Destratification can resolve a number of heating and air quality issues.
March 1, 2013 by Erin Hsu
Many facility managers dread the costs of keeping their workforce comfortable in spacious workplaces every winter. In other spaces, condensation can wreak havoc on products. It may seem counterintuitive, but for many facilities the answer to both these issues is better known for its summer cooling power: ceiling fans. More specifically, large diameter, low speed overhead fans can significantly reduce energy consumption, lowering energy costs by up to 35 per cent. The air movement from fans also clears condensation and the accompanying safety and product integrity problems.
“In large spaces, HVAC systems are generally not capable of efficiently distributing air to all occupants,” explains Christian Taber, senior applications engineer and ASHRAE High-Performance Building Professional for Big Ass Fans. “Large circulator fans mix the air in the space, ensuring good air distribution and creating uniform temperatures that increase occupant comfort and minimize stratification.”
Large diameter, low speed fans produce a column-shaped jet of air roughly equal to the diameter of the fan. As this jet strikes the floor it spreads out in all directions, displacing the stagnant air and setting up a convection-type circulation pattern. Air from the floor level is moved out towards the walls or obstructions and then moves up back into the fan to be recirculated. During cooling seasons the fan speed can be increased, so that faster-moving air working in concert with the body’s natural cooling process produces a cooling effect of up to 10F. Conversely, during heating seasons the fan speed can be reduced so that the air continues to mix, moving hotter air at the ceiling down to occupant level without causing a draft (see Figure 1). This destratification keeps occupants comfortable in heating seasons and may significantly reduce heating costs.
Let’s take a closer look at how these large fans solve two major wintertime issues in large facilities.
In the winter, stratification, or layering, occurs because the hot air from a heater is approximately five to seven per cent lighter than cool air in a space and tends to rise to the ceiling. In the winter, large diameter fans can be used for destratification by gently pushing the warm air trapped at the ceiling down to occupant level, creating a more uniform temperature throughout the space without causing a draft. In addition to providing comfort to occupants, destratifying the air in a space has the added benefit of allowing for a lower thermostat set point.
A major focus for estimating winter energy savings is the difference in air temperature between the floor level and the underside of the roof, as well as taking into consideration outdoor air temperatures, with the assumption that after thermal destratification the air temperature will be essentially uniform within the space. An estimate of energy savings would consider the building envelope and the difference between heat lost through the roof and the two temperature differences before and after destratification, among other factors.
Occupants of existing buildings can easily access data through their local utility companies regarding the amount of gas or other fuel used during a heating season, while local climate records can provide average outdoor air temperature during a particular heating season. This data can be used to calculate the overall rate of heat loss through the building envelope in a stratified condition over a given period in Btu/ft²/degree day. [A degree day is used to relate the day’s temperature to the energy demands of the heating system, using 65 degrees as a baseline.]
Federated Co-operative Ltd., a retail cooperative with members throughout western Canada, operates retail shops, fueling stations, building material supply centers and refineries, occupying numerous warehouse spaces to store their products. Federated used large diameter, low speed fans to decrease the rate at which their buildings were shedding heat.
Trevor Carlson, manager of environmental and technical services, decided to conduct pilot tests in the 80 000-sq.-ft. loading dock section of their 300 000-sq.-ft. Saskatoon warehouse. These tests included the installation of five 24-ft. diameter fans.
Carlson tracked data concerning average energy consumption and daily temperature changes throughout two full years. The heating index of this facility before the installation of the fans was 4.49 Btu/ft²/degree day. Within the first year after installation, Federated saw a decrease to 3.99 Btu/ft²/degree day and the following year, a full year of using the fans, their heating index was 3.61 Btu/ft²/ degree day.
“We noticed a pretty significant decrease in natural gas consumption,” Carlson said. “We believe we saved $19,800 in the first year in natural gas consumption as a result of the fans. That’s a payback of approximately three years on the project.”
It is easy to assume cold storage is elementary; seal and insulate the walls, install a heavy door and turn the thermostat low. However, there is much more involved in maintaining a well-chilled storage facility. Sometimes even the most carefully designed facilities can encounter problems including dead spots and ice buildup. Introducing air movement has proven to be an effective way to curtail these issues and provide a more uniform cold storage environment.
Condensation was a big problem at McCain Foods production plant in Carberry, MB. Drops of water forming frozen stalactites had the potential to contaminate product, leading the Canadian Food Inspection Agency to express concern about ice buildup.
“We have different air pressures in the factory. In the process area you have very moist air that is at higher pressure than the air in the freezer. What happens is that moist air comes down through the freezing tunnels and out the conveyor holes in the wall and infiltrates the freezer,” explains chief engineer Geoffrey Aitchison. A 24-ft. diameter fan proved to be a simple solution.
“After two weeks the ceiling was completely clear and it has been ever since,” Aitchison said. “The air is circulating, plus we’re warming the ceiling to the same temperature as the air, so the frost isn’t going to stay there.”
With great success in its freezer, McCain Foods added another 24-ft. fan to its processing area, where condensation was forming on the ceiling above a processing tank and constantly growing mold.
Just like in the freezer, the tremendous air circulation abilities of the fan kept the ceiling and air temperature consistent, preventing the moisture buildup that allows mould to form.
Along with reducing condensation and creating consistent temperatures, large diameter fans also aid McCain Foods’ energy efficiency plans. Since the installation of their first fans in 2007, McCain Foods Canada has installed additional fans in other processing facilities. <>
Solving issues in atria designs
Manitoba Hydro moved into its 700 000 sq. ft. high-rise headquarters in September 2009. Nearly all of the building’s electricity comes from self-renewing water power, and the structure takes advantage of advanced technologies that respond to the harsh local climate. Located in downtown Winnipeg, the tower consumes only 88 kWh/m²/a–less than a quarter of the 400 kWh/m²/a typical of a large-scale North American office tower located in a more temperate climate.
Instead of recycling air, Manitoba Hydro introduces 100 per cent fresh air year round, regardless of outside temperatures. During the colder months, the outside air must be heated. As a result, the hot air rises to the ceilings in the three atria. At times, the floor temperature remained 10C (50F) while the ceiling temperature settl
ed at a toasty 30C (86F). To eliminate this stratification in the 75-ft. high atria, three 14-ft. fans recirculate the warmed air, minimizing the discrepancy to five degrees or less.
Even though energy conservation was a focal point of the overall design, employee comfort and satisfaction was the primary concern. Mark Pauls, hydro energy engineer with Manitoba Hydro, notes that all employee costs were considered in the development of the tower. “Because our employee costs are roughly 100 times our utility bill, if we improve productivity and decrease absenteeism by one per cent each, that dwarfs any energy savings we would ever see.” explains Pauls.Erin Hsu is with Big Ass Fans. Based in Lexington, KY, the company is a designer and manufacturer of large diameter, low speed fans for industrial, agricultural, commercial and residential use.