Humidity Control Calculations

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Humidity Control Calculations

  • Load Calculation
  • Pool Evaporation
  • Occupant Load
  • Outdoor Air
  • Load Estimation Software

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As mentioned previously, 50-60% relative humidity levels are ideal for bather comfort but they can lead to condensation problems and serious damage to the building structure. If the building structure itself has not been properly designed for this higher humidity application catastrophic results may occur. The Architect should design and protect the building enveloped based on an indoor dew point design condition.

Controlling humidity to provide this stable dew point condition year round requires that a total moisture load be accurately calculated. This moisture load must be removed from the space at the same rate it is generated in order to maintain stable space onditions.

Load Calculation

Every building's moisture (latent) load is calculated in the same way. There are generally three sources of moisture that are considered:

Pool Evaporation

The internal load in a natatorium is the evaporation from the pool water and wet deck surfaces. In a natatorium this represents the majority of the total dehumidification load. Consequently, it is essential to accurately predict the pool evaporation.

There are 5 variables used to calculate the evaporation rate:

  • Pool water surface area
  • Pool water temperature
  • Room air temperature
  • Room air relative humidity
  • Pool water agitation and Activity Factor

The first four variables are straightforward and should be dictated by the owner. They are used to calculate the baseline (unoccupied) evaporation rate in the natatorium.

The Activity Factor is the fifth variable. It is a water agitation factor. The Activity Factor is used to evaluate how much water agitation and splashing is expected when the pool is in use and how that increases the evaporation from the baseline value. Chapter 4 of ASHRAE's 2011 HVAC Applications Handbook publishes an Activity Factor table (Table 2) based on years of empirical field and test data.

Type of Pool Activity Factor
Elderly Swim 0.65
Fitness Club - Aquafit 0.65
Hotel 0.8
Institutional - School 0.8 - 1.0
Physical Therapy 0.65
Public / YMCA 1.0
Residential 0.5
Swim Meet 0.65
Wave Pool 1.5 - 2.0
Whirlpool 1.0
Table 2 - Activity Factors

Evaporation Rate Equation:

Equation #2 in chapter 4 of ASHRAE's 2011 HVAC Applications Handbook calculates the evaporation rate in pounds of water per hour (lb/h) for air velocity over water @ 10-30 fpm. The Vapor Pressure values can be found in steam tables.

ER = 0.1 x A x AF (Pw - Pdp)
ER = evaporation Rate of water, lb/h
A = area of pool water surface, ft2
AF = Activity Factor (see Table 2)
Pw = saturation vapor pressure at water surface, in. Hg
Pdp = partial vapor pressure at room dew point, in. Hg

It can be seen from the equation that the following factors increase the evaporation rate:

  • Increasing water temperature
  • Lowering air teperature
  • Lowering air relative humidity
  • High activity/agitation

Once equipment has been selected and installed, any change of the variables that increases the evaporation rate can result in equipment no longer being suitable for the new larger load.

Occupant Load

Swimmers are not usually considered occupants as they are submerged in the water. Swimmers and their water agitation are included in the Activity Factor. Spectators, especially in facilities that host swim meets can total several thousand, and add a significant moisture load, (see Table 3).

It is important to understand that when a facility is hosting a swim meet the Activity Factor of the water is considerably reduced. Typically there is only one swimmer per lane and while they agitate the water considerably, the overall agitation is much less than a densely occupied pool during a public swim.

Activity Level Lb/h per Spectator
Quietly Seated 0.155
Moderate Activity 0.205
Enthusiastic 0.250
Highly Enthusiastic 0.530
Table 3 - Occupants Latent Load

To evaluate the dehumidification load during swim meets an Activity Factor of 0.65 is used to calculate the evaporation rate. The total number of spectators and competitors on the pool deck must also include in the load. Codes also generally require that each spectator be provided with 7.5 CFM of outdoor air. The load impact of the outdoor air must also be calculated.

Seresco recommends that facilities hosting swim meets size equipment based on the larger of the two main operating modes, normal operation load or swim meet load.

Outdoor Air

The introduction of outdoor air is essential to maintaining good air quality in any facility. The impact of this outdoor air ventilation on a natatorium changes with the weather and the geographic location of the facility. Introducing outdoor air during the summer generally adds moisture to the space and in the winter removes moisture from the space. For maximum dehumidification load calculations the Summer Design conditions are considered.

Construction codes generally require that outdoor air be introduced into a commercial building during occupied hours. ASHRAE Standard 62-2004 Table 6.1 recommends the introduction of outdoor air into a natatorium at the following rates:

  • 0.48 CFM/ft2 of pool and (wet) deck area as base line
  • 7.5 CFM per spectator added to the base line during swim meets

ASHRAE interprets the wet deck area as no more than six foot perimeter around the pool. The purpose of this outdoor air is to help dilute chemicals off-gassed from water. A predictably dry portion of the deck will not factor into the IAQ issues. Additionally, outdoor air requires considerable heating in the winter.

More is not better. Exceeding code requirements is not recommended. In winter it will significantly increase the operating expenses and in summer may increase the dehumidification load.

Load Estimation Software

Seresco has developed software that calculates all moisture loads in a matter of minutes. Figure 2 gives a snapshot of the basic data that would generally need to be entered to calculate a load.

Figure 2 - Sample of Seresco's Load Estimating Software
Figure 2 - Sample of Seresco's Load Estimating Software