Specifications

7
HUMIDITY CONTROL CALCULATIONS
General Notes:
• Discuss the planned operation of the facility with the
operator to establish operating temperatures.
• Facilities with warmer water temperatures tend to
have warmer space temperatures.
• Physical therapy facilities will often cater to therapist
comfort rather than the patient. The patient is
generally not in the space for more than an hour,
whereas the therapist is there all day. The designer
should consult local codes. Some States require a full
purge of the clinic room air with 100% outdoor air for
every hour of occupancy.
• Elderly swimmers tend to prefer much warmer air and
water temperatures.
• Maintain Relative Humidity between 50% & 60% RH.
Do not go below 50%.
Humidity Control Calculations
While warm space temperatures and 50-60% relative
humidity levels are ideal for bather comfort they also
translate to high dew point conditions which 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 indoor dew point
application catastrophic results may occur. The rchitect
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 conditions.
It is imperative that the designer know what the owner’s
operating temperatures will be in order to properly
establish loads.
Load Calculation
Every building’s moisture (latent) load is calculated in the
same way. There are generally three sources of moisture
that are considered:
• Internal load (pool evaporation)
• Occupants
• Outdoor air load
Pool Evaporation
The internal load in a natatorium is the evaporation
from the pool water and continuously wet 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.
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
Table 2 – Activity Factors