Sizing and selection of equipment for dehumidifying and air-conditioning swimming pools

How to Choose a System for Dehumidifying and Air Conditioning Pools

When selecting and sizing a dehumidifier for indoor pools, it is essential to carefully consider the moisture absorption capacity of the outdoor air that will be injected into the space.

On one hand, the Technical Building Code for Thermal Installations in Buildings (RITE) (R.D. 1027/2007 of July 20) clearly specifies that the ventilation airflow rate in indoor pools must be 2.5 dm3/s•m2 of the space, excluding stands. Whether or not this value is appropriate for all pools, and regardless of the method (fully documented) used to determine it, it is a standard that must be met and ensures that pollutant concentrations (chloramines, CO2, etc.) remain within limits approved by health authorities.

On the other hand, a mixture of indoor/outdoor air supplied to the pool space can absorb some of the water vapor generated, as long as the specific humidity of the supplied air is lower than that of the space by an amount that depends on the airflow rate and the specific humidity of the environment. As detailed by Pedro Torrero Gras in his DTIE 10.04 ‘Indoor Heated Pools with Outdoor Air as the Only Dehydrating Medium,’ published by Atecyr, it is even possible for all the water vapor to be absorbed if the airflow is sufficient and the specific outdoor humidity is low enough.

Choosing a System: A Question of Profitability

Using outdoor air to aid in dehumidification in indoor pools is possible throughout Spain and Europe; the question is how many hours per year it can be used, and whether it is profitable to use it during all those hours.

In locations within Spain, particularly in the central plateau, the outdoor air supply mandated by RITE may be sufficient to dehumidify the indoor air for thousands of hours each year. The low specific humidity of outdoor air during cold periods allows it, when mixed with indoor air, to absorb the humidity generated by the pool and its occupants.

In these cases, conventional dehumidifiers, which are not designed to utilize outdoor air support, shut down their compressors and become a simple (and very expensive) air handling unit (AHU), with their hot water coil responsible for heating the area.

Furthermore, the number of hours per year during which dehumidification can be achieved by increasing the outdoor airflow rate is considerably higher. Referring again to DTIE 10.04, I fully agree with Pedro Torrero’s recommendations: a ventilation rate between 4 and 7 air changes per hour and an outdoor air specific humidity 2 to 3 g/kg lower than the pool area’s interior specific humidity allows dehumidification at almost any location in Spain under “average” or “typical” conditions. Only in particularly warm and humid areas, such as the Mediterranean coast, and only during summer (if the indoor pool is in operation), would it be necessary to use mechanical dehumidification during the daytime period.

Temperature: An Essential Factor to Consider

In this approach to dehumidification using outdoor air, it is crucial to consider temperature. Introducing outdoor air can negatively impact the indoor air temperature during colder months. Therefore, using this system requires a high-efficiency heat recovery system.

If the recovery system consists of air-to-air exchangers and a heat pump integrated with them, forming a regenerative refrigeration circuit, it creates an exceptionally energy-efficient and cost-effective system. In addition to using the drying effect of lower outdoor humidity, it also harnesses the latent heat of condensation to produce sensible heat, minimizing the need for thermal groups to maintain thermohygrometric conditions in the pool area.

Dehumidification Approach

To achieve energy savings, it is crucial to move away from the “brute force” approach of compressors and adopt a more technologically advanced approach where energy efficiency (or the lack thereof) plays a significant role. This strategy allows refrigeration equipment to be more versatile, adapting its dehumidification function to produce heat at optimal times. For this, it is also necessary to incorporate a fully integrated control system that maintains balance among the equipment’s various functions under each climate condition and at each operating point.