Fig.1: Due to attire and activity differences, comfort criteria is different for indoor pools than traditional spaces

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1 Sizing and Selecting Indoor Pool Dehumidification Units Just like for any indoor environment, designers of indoor pool areas (natatoriums) have the goal of designing a system that will provide good indoor air quality, energy efficient operation and environmental comfort for the occupants. The criteria for these spaces however is somewhat different and more complex than traditional indoor building spaces since the occupants are in bathing suits with evaporation effects occurring on the skin, there is an irritating chemical concentration in the air and finally there is a large tank of water evaporating and releasing huge amounts of water vapor into the air which can cause severe damage and corrosion to the structure due to moisture condensation. Fig.1: Due to attire and activity differences, comfort criteria is different for indoor pools than traditional spaces Similar to the traditional HVAC project, one of the first steps towards achieving the design goals above is to accurately calculate the load on the mechanical system so as to be able to determine the equipment capacity

2 required and the optimal equipment size and configuration needed. The load calculation methodology and parameters used are more numerous and somewhat different than for calculating traditional HVAC loads and engineers may not always be aware of how the various design parameters affect the resulting heating, cooling and most importantly, dehumidification load and equipment size. With a typical desired space of temperature of 80F 85F, cooling loads tend to be less because the natatorium space is kept at a warmer temperature and conversely heating loads will be higher. Furthermore, when calculating the critical dehumidification load, there are additional, important parameters that affect the load such as water temperature, relative humidity, pool activity level, number of spectators and additional outside air requirements. DESIGN SPACE CONDITIONS Since designers are well acquainted with how to calculate heating and cooling loads, we will focus on the dehumidification load. When designing to prevent condensation from occurring and damaging the building structure, one of the first concepts to keep in mind is that the designer really needs to evaluate and then design for the space dew point required to ensure that the vapor barrier location will be at a point in the wall that will be warmer than the dew point so that moisture will not be able to travel through the wall and condense inside it. Figure 2 below shows a wall section with the vapor barrier installed properly and at a location which provides a comfortable 5.5 degree safety margin between the temperature at the location of the vapor barrier and the location in the wall where the wall temperature would drop to the 64.5F dew point corresponding to the space conditions of 85F & 50% RH. Fig.2: Vapor barrier location relative to dew point

3 The key point is that the mechanical system designer must coordinate with the architect and determine the wall temperature at the location of the vapor barrier and then select space conditions that will provide a design dew point at least 3 5 degrees lower. If the vapor barrier in the example above was instead installed at a location in the wall that resulted in a 64F temperature at the vapor barrier, then the designer must adjust the design dew point by lowering either the design space temperature, design relative humidity or both to prevent moisture condensing in the structure and causing corrosion to the point that catastrophic structural failure can even occur. Basic psychometrics quickly shows that in order to have a design dew point of 60F at 50% RH, the space temperature would have to be lowered to 81F. At the windows, depending on the quality of frames, number of panes and U value, the surface temperature at least in some areas of the window assembly may still occasionally be colder than dew point on cold winter days and surface condensation will occur. The way to address condensation on the windows and metal window and door frames is not to design for an impractically low dew point that would prevent condensation on these surfaces at all times but to instead ensure that conditioned, dehumidified air is blown across these surface. Three to five CFM of supply air should be directed at these surfaces to prevent condensation. For any indoor environment, relative humidity is always recommended to be kept between 40% 60% to minimize bacteria, fungi, and virus growth. However for pool environments, the requirement is a little tighter with 50% 60% RH being the recommended limits for occupants to feel comfortable and space temperatures are likewise generally kept between 80F to 85F. Often times there are owner preferences that need to be inquired about when it comes to the pool area temperatures but lacking any specific information from the owner, table 1 below can be used for selecting the indoor space temperature and relative humidity (and thus design dew point). Typically the space temperature is selected to be several degrees higher than the water temperature to minimize evaporation. Activity Air Temperature Water Temperature Relative Humidity Conditions Residential 72~ 85 (22~29) 75~ 90 (24~32) 50 to 60% Therapeutic 80~ 85 (27~29) 85~ 95 (29~35) 50 to 60% Competitive Swim 78~ 85 (26~29) 76~ 80 (24~27) 50 to 60% Whirlpool / Spa 80~ 85 (27~29) 97~104 (36~40) 50 to 60% Elderly Swimmers 82~ 85 (28~29) 84~ 88 (29~31) 50 to 60% Aquafit Programs 82~ 85 (28~29) 82~ 86 (28~30) 50 to 60% Hotels 82~ 85 (28~29) 82~ 86 (28~30) 50 to 60% F ( C) Table 1: recommended air, water and relative humidity levels for various pool types. OUTSIDE AIR & INDOOR AIR QUALITY ASHRAE revised the amount of outside air recommended for Natatoriums down from 0.5 CFM per square foot of pool surface and wet deck area and 15 CFM per person down to 0.48 CFM per square foot of pool water surface and some other rate, generally 0.06 CFM per square foot of overall deck and floor area. Outside air for spectators is also now to be included at 7.5 CFM per spectator. An exhaust fan, either unit mounted or remote should then exhaust enough air so that the pool area is kept at a 0.05 to 0.10 w.c. negative pressure relative to the rest of the building areas. For indoor pools however, bringing in the code required amount of outside air, the so called dilution solution is just part of the strategy needed to maintain good indoor air quality. Designers must also be concerned with a

4 unique situation that occurs in natatoriums where the byproducts of chlorine based disinfectants can cause significant irritation to pool occupants. Trichloramine vapor is produced when chlorine combines with nitrogenous organic material in the pool water (primarily given off by humans) and is the primary compound responsible for pool IAQ problems causing significant physiological responses in humans ranging from sneezing, coughing, eye irritation to breathing difficulty and increased risk for asthma. Concentrations as low as 0.5 mg/m 3 are noticeable and irritating to humans and the concentration of trichloramine increases as the activity level and agitation of the pool surface i.e. kicking, splashing increases in the pool. One of the difficulties with effectively removing trichloramine is that it is also denser than air and accumulates directly at the surface of the pool and below the deck creating a bubble in this area which coincidently is also the breathing area of the swimmers in the water. Outside air dilution along with water based methods such as UV treatment of the pool water can help reduce the levels of trichloramine and other types of combined chlorine but research has shown that these methods alone are generally not completely effective. One proven and effective solution offered by Dectron is the use of their gas phase chloraguard filtration system (fig.3 below) combined with optimizing the air distribution over the pool surface to maintain a low velocity, unidirectional flow of air across the pool surface to a low mounted return located as close to the pool surface as possible. Fig. 3: CHLORAGUARD FILTER MODULE Often a 1/3 & 2/3 split is used for the return air grille placement where 1/3 of the return air is through a low wall return and the other 2/3 of the return air is through a high wall return. Directing supply air across the pool surface is a very fine balancing act however and air of too high a velocity can significantly increase evaporation and drive up the dehumidification load. The average velocity over the pool surface should be kept at below FPM. For the entire pool enclosure, an overall supply air rate of 4 6 air changes for non spectator pools and 6 8 ACH for spectator pools is generally recommended to help prevent stagnate zones of air. For jobs of sufficient size and scope, Michigan Air working with Dectron can perform CFD modeling of the natatorium air distribution to ensure good airflow patterns across the water surface, windows, doors and the entire space. Finally if the pool area includes a whirlpool, the high degree of water agitation also generates high trichloramine concentrations at the whirlpool and the pool exhaust air should be taken from directly over or next to the whirlpool (the warmer air over the whirlpool surface tends to help carry the trichloramine vapor upward toward an overhead exhaust outlet in this case). DEHUMIDIFICATION LOAD CALCULATION Now that we know our design room conditions, pool water temperature, outside air required, type of pool and overall approximate supply air amount required (3 to 5 CFM per square foot of glass, skylight, exterior door and enough to provide 4 8 air changes an hour) we can proceed to calculate our dehumidification load which depends on three variables; 1) evaporation from the pool surface 2) moisture from outside ventilation air and 3) moisture from occupants. Note that in the winter, the dry, cold outdoor ventilation air is often a credit against the dehumidification load and in the summer it will add to the load.

5 Although a hand calculation can be done, Dectron relies on equipment sizing software to calculate the dehumidification load and the following discussion is meant to provide a general understanding of what is involved in the load calculation process. The ASHRAE handbook includes evaporation rate tables that are used to estimate pool evaporation. The evaporation tables used by ASHRAE are calculated based on the model developed by Dectron. Furthermore, Dectron s Activity Factor table of measurement, shown in table 2 below has become the industry standard. Table 2: Activity Factors For Different Pool Types The Activity Factor is one of the most important factors used to accurately predict the dehumidification load. When combined with the evaporation rates below and the equation: EVAPORATION RATE (lb/hr) = ERF x AF x Pool Water Surface Area (ft2) Where ERF = evaporation rate factor (from table 3) & AF = activity factor (from table 2) Table 3: Evaporation Rate Factors There are several relationships that quickly become apparent and are worth keeping in mind: The Evaporation Rate increases as water temperature increases The Evaporation Rate increases as space RH decreases The Evaporation Rate increases as activity level increases The Evaporation Rate increases as space temp decreases The Evaporation Rate increases as the difference between water temp and the warmer room temp decreases

6 In additional to the evaporation rate as determined above, the effects of the occupants and outside air then needs to be calculated and added to the evaporation rate to determine the total dehumidification load. Moisture contributed by people can be found in any design manual but recognize that the latent moisture shown is for indoor conditions below 80F and that for indoor pool occupant latent values should be increased by 20% due to most pools being several degrees or more above 80F. 190 btu/h per person is generally a good value to use. The amount of moisture contributed by ventilation air can be calculated by the formula: Ventilation Moisture = [CFM OA x (grains OA grains RA )] / 1555 ENERGY EFFICIENCY As water evaporates, it cools the water in the remaining body thus requiring the pool water to be heated in order to retain it s temperature. In this manner, the energy a pool loses through evaporation represents approximately 95% of its annual water heating requirements. Dectrons Dry o tron model captures this heat by rejecting the compressor heat to a co axial heat exchanger that heats the pool water rather than rejecting it to the atmosphere. A thermal fly wheel thus occurs that returns the energy lost through evaporation back to the pool water. The process s energy cycle is 100% efficient since all the moisture is converted into sensible heat for recycling. When the pool water is already at temperature and dehumidification is occurring, the unit will instead use the hot gas for free reheat (or can simultaneously heat the pool water and recycle heat back into the air for reheat). If only straight cooling is required by the space, and the pool does not need heat, only then will the heat be rejected to either an air cooled or water cooled condenser. Typically 80% of the pool water heating cost can be eliminated in this manner. The efficiency of this system is far superior to any of the 100% outside air systems that are also occasionally used for conditioning pool areas. At first glance, the thought of using 100% outside air to condition a pool might seem advantageous but let us dig a little deeper into this. First keep in mind that ASHRAE recommends that a pool area be kept between 50%RH and 60% RH. At humidity levels below that, the evaporative effect on swimmer s skin as they exit the water will have a chilling effect. Humidity levels above that will be uncomfortable to occupants, promote the growth of fungi and bacteria and make condensation very difficult to control. If 100% outside air is used, there will be periods of time when the outdoor air will have more moisture in it than the indoor environment and which will then cause room RH to rise to potentially damaging levels. Likewise, winter air may contain so little moisture that the indoor environment will drop below 50%. Beside the inherent inability to control the indoor environment during the full range of outdoor weather, there is also a huge energy penalty. Outside air in amounts required to provide good air circulation for pool area (4 8 air changes an hour of supply air) represents a very large heating load and energy input, even if energy recovery is used. Secondly, the dry winter air will drive up the pool evaporation rates to very high levels. For example if we had a pool that measured 75 x 25 in a dry winter climate with an average of 30% RH, the pool would evaporate at a rate of 85 lbs/hr. If the space was maintained instead at 50% RH, the evaporation decreases to 52 lbs/hr. The high evaporation rates associated with 100% outside air systems will therefore result in higher pool water heating costs, larger amounts of make up water required and higher chemical treatment costs. The most effective, energy efficient way to maintain appropriate relative humidity in a pool environment is a refrigeration based dehumidification system that rejects it s heat to the pool water providing free pool water heating up to 80% of the time. Your Michigan Air Products Sales Representative can provide a project specific energy analysis for your pool project demonstrating the savings. CONFIGURATIONS

7 Once all the dehumidification, cooling and heating loads are determined along with the required amount of supply air, designers must finalize the unit selection by deciding upon the unit configuration best suited for the project. Dectron offers a wide variety of unit configurations to suit any Natatorium application. The equipment drawings below illustrate some of the different styles of units that are available. A brief description is provided for each. Further unit customization is also possible to suit special project requirements. Since its first unit installed in 1977, Dectron has seen it all and done it all when it comes to pool units and their un equaled experience designing the most energy efficient units for this demanding application makes them the best choice for your pool projects. If you are involved with a pool project, please contact Michigan Air Products so we can assist you! BASIC DRY O TRON UNIT: EXHAUST FAN IS REMOTE AND HEATING MAY BE EITHER UNIT MOUNTED OR IN THE DUCT SMART SAVER: UNIT MOUNTED EXHAUST FAN WITH A PASSIVE OR GLYCOL HEAT RECOVERY LOOP BETWEEN OA AND EA

8 ECONOMIZER: AIRSIDE ECONMIZER COOLING IS PROVIDED TO REDUCE COOLING ENERGY REQUIRED ECONOSAVER (ABOVE LEFT): REDUCES THE ENERGY REQUIRED FOR HEATING BY RECVOVERYING HEAT FROM THE REFRIGERATION SYSTEM. PURGE (ABOVE RIGHT): A SEPARATE PURGE FAN IS USED TO QUICKLY PURGE THE SPACE FLOOWING SUPER CHLORINATION TO MINIMIZE THE TIME SWIMMERS MUST BE OUT OF THE NATATORIUM

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