Article: Winter Operation
CHANGING OVER TO WINTER OPERATION NEED NOT CREATE A CHILL
Swimming is no longer an activity relegated to the warmer, summer season, and a greater number of indoor pool facilities and waterparks are being built, especially in the colder northern part of the country. Today’s newer facilities are designed and constructed with high ceilings, glass walls and dramatic architecture, and include water slides, flumes and many spray-type toys for year-round enjoyment. The key is to maintain summer-like conditions on the inside regardless of the weather outside, and to do it cost-effectively.
Studies have shown that bathers are more tolerant of higher humidity and air temperatures. As the space and water temperature drops, even with ideal humidity, discomfort sets in. That’s why it is critical to manage these variables closely during colder seasons or risk unhappy bathers.
Understanding your pool environment and equipment can reduce operational cost
The poolroom environment is in a constant state of flux as outside and inside variables change season-to-season. Outside temperatures affect the inside heat loss/gain sometimes resulting in the need for more energy to maintain occupant comfort. For a majority of indoor pools, regardless of geographic location, water and space heating are required 70% to 90% of the year.
Water/air temperatures and humidity levels must be within a narrow range for occupant comfort. Ideal water temperature is around 82° with the air temperature slightly higher to prevent chilling once exiting the pool. The desirable humidity range is generally 50% to 60%--greater than 60% creates a sticky feeling and/or difficult breathing, and low humidity results in evaporative cooling on the bather’s skin, resulting in a chill.
Maintaining ideal environmental conditions has a fairly high cost of operation, even during the best of times. Operation during the very cold winter months can greatly increase heating costs due to rapid evaporation rates and heat loss through the structure.
Controlling Evaporation
Evaporation takes place as the pool water is heated: the higher the temperature, the greater the evaporation rate. That evaporation rate is also influenced by the difference in temperature between the water and air, or vapor pressure. The difference in the vapor pressure of the water molecules of the warm water compared to the air vapor pressure creates evaporation. If the air is cooler than the water, the evaporation rate is higher. A good example can be seen in nature, when a warm pond steams on a winter morning. The evaporation rate is also increased through agitation. As you increase the water surface area exposed to air, you increase evaporation.
Evaporation removes heat from the water, which causes the pool water to cool. To maintain water comfort conditions, additional energy is needed to sustain the water’s original set points. In most pools, heat energy must be added back into the pool water or it will cool down below a comfortable temperature.
And of course, with evaporation there lies the possibility of condensation formation. Water vapor will return to a liquid as it comes in contact with cold surfaces. In most cases, the condensate includes chloramines, a corrosive byproduct of the pool chemistry. Chloramines are very destructive, and aggressively attack metallic surfaces including furniture, fixtures and structural components.
Your dehumidification system type will dictate your options
The most common technologies employed in pool room dehumidification equipment are outside air-based (OA) and mechanical refrigeration-based systems. These packaged dehumidification units are built for pools and incorporate more sophisticated mechanical design and software than conventional systems to minimize the operation cost.
Outside air- based dehumidifiers use ventilation to remove moisture. The volume of air needed is a function of the amount of moisture to be removed and the moisture content of the incoming air. Depending on your geographic location, summer conditions within a pool facility will generally be warmer with reasonable control of moisture.
During winter months, the moisture content of the outside air is generally lower, making it very suitable for dehumidification. However, the outside air temperature is much colder and requires constant heating to maintain comfort conditions within the facility. If left unchecked, the heated air will be constantly exhausted while the incoming air will have to be heated, escalating operational cost.
To be practical for winter operation, OA systems should be equipped with a heat exchanger within the exhaust circuit to capture heat for tempering the incoming air stream. The cost savings can be significant, with improvements up to 50% as compared to units without a heat exchanger.
Another winter operational strategy is to reduce the ventilation rate during off-peak hours. Humidity setpoints are generally established to maintain desirable levels during peak operational times where the rate of evaporation is high. By reducing the amount of ventilation to meet the lower evaporation rate during off-peak hours, you reduce fan operation as well as the heating requirements. Savings generally are significant, realizing up to 50%, as compared to constant operation day and night.
Dehumidification systems utilizing a refrigeration system have better overall control of dehumidification and temperature compared to OA systems but generally have a higher first cost. The mechanical dehumidifier method uses electricity as the primary energy source and offers a reclaim mechanism in the form of a heat pump cycle, which adds the heat back into the air and water. Thermal COP’s (coefficient of performance) of 5 are not uncommon, and this method offers the best control regardless of location and climate.
Mechanical dehumidifiers recover more of the latent heat from pool evaporation than passive heat exchangers, offsetting the initial cost of the unit. It is not uncommon for a heat pump system to satisfy the heating requirements for a majority of the year, using an auxiliary heating system only when outside temperatures are very cold. Where electricity cost is low compared to other fuel types, mechanical dehumidification is attractive.
Like an OA system, mechanical dehumidifiers need to ventilate the space to meet code requirements. These systems can likewise benefit by using a heat exchanger to capture heat from the exhausted air stream. Furthermore, mechanical refrigeration units using sophisticated control systems can constantly monitor inside and outside temperature and humidity setting, choosing the least costly method to control the setpoint conditions.
During winter operation, heat loss is high through walls, windows and roof, and must be replenished to maintain user comfort. Cold surfaces also cause condensate formation as humid air comes in contact with them. The method of controlling condensate formation is to maintain a humidity level lower than the dew point, that temperature at which water changes from vapor to liquid.
Cold temperature sensors mounted on the coldest surfaces, and working in conjunction with the control system, monitors the surface temperature and automatically lowers the dew point when required. In that way condensate formation is prevented, and the system need only dehumidify to the minimum level saving on operational cost. Cold temperature sensors work equally well in OA and heat pump systems.
Other operating strategies for maintaining occupant comfort at the lowest cost
In an effort to save energy during the winter season, de-activating the dehumidification system during off hours is not a viable strategy. The reason is the air temperature drops rapidly due to heat loss through the structure while the water temperature remains reasonably constant due to its mass. It is not uncommon for the space temperature to drop 10 to 15 degrees at night during the colder seasons. This greatly increases the rate of evaporation due to the large difference in vapor pressure. And since the air distribution systems is off, the moisture rapidly rises vertically to the cold ceiling, condensates and drops back into the pool, resulting in cosmetic and structural damage. A better strategy is to dehumidify at the minimum level and maintain proper air temperatures to minimize evaporation.
Another method to reduce evaporation, and one of the simplest energy saving strategies when the pool is unoccupied, is the use of a thermal blanket or cover over the pool surface. It eliminates the water-to-air interaction by providing an impermeable membrane over the pool surface. With a cover in place, a temperature set-back can be implemented during off hours saving energy on space heating.
This strategy can be impractical for larger pools, however, because of the time and manpower required. If the pool cover is not in place, even for one evening, and the space temperature is automatically set back, rapid evaporation and condensation will take place as noted earlier.
Another factor when managing water evaporation is pool water agitation. Pool water agitation creates more surface area, resulting in increased evaporation. It can be as simple as occupants splashing to more sophisticated man-made toys like waterslides, cannons and sprinklers. Careful management of peak load dehumidification along with shutting off unused water features can help.
Avoid the use of absorbent deck coverings such as carpet within the pool area because it absorbs and holds water. Also limit wet deck areas by installing adequate drainage and/or reducing deck over spray.
The changeover from summer to winter operation should be seamless, especially if you have a dehumidification system in place that is designed for pool applications. Built-in operational features will go a long way in maintaining the desired conditions at the lowest possible cost.
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