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Growing up as a rink rat on the outdoor ice hockey rinks of Duluth, Minn., I learned to love the hard, fast ice that Mother Nature provided. The clap of a puck hitting flat on the ice or the sharp grinding of a skate blade digging in — these are the sounds that told you the ice would be good.
The cold air temperatures and low dewpoint of a Minnesota winter made it easy to keep hard, fast ice surfaces, but moving that ice surface indoors and maintaining high-quality ice is much more challenging. This article will focus on the HVAC design approach for indoor ice rinks that provides a high-quality ice surface while also being energy efficient and maintaining comfort for spectators.
Managing Heat Loads
The heart and soul of every ice rink is the ice plant. At 45% of the building’s total energy consumption, it is generally the focus of any energy conservation measure. However, often those efforts are wasted due to other factors or loads causing the ice plant to work harder than necessary: the building envelope, lighting, spectators, resurfacing, etc.
When those heat loads are not considered holistically with the ice plant, it will need to work harder than necessary. Efficient design starts with managing the heat loads on the ice plant.
Those heat loads can be broken down into three basic components: conductive loads, radiant loads and convective loads. Conductive loads include heat gain from the ground into the piping, the floor pumps and ice resurfacing.
Radiant loads include heat gain from lighting and radiation losses to the building structure or roof above the ice surface. Convective loads include air temperature and humidity in the rink, the ice’s surface temperature and air velocity at the ice surface.
In addition to improving the convective heat loads through the HVAC system, energy-efficiency improvements concerning the other loads also should be considered when designing ice rinks. These improvements could include low-e ceiling or dimmable LED lights to reduce radiant loads, and variable-speed pumping or condenser heat recovery to reduce conductive loads.
According to ASHRAE research project RP-1289, the convective heat transfer may be as much as 28% of the total heat load on the ice plant. These loads are directly related to the air temperature, relative humidity and air velocity at the ice surface. Controlling each of these factors is important in maintaining ice quality and minimizing energy costs.
In convective heat transfer at the ice surface, Newton’s law of cooling defines the rate at which the ice gives up heat to the surrounding air volume. This can be estimated by the equation in Figure 1 from the ASHRAE Handbook—Refrigeration, Chapter 44.
Evaluating the components of Newton’s law, the ice system designer can optimize three parts of the equation: temperature differential, heat transfer coefficient and condensation.
Optimizing Temperature Differential
As given by Newton’s law, minimizing the temperature difference between the ice surface and the air will reduce the convective heat transfer and increase efficiency of the ice plant. This can be accomplished either by increasing the ice surface temperature or decreasing the ambient air temperature.
Ice rinks set up for hockey will operate the ice surface temperature at approximately 20 F, but warmer ice temperatures are typical for figure skating and recreational uses. Ice rink designers should provide an ice plant control system that will allow the operator to reset the ice surface temperature based on the usage of the rink.
When looking at the optimal air temperature to increase efficiency, it would seem that the coldest air temperature possible would result in the most efficient system. However, occupant comfort must also be considered. Ice hockey players and figure skaters will be under constant physical activity; colder air temperatures may not be a problem for them. However, the spectators in the stands will be sedentary and the cold will be very uncomfortable.
In small recreational facilities, infrared radiant heaters can be provided for spectator seating. Radiant heaters are an excellent solution in this application because they can provide occupant heating without increasing the ambient air temperature.
Radiant heaters should only be placed over the seating area and always directed away from the ice surface to prevent any impact on ice quality. Supplemental heating will be needed at the rink dehumidification unit to prevent the overcooling of smaller facilities. (Dehumidification system design will be covered later in this article.)
Application of radiant heaters in larger arenas is not feasible, so occupant comfort will need to be accomplished through the air system. The National Hockey League standard for arenas is to maintain an air temperature of 60 F with a dew point of 32 F.
In arenas, temperature control will be accomplished through the arena bowl dehumidification units with the referenced temperature sensed at ice level, ideally located near the return air inlets. The bowl air systems should be designed to limit temperature creep during hockey games to 5 F, with a maximum dew point of 40 F for good ice quality in the third period.
If the reference temperature exceeds 65 F by the end of a game, pre-cooling the bowl before the next game should be considered. Pre-cooling to 55 F pre-game will help ensure good ice quality late into the third period.
Reducing Heat Transfer Coefficient
Designers of the arena bowl air systems will need to consider air distribution patterns for optimal efficiency and ice quality. From the convection heat transfer equation, the heat transfer coefficient (hc) is a factor of the air velocity over the ice surface. Preventing unnecessary air movement will help minimize the heat transfer coefficient and reduce the load on the ice plant.
With arena bowl air distribution systems, air outlets should never be placed over the ice surface; all outlets should be placed over the seating area. Air outlet design should consider the velocity and throw from the outlet. If an air outlet placed over the seating area can potentially throw air over the ice surface, the outlet should be moved farther from the ice surface, or directional capability should be added to throw the air away from the ice surface.
The designer should also consider the possibility of condensation at the building roof structure; a portion of the supply air may need to be directed up toward the structure. An example of a typical arena air outlet is provided in Figure 2.
Low air returns located at ice level in the four corners of the arena encourage airflow down and away from the ice surface. The designer can also reduce air velocity at the air outlets by drawing air down and across the seating area.
Preventing Condensation with Dehumidification
The final factor in the convective heat transfer equation is the humidity within the ice rink. This is represented in the convective heat transfer equation by the delta mole fraction of water in the ice and the air (Xa - Xi ). This portion of the equation accounts for the heat transfer associated with water vapor from the ambient air condensing on the ice surface. Condensation is Enemy No. 1 when it comes to energy efficiency and ice quality.
The phase change of water represents a significant heat load that can be minimized through the design of the HVAC systems. As noted previously, the environment within the ice rink should be maintained at a dew point between 32 F and 40 F. Even with an ambient air dew point of 40 F, the localized dew point at the ice surface will be lower; condensation should not be a factor.
At higher dew points, water vapor in the air will be drawn to the cold ice surface and condense on the ice surface or in the air slightly above the ice surface, resulting in a fog over the ice. The most effective way to maintain these low dew points is through active desiccant dehumidification systems.
Designers of ice rink dehumidification systems must complete a full accounting of all the moisture sources within the ice rink to ensure adequate dehumidification capacity. These sources include permeation through the building envelope, occupant load, water vapor from ice resurfacing, infiltration from uncontrolled areas and outdoor air ventilation.
In small recreational facilities with few spectators, the bulk of the moisture load will be associated with permeation, ventilation and ice resurfacing. In general, this can be handled with a small, packaged desiccant dehumidifier. These packaged dehumidifiers are provided with an active desiccant wheel and gas heat for both space heating and desiccant regeneration.
Depending on the rink’s size, the cooling effect from the ice surface can be enough to cover the space cooling requirements; a cooling coil in the dehumidification unit may not be required. As noted previously, air distribution over the ice surface should be minimized, so the dehumidifier should be placed in the corner of the rink, with air distribution along the long side of the rink and directed away from the ice surface.
In arenas where the quantity of spectators is significant, the bulk of the dehumidification load will be associated with the moisture let off by the spectators and the outdoor air ventilation required for the spectators. Arena dehumidification systems will generally consist of one or more large dehumidifiers with active desiccant wheels, gas heating for regeneration and heating/cooling coils for air temperature control.
Enthalpy energy recovery wheels should be considered, depending on the number of spectators and ventilation air required. Not only do they provide sensible energy recovery on the outdoor air, but they also can remove some of the incoming moisture in the outdoor air and reduce the work required by the desiccant wheel.
The number of hockey games with spectators in every seat represents a small portion of overall arena usage, so the designer should consider the use of a demand control ventilation sequence to reduce the quantity of outdoor air provided during low-occupancy practices and events. An effective demand control ventilation strategy will significantly reduce the energy cost associated with the dehumidification systems and increase ice quality.
Indoor ice rinks are energy-intensive buildings; when operators look for ways to cut energy costs, they typically focus on the refrigeration system, which could unintentionally hurt ice quality. Often, however, a poorly designed HVAC system is the culprit and makes the ice plant work harder than necessary.
High-performance, energy-efficient ice rinks come from a blend of thoughtful refrigeration and HVAC systems design.
Kyle Wilson, PE, LEED AP BD+C, is the market leader for sports and recreation at IMEG Corp. He is responsible for designing sports arenas and other building types and has expertise in designing ice rink refrigeration systems, building HVAC systems, building management systems and plumbing systems for ice rinks and ice arenas.