Air Apparent - Engineering Ice Quality
March 01, 2002
Appeared in Stadia
Investors have put billions of dollars into indoor ice arenas to meet the growing popularity of both professional and amateur hockey. However, for owners to realize the greatest returns from their investment, the condition of the ice must be perfectly attuned to the level of play. Ice for a hockey game must meet higher standards than that for recreational skating, but every ice arena must satisfy some level of performance requirements.
Producing and maintaining quality ice begins with sound floor construction and the proper selection and use of ice-making and maintenance equipment. For professional hockey, the concrete floor must be almost perfectly flat the deviation can be no more than 3/16th of an inch across the entire floor to produce consistent freezing of the ice and consistent thickness. If the concrete below the ice is too thick in some areas, less heat transfer will occur from the piping to the ice sheet, resulting in softer ice or inconsistencies in ice thickness
Indoor air environment
Even with the best designed and installed floor, and optimal ice making and maintenance equipment, consistently good ice cannot be assured without an ideal indoor air environment temperature, humidity and air flow. As the air goes, so goes the ice.
There are a number of examples of poor arena air conditions resulting in sub- standard conditions for hockey competitions or other events. The problem is manifest in soft ice, fog, condensation and poor playing conditions. Soft ice will slow down play and hamper players from performing at their best. With soft ice, puck and player movement can become sluggish.
Consider the problem of condensation on the ice. If not properly dealt with, the increased humidity levels within an arena from the spectators, players and outside air will cause the water vapor in the air to condense on the ice. If condensation builds on the ice faster than the sheet can re-freeze it, the quality of the ice is compromised and softening occurs. If the condensation refreezes, a thin secondary layer of ice is created. This process continues to build up layers and the quality of the ice deteriorates. This is when the players get upset, because their ability to perform decreases.
Fog, obviously, is not conducive to good play. It too is a product of humid air, but occurs upon mixing the bowl humid air with the cold air directly above the ice sheet. This has been a problem even at the highest level of competition, because appropriate engineering and equipment installation measures were not implemented.
In the construction of ice arenas, the challenge for mechanical engineers is to design Heating, Ventilation and Air Conditioning (HVAC) systems that create ideal air conditions for the ice. The design of such systems begins with an analysis of arena usage. Today, many arenas are used for multiple purposes a professional ice hockey game one night could be followed by a professional basketball game the following night. So the ice must be prepared for the hockey game, and then removed (or covered) for basketball. The following day, ice may again be required for hockey competition. The making and maintaining of ice for such different back-to-back activities requires time which is always at a premium. Reducing the humidity above the ice will facilitate the transition from one activity to another.
For many fans, attending a hockey game means more than watching a sporting event. It means enjoying entertainment before the game and during intermissions. The ice is resurfaced during these interludes, but the process is often delayed as long as possible so that resurfacing is finished just before play resumes. The HVAC system can facilitate the process by maintaining low humidity so the ice freezes faster eliminating the possibility of soft ice.
Counting the cost
Maintaining the right temperature in an arena is another concern for engineers in designing HVAC systems. However, unlike most indoor air environments, ice arenas require engineers to focus on two types of temperatures: the temperature of the dry air (dry bulb temperature) and the temperature related to condensation and humidity, known as the dew point temperature.
Theoretically, to produce perfect ice (no
condensation present), the dew point temperature of the air
immediately above the ice sheet must be lower than the surface
temperature of the ice. For professional hockey competition,
the desired ice temperature is 22°F
Although significant amounts of condensation on the ice will soften the sheet, the cost of reducing it to zero is prohibitive, not only for initial capital costs but also for ongoing operations.
Clearing the fog
What, then, are the acceptable indoor air conditions? The answer depends on the venue but there are two minimum considerations for the quality of the ice and the fans event experience: firstly, fog, and secondly, condensation on the arena bowl surfaces (including seating and the underside of the roof). Both are products of humidity and temperature, and can be avoided if engineers do their homework.
Fog occurs when very cold, saturated air immediately above the ice mixes with warmer air approximately eight feet over the ice, resulting in a temperature that condenses the warmer airs water vapor. The challenge for the mechanical engineer is to ensure that the warmer air stays dry enough that fogging does not occur when it mixes with the colder air.
The effects of mixing these warm and cold air streams are clearly shown on a psychrometric chart that engineers use as a tool to demonstrate that fogging will be avoided. If your engineer cannot do this, find another one!
Fog can have dire consequences. One of the most infamous games in US National Hockey League (NHL) history the third game of the 1975 Stanley Cup finals between the Buffalo Sabers and the Philadelphia Flyers became known as the Fog Bowl. As the story goes, the game had to be stopped no fewer than 12 times because of poor visibility. During these interludes, the players skated the ice sheet with unfurled towels and fanned the air to try and disperse the fog. Such playing conditions are not acceptable to players or fans and too many dollars are at stake to tolerate delays in broadcasting such events. The good news is that less air conditioning is required to avoid fog than to reduce surface condensation.
For the HVAC engineer, condensation is the more difficult concern. Condensation on the building and ice surfaces, as previously mentioned, results from the airs dew point temperature exceeding the surface temperatures.
During a game, increased humidity and condensation can occur for various reasons perhaps the body heat of energetic players and screaming fans, or because of outdoor air flowing into the building through ventilation intakes or open doors. Condensation can also occur when the cold outdoor temperature causes interior surface temperatures (such as steel roof joists) to drop lower than the indoor dew point temperature,
In the past, the NHL has set criteria for the indoor air environment during playoffs. The guideline indicates stabilizing the indoor air temperature at 60°F (15.6°C) with 40 per cent relative humidity.
Alternative HVAC systems
Generally there are three possible methods for HVAC systems to maintain the right indoor conditions for quality ice:
Sub-cooling with reheat
Inherent to air conditioning is removal of water from air. Taken to an extreme, a traditional central plant can lower the air temperature enough to extract humidity from the air that would otherwise condense on the ice surface. However, at these temperatures, the bowl becomes a refrigerator too cold for fan comfort. In order to maintain comfort levels, the air is re-heated. Since significant equipment is not added to the HVAC system, this method can have a lower initial capital cost. However, the systems capacity must be increased and it uses more energy for sub-cooling and re-heating simultaneously, so it has greater operating costs.
The sub-cooling process can be adequate in colder regions of the world, especially during winter, because there is less humidity in the outside air. In warmer, more humid climates, the sub-cooling and re-heating taxes the system much more.
Dry desiccant dehumidification utilizes a rotating wheel, not unlike heat wheels used for energy recovery. However, unlike a heat wheels velocity of 10-15 rotations per minute, the desiccant wheel spins at 8-16 rotations per hour. The desiccant wheel is impregnated with a silica gel that removes moisture from air passing through it. As the arena air is drawn through the wheel, the desiccant soaks up moisture from the air like a sponge. As portions of the desiccant wheel become filled with moisture, the wheel continues to rotate through a heating stage that bakes out the moisture and then the process begins all over again.
The desiccant wheel can be installed in two ways, either as an integral part of the HVAC airside system, with the moisture removed as air flows through the system, or as stand-alone equipment. This system uses less energy than the sub-cooling process but does not have the same initial cost impact.
The liquid desiccant system provides a similar effect as the desiccant wheel. However, instead of a heat wheel, chilled liquid brine made up of lithium chloride is cascaded over fill in a manner similar to a cooling tower. The brine picks up moisture from the air passing over it. The brine must then be heated to remove the moisture and must also be re-cooled prior to being circulated over the fill as it begins a new cycle.
This technology has been perfected in the pharmaceutical and process manufacturing industries but due to the equipments bulky nature, greater cost and more complex subsystem; it has not been embraced by the arena industry. However, payback analysis can prove its long-term viability.
Dont skate the issue
For arena owners and designers there are a number of considerations that go into the selection of an HVAC system. As might be expected, cost is often the first consideration. But there are trade-offs: a premium investment up front may result in lower operating costs and in better ice quality. Conversely, initial cost cutting could result in poor ice quality a condition that could become readily apparent not long after the arena opens. And the cost of correcting mistakes could wipe out any savings realized from cost cutting. Furthermore, arenas have long lives generally at least 30 years is planned for public buildings. So owners and designers will have to live with their investment and design decisions for a long time.
For new arenas, architects and engineers should decide early in the design process on the best HVAC approach to ensure ice quality. Timely decision-making will facilitate the process of integrating the HVAC equipment with the building architecture and completing its installation on schedule. Designing the right HVAC systems for an arena can ensure that the ice is properly prepared and maintained for every performance providing the best for the players and fans.
Bill Larwood leads the Public Assembly and Sports Venue team in Los Angeles for the Syska Hennessy Group, a leading US consulting, engineering, technology and construction firm. His professional engineering experience includes over 12 million square feet of arenas and convention centers world-wide, including the Arrowhead Pond Arena of Anaheim, California and the Coyotes Arena in Arizona, which is currently under design.