Going swimmingly: as for indoor/outdoor environmental contrasts, it's hard to beat a tropical indoor water park amid what is otherwise a subzero Alaskan day. Yet that's precisely the scene in Anchorage, where this major attraction navigates its humidity challenges and the sometimes extreme outside air conditions. Heat exchange and opportunistic plenum design also ensure that comfort and costs don't go slip-sliding away

Engineered Systems, May, 2006 by Raj Bhargava

Alaska is known for the majestic outdoor beauty of Mount McKinley, its vast lands of forest, and abundant wildlife. Indoors, Alaska is also home to one of the largest enclosed water parks in the United States. H2Oasis, on a three-acre site in Anchorage, has proven to be a paradise in the midst of the arctic tundra for people to escape into a tropical destination without spending a bundle. This tropical indoor water park has the capacity to hold over 1,000 occupants in the 55-ft high building, encompassing 40,000 sq ft in south Anchorage.

This project started with an idea that grew into what is now literally the "hottest" spot in Alaska. The water park boasts a lazy river, pirate ship, wave pool, water cannons, water coaster, body slides, and pools for different groups. The park received a permit to start construction in July 2001 and officially opened to the public on March 2003. The cost to build this recreational facility in the northernmost state was $7.5 million. Over 140,000 guests a year come to enjoy the simulated tropical oasis in the sub-arctic Anchorage climatic region, an incredible attraction for a city with just 300,000 residents.

Running H2Oasis is no easy task, and keeping it moving everyday takes considerable resources. The electric utility bill for this oasis in the snow ranges between $16,000 to $18,000 a month. The monthly gas utility cost varies between $6,000 and $12,000. The water bill to run H2Oasis is $1,500 to $2,000 a month.

There were many architectural firsts that make this building unique in North America. H2Oasis was the first water park in North America to have an enclosed Master Blaster, a 505-ft long twisting water coaster. There are presently 50 Master Blasters in North America, and only three of these are part of an indoor water park. The modern marvel also makes a 450-degree turn around the south tower. This Master Blaster stands four stories high in one corner of the water park. The main pumps supply the 5,500 gpm of water required to propel humans down the undulating chute.

Designing the ventilation system for the water park created some unique challenges. Customer comfort was paramount, and that required maintaining the right mix of temperature and humidity. Here in Alaska, the summer temperatures rarely get over 70[degrees]E However, customer comfort dictates that the water park environment be kept at a balmy 86[degrees], and 60% rh year round. This means that even in summer, the cool, dry outside air has to be heated. In winter, the conditions require even more heating, as the outside air can be as low as -25[degrees], containing virtually no moisture.

With the required year-round heating of the outside air, proper equipment selection was necessary to meet targeted indoor conditions and minimize energy consumption. The major variable in choosing equipment was the high quantities of moisture generated indoors. Preliminary calculations indicated that up to 944 lbs of water an hour could be generated, a large load for any ventilation system. Between the heated pool, the even hotter spas, water spraying from various rides, and damp towels and bodies everywhere, humidity control was a serious problem to overcome.

TECHNICAL ISSUES

Initially, the developers sought to mimic other water parks, swimming pools, and natatoriums around the country. Their assumption was that a dehumidification system, with mechanical refrigeration and reheat, would be needed because it was the most commonly used equipment from North Dakota to Florida. However, Anchorage has a fairly cool, dry summer and the calculations led to a different result. Our analysis was based on the following factors:

* ASHRAE states that recreational pools need to be maintained at 84[degrees] water temperature.

* ASHRAE also states that the room air temperature has to be at least 2[degrees] warmer than the water, to keep condensation in check. (Air, if cooler than pool water, will cause condensation and "misting").

* The ambient summer 1% design conditions are 73[degrees] db and 60[degrees] wb, according to ASHRAE weather data tables.

With these variables, a spreadsheet was created, (Table 1), to determine the outside air needed to dehumidify the air and restrict the indoor humidity from exceeding 60% rh. Two conclusions were derived: (1) The outside air always has to be heated, winter and summer, because the 86[degrees] indoor setpoint is well above ambient; and (2) The heating causes the air to "dry" and thus no mechanical refrigeration was needed to dehumidify the space. The more outside air is brought in, the more it reduces the humidity indoors, as long as the Anchorage temperature regime is applied.

Thus, the control sequence was set to reduce outside air based on energy considerations, but sufficient to keep the humidity from exceeding its mark.

Beyond the standard design elements for any large assembly area, the quantity of water in the environment presented another consideration. The pertinent calculations used to determine the effect on the indoor environment were performed as follows.