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The principles of tent heating and cooling

August 1st, 2007 / By: / Feature, Lighting, Power & HVAC

Learn the principles of tent heating and cooling to deliver more comfortable events for your clients.

The event business never gets any easier, and heating or cooling our temporary structures and tents is no different. The standards and codes for heating, ventilating and air conditioning (HVAC) are primarily developed by the American Society of Heating, Refrigerating & Air-Conditioning Engineers (ASHRAE) for permanent structures such as houses, commercial buildings, arenas and more. For these permanent structures, values have been assigned for the type of construction and associated heat gains and losses for ceilings, walls, floors, windows and doors. These values are then used in a heat loss/gain formula to calculate the required HVAC. However, there are no values set for membrane structures, so applying the same formulas to tents is difficult at best.

Temperature differences

The first factor in designing environments with appropriate temperatures is known as Delta T. Simply put, this is the difference between the outside average temperature and what’s desired for the inside. For example, if the outdoor average low temperature is 0 degrees Fahrenheit, and the desired inside temperature is 70 degrees, then the Delta T is 70. ASHRAE assigns lowest and highest outside temperatures for different parts of the country for permanent structures. But we need to design for the event date, so these values will be of little help in the fall and spring. A local National Weather Service office, the Convention & Visitors’ Bureau, or weather Web sites are good resources for finding the average outside high and low temperature at the time of year the event will be held. The inside is easier: Simply find out what temperature the client wants to maintain.

Your HVAC equipment also has a Delta T. A propane heater cannot have any touchable surfaces exceeding 185 degrees. Heaters are tested at 77 degrees for seven hours to make sure this code is met. This limits the temperature rise, in the case of a heater, to about 100 to 120 degrees. So if your heater is outside (as it should be) sucking in zero-degree air, the best you would get is 100 to 120-degree air mixing with the air already inside the tent, bringing the inside temperature down. But if your heating (or cooling) equipment recirculates already conditioned air, you will need less HVAC. If we use the same example, recirculating 50-degree air inside would essentially allow the exit air of the heater to rise to 150 to 170 degrees.

It is important to note that not all propane or liquid-fuel-fired heaters are designed for (or capable of) air recirculation. Many are direct-fired and blow the products of combustion in with the hot air. Check with the equipment’s manufacturer or the installation instructions for the equipment to see if air recirculation is a possibility.

Resistance and infiltration

The next major factor in temperature control is the R factor. This refers to the thermal resistance of the material or, in the case of a permanent structure, the complete construction package. When you buy insulation, you might find a label denoting R-13, R-24 or something similar, which indicates the material’s thermal resistance. The higher the number, the higher the insulating value. Unfortunately for tents, the R factor for vinyl is not defined and is probably less than R-1. This means that the heating or cooling within the tent will be readily lost to the environment, which increases the amount of heating or cooling required.

Just when you think the R factor is against us, you’ll find that the next factor—infiltration—is even worse. In a permanent structure, the infiltration, or air change, varies depending on the construction and use of the building. Section 29 of the ASHRAE Handbook contains formulas for computing infiltration. If you have a 2,500-square-foot older house with 8-foot ceilings, or a cubic volume of 20,000 cubic feet, and the effect of 15 mph winds, you will see a complete air change (infiltration) once every hour, like leaving the front door open. Newer homes had about one-half an air change per hour, and the newest homes only had about one-quarter air change per hour (like leaving a double-hung window wide open). When a tent has an 8-by-10-foot entry opening, this causes an air change around four to five times per hour, and this doesn’t include the tent’s other openings in the top and sides. There are around four to five more air changes at 25 mph winds and another four to five at 35 mph. Remember, too, this is assuming there is only one entrance; codes will require at least two.

Let’s compare an older, 2,500-square-foot house (20,000 cubic feet) to a 40-by-40-foot frame tent (1,600 square feet or around 18,000 cubic feet). The house (with one air change per hour) would require between 35 and 50 BTU per square foot for heating, if the Delta T in winter is 85. In the summer, with a Delta T of 20, the same house would require 125,000 BTU per hour or 6 tons of air conditioning (1 ton of air conditioning equals 12,000 BTU per hour).

The tent, on the other hand, would require around four to five times the HVAC of the house. For the whole tent, this would mean around 500,000 BTU per hour for a winter day in Buffalo, N.Y., or around 25 tons of air conditioning in the summer—and that’s with doors in the tent. Leave one or two 8-by-10-foot openings, and you won’t be able to blast enough heat or cooling into the tent without driving everyone out with the powerful air movement from the HVAC equipment.

Delta T, R factor and infiltration are all used in a complex heat loss/gain formula to calculate the required HVAC for the structure. The chart in the image box above gives the results of this formula for different temperatures in a 1,600-square-foot tent. It is by no means definitive but rather provides a useful starting guide. Note that the values assume a tent with doors; they can be used for any size tent starting from 20 by 20 feet. The air change values vary based on the type of tent and how well the sides are sealed.

The wind speed given in the table is indicative of the effect on air changes and is not necessarily the air change caused by that wind speed. More openings along with the location of openings compared to wind direction could increase air changes even at lower wind speeds. When calculating your required heating or cooling, be sure to increase or decrease the infiltration per hour to account for how well you’ve sealed the tent and the number of openings.

People and lighting add heat to a space, and these factors need to be accounted for, especially when designing an air conditioning system. The average person adds around 350 BTU per hour of heat to a space. So, 100 guests will add 35,000 BTU of heating to the tent. Similarly, each watt of lighting adds 3.5 BTU per hour; 2,000 watts of lighting adds 7,000 BTU per hour to a tented space. When you calculate the required air conditioning using the chart above, be sure to factor in the effect of people and lighting to get an accurate total requirement.

Maximizing HVAC

Tightening up the tent to reduce infiltration proves to be the best method for reducing the amount of HVAC required. Keder or hard sides and doors are the first step. Next would be sealing up valances or placing foam to plug the gaps between the tension bars and perimeter beams. Liners also cut the cubic volume down and increase the R factor (maybe even to R-1), which decreases heat gain or loss. Raised floors will also help, as will sealing the floor with carpeting and sealing the sides to the floor to prevent infiltration.

To optimize the effectiveness of the HVAC system, it should be properly designed. ASHRAE standards state that if cooling is the primary mode of temperature control, then the outlet vents should be raised off the floor and return air (if any) should also be high off the floor. This arrangement allows the cold air entering the room to sink to the floor, and as it does, it mixes with the hotter air rising to the top of the room. This reduces what is called stratification, where the air forms layers, with the hottest on the top and coldest on the bottom. The reverse is true for heating. The vents should be low, blowing the hot air across the floor. The air will rise and mix with the cold air sinking toward the floor. Likewise, the air return (if any) should be low, near or in the floor. Using a diffuser with your propane heater helps spread the air across the floor, making the heater more effective.

There are many different types of equipment available aside from propane heaters and air conditioning units, such as radiant end electrical heaters, desiccant moisture-removing and evaporative (swap) coolers. These different systems take more knowledge to use and, in most cases, have limited applications in our business.

Having some basic propane heaters can certainly extend your season so that it starts in mid-spring and ends in mid-autumn. With more knowledge and experience, tenting in the winter can be added to the list of services you offer. Finally, starting with some smaller air conditioning units (one, two or five tons) for summer applications can add to your bottom line.

Have a conversation with your customers to let them know what it will cost and what they can realistically expect from the HVAC you are going to install. A 95-degree day in mid-May in Buffalo (or a 20-degree day in Miami in February) is unlikely but possible, so you need to inform the client what the parameters are for wind, outside temperature, cost of sealing the tent, etc. Spelling these out in the contract never hurts, either. Knowing the basics of heating and cooling will help you sell your client the most comfortable option available for a crowd-pleasing event.

Tom Markel is the owner of Bravo Events Expos Displays in Buffalo, N.Y.

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