons for cooling large rooms, in particular industrial halls: 

Production
In some cases, production requires that the room temperature remain within a low tolerance. This is the only way to ensure the required precision of the parts to be manufactured. This means that cooling is necessary to regulate the hal temperature. 

Work output
The work output of human beings falls demonstrably at higher temperatures (Fig. 1); at the same time, the frequency of accidents rises. Economic considerations, therefore, beg the question as to whether room cooling makes good sense. The lower the costs involved, the greater the possibility to implement it. 

Work output falls with rising temperature
Fig. 1: Work output falls with rising temperature.

Comfort
Given the positive experience with air conditioning systems in other applications, in particular in private cars, the wish on the part of employees and workers for room cooling, also in production facilities, is understandable. On the other hand, investments are necessary. A compromise between what is desired and the costs involved makes good sense, which once again throws up the idea of using recirculating air heaters for cooling.

Cooling with recirculating air heaters

As a general principle, every heating system can also be used for cooling if, instead of the medium that supplies heat, a cooling medium (e.g. cold water) is used. Here, however, a number of general conditions must be met first or, if this is not possible, resulting problems such as condensate or cooling distribution have to be solved. This also applies to the solution that dominates with regard to heating large rooms: recirculating air heaters. The following must be considered and/or taken into account: 

Free cooling
With a mixed air box upstream of the heating unit, it is possible to feed cool fresh air into the hall and thus provide cooling. The cooling capacity depends on the fresh air temperature, which means that this can only be a supplement to the reduction of operating costs (Fig. 2). 

With a mixed air box, free cooling can be used

2-pipe or 4-pipe system
The consideration here is whether the coil necessary for the heating can also be used for cooling or whether a separate cooling coil is installed. The advantage of the 4-pipe system (= 2 coils) is that each coil has optimised dimensions, but this is offset by higher costs. This is why in the case of industrial applications the 2-pipe solution dominates. 

Condensate
When air is cooled, the saturation limit of the water vapour can be reached and condensate occurs. This condensate has to be collected and drained off; a leak could lead to considerable damage to the building, machines or to production stoppages. While the collection of condensate on units with horizontal air flow is relatively easy using droplet separators, separation on units with vertical air flow is technically challenging. However, as this arrangement dominates for reasons related to energy consumption and the air flow, inventive spirit is required to find a low-cost solution. Practice has shown that – even though it involves years of development and has to be paid for dearly – a solution is possible, proving itself even under extreme conditions, e.g. in the hot and humid Asian climate (Fig. 3). 

Alongside the condensate that arises when the air flow cools down in the cooling coil, there is the additional possibility that room air condenses on the outside of the cool section of the unit. Under certain circumstances, depending on the room air conditions, the unit must be insulated accordingly. Cold bridges should also be avoided. 

Separation of the condensate in the vertical incoming air flow is difficult, but possible

Air distribution
As already mentioned, recirculating air heaters in high halls are usually arranged in such a way that the air flow goes vertically downwards (Fig. 4). This results in a number of advantages:

  • no obstructions for production resources, e.g. cranes
  • no additional space required by the units
  • draught-free air flow
  • low temperature stratification
  • low operating costs 
Air heaters

Fig. 4: Recirculating air heaters with air outlet downwards are usually arranged decentrally and in groups.

If these units are now also to be used for cooling, the problems that can arise are draughts and inefficient cooling distribution. 

However, it is possible to solve or at least minimise these problems:

  • With a (where possible automatically) adjustable air distributor, the air flow can be set as draught-free depending on the supply air temperature; this requires high induction.
  • For efficient cooling, the units should be installed as low down as possible so that the room volume to be cooled is reduced.
  • If possible, the cool supply air can be blown directly vertically downwards to achieve an air flow that is similar to displacement in the occupied area.

As a general principle, however, contrary supply air temperatures (heating and cooling) require a compromise with regard to the air flow.

How to regulate?

There are no problems here, but well-conceived regulation can improve comfort and increase economic efficiency:

  • The operating modes 'heating' and 'cooling' should be separated by a cascade.
  • Whether 2-point or steady regulation is used is a question of comfort and operating costs; the tolerance of the target value is also important.
  • Important is that the target value can be changed depending on the operation time.
  • It makes good economic sense to group together several units into regulation groups in order to be able to better utilise the energy released in the room, e.g. machines.

Summary

Cooling large rooms and halls, in particular industrial halls, must be low-cost; the use of heaters - which are necessary anyway - for cooling is therefore a good idea. In comparison with ventilation systems with central units, no ducts are necessary (even insulated in the case of cooling). The decentralized arrangement that is usual for such units offers the highest flexibility. This applies both to any extension and to the regulation and type of units (power output). In the meantime, more than 20 years of operating experience protect against unpleasant surprises.

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Author
Loris Basso
 
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