In addition, the outside air that is brought into a building to "make up" the amount of air that has been exhausted has a very significant impact on cooling and heating capacities.
Indeed, insufficient makeup air can lead to increased building infiltration and higher energy costs, not to mention unhappy building owners and uncomfortable occupants.
While there are no facts and figures to show how many buildings in the United States provide the correct amount of makeup air, it can be surmised that there are probably a great many that do not have sufficient makeup air. And there's no one solution that is available for every building. Each building must be analyzed with a careful air balance calculation and evaluated based on the diversity of sources of exhaust air and infiltration.
Only when those calculations and evaluations are made can the engineer confidently recommend a particular makeup air system for an application.
Myriad Methods
Over the years the trend has been to construct buildings that are as airtight as possible, in part to cut down on the amount needed to be spent on heating and cooling. In the 1990s, IAQ became the buzz term, and building owners started seeing their tight buildings as liabilities. They began experimenting with ways to introduce more outside air, or makeup air, into occupied spaces, especially in response to ASHRAE's Standard 62, "Ventilation for Acceptable Indoor Air Quality."Some building owners started pumping in unconditioned outside air, thinking the more "fresh" introduced, the better. That didn't always work well because the existing hvac equipment would have to heat or cool a tremendous amount of air for which it was not designed. That is especially true when the weather was very hot and humid or very cold and dry.
For this reason, it is now more likely to find a building in which the makeup air is treated separately from the rest of the system. As Ralph Kittler P.E., vice president, sales, Dectron division of Dectron Internationale (Roswell, GA) notes, this decoupled approach involves a dedicated unit specifically designed to address the high latent outdoor air load.
"These units are designed to dehumidify the outdoor air and generally deliver the air into the building at a neutral humidity level and temperature. A unit of this type will operate specifically based on the outdoor conditions. That allows the air conditioning equipment to deal only with the indoor loads," said Kittler.
But which type of unit to choose? There are many types of equipment available to provide makeup air to a space. Indeed, a regular rooftop unit can be used to bring in additional air. But in many cases makeup air units are dedicated direct gas-fired units and indirect gas-fired units. These units normally do not incorporate cooling, but might offer the option of a factory-installed cooling coil to be mated to a remote condensing unit.
It is also possible to provide makeup air using electric coils, hot water coils, or steam coils, but these methods are not normally used due the cost of operation (electric coils), or the absence of a boiler system (hot water or steam).
In the case of buildings with almost no exhaust and very tight construction, it might be possible to provide the small amount of makeup air required through a commercial rooftop package but this tends to apply almost exclusively to office buildings and retail.
"The typical warehouse, distribution center, or factory has more infiltration than a commercial rooftop can offset, or has too many exhaust sources for the limited outside air capacity of commercial rooftops. Even in the retail arena, a restaurant will have more exhaust in the kitchen area than a conventional rooftop can handle reliably," noted Mike Kaler, marketing manager, Applied Air (Dallas).
Climate Matters
The type of makeup air equipment used, as well as its size, is usually dependent on the climate. Humid areas of the country require considerably larger-capacity units to dehumidify outdoor air. "The supply air requirements for occupant comfort are generally air temperatures of 70 degrees to 75 degrees F with an rh of 50%," said Kittler.He added that a good design criteria for engineers is to specify the unit performance at two operating conditions - a design day and an off-peak period such as a 70 degrees rainy day. The off-peak times of the year are actually more critical to evaluate the unit performance because this is the lion's share of the unit operation. "Engineers also need to specify the maximum allowable supply air dewpoints," added Kittler.
Kaler believes that direct gas-fired equipment is the most efficient choice for makeup air. "While indirect gas-fired equipment might have a nominal energy efficiency of 76% to 80%, a direct-fired unit will operate at a nominal 92%. Thus, the further north or more extreme the climate, the more valuable a direct-fired piece of equipment becomes."
It is also possible to experience condensation within the heat exchanger sections of indirect- fired units in very cold climates, said Kaler. "If the heat exchanger is not properly pitched and drained and not constructed of stainless steel through all sections, then corrosion and premature failure can occur."
That's not to say that indirect gas-fired makeup air equipment doesn't have its place, but again, it comes down to the climate. Climates with high concentrations of combustible or potentially toxic elements in the air might not be suitable for direct-fired equipment as the air is directly exposed to the flame. "Many applications allow the recirculation of some of the air from the space and return air that is laden with paint, solvents, or very fine powders would be better handled with an indirect gas-fired piece of equipment," noted Kaler.
Huge Users Of Makeup Air
No discussion of makeup air systems would be complete without including laboratories, cleanrooms, and other similar environments, which have always been big users of makeup air. Many such applications require a high percentage of outside air in order to comply with building and safety codes.Dennis Schoener, senior project engineer, CUH2A (Princeton, NJ), designs hvac systems primarily for laboratory spaces. "When you get into laboratories, usually it's 100% outside air or sometimes we mix some office areas, and I might return a little bit from the office areas, but the lab itself is 100% exhausted. There's a couple ways of handling it and minimizing impact."
Schoener likes to think outside the box, and he recently demonstrated that ability with a set of chemistry labs that were located within a pharmaceutical company in Connecticut. The building contained 575,000 gross sq ft. While the chemistry labs only constituted 19% of the square footage of the building, those labs required almost 50% of the outside air requirements for the building.
"We had a typical chemistry lab module, which is about 835 sq ft/module, and there were 48 of them in the building. In each of those lab modules there were about 56 linear ft of benchtop fume hoods. It's real hard to introduce that quantity of air to the space without causing turbulence in front of the hoods," noted Schoener.
To minimize the turbulence, the cooled air that was used for the office area just outside the lab was transferred, via ductwork, into the lab. Constant volume air was provided to the perimeter corridor, which was open to the lab, and that air was swept back into the lab proper.
Schoener still needed to determine the air quantity needed in the lab, so he considered the makeup air to the fume hoods, the heat load to the space, and the minimum air change rate. In this particular case, the hoods were so intensive that it was clear a lot of air would have to be moved - much more than the minimum air change rates.
"Out of the 9,800 cfm that we needed to make up air into that lab, 770 cfm came from the offices, and 1,420 cfm came from the perimeter. I still needed to make up 7,600 cfm directly into the lab, which is still a high air change rate for that square footage," said Schoener.
Instead of cooling all the air down to 55 degrees then reheating it back up, as is the usual lab scenario which wastes a lot of energy, Schoener created two air systems that fed each of the chem labs. There was an AHU system that produced 55 degrees air, then there was a makeup air unit that produced room temperature air. These two airstreams were mixed before the air entered the lab and produced 64 degrees supply air, so the maximum heat gain in that space was enough to keep the temperature in that lab at 72 degrees . By doing that, the makeup air was tempered down to room conditions.
Schoener used heat pipe technology in the makeup AHUs. "We wrapped the heat pipes around the cooling coil, so as air came in it was precooled by the refrigerant, then it went through the chilled water cooling coil, which took the temperature down even a little bit further. Then it went through the second heat pipe, which rejected the heat back to the airstream. That allowed us to basically get free reheat. It was a good way to reduce the tonnage and also reduce the reheat needs."
And talk about reducing tonnage, this design required 627 fewer tons of cooling than a conventional system and also 3,600 MBtuh less of reheat at peak load. This just goes to show that for every application, there's a different way to design a makeup air system. ES
New Standards For Gas-fired Equipment
New versions of the existing ANSI standards for gas-fired equipment have become effective, or will become effective on Jan. 1, 2003. The old makeup air standard, Z83.4, "Non-recirculating direct gas-fired industrial air heaters," has been changed to make recirculation of room air through these units noncompliant with the standard. ETL (ITS) and CSA are both now certifying to this version of the standard.Other changes, according to Mike Kaler, Applied Air, include:
- A more stringent certification process for products of combustion at various points in the potential operating range of the equipment;
- Additional flame sensors for larger burners in order to ensure total combustion over the entire length;
- Tighter control over high and low burner airflow limits; and
- Adherence to the 160 degrees F limit on discharge air temperature.
"While all of this adds up to some additional cost in the units, it also assures owners of this equipment that it is providing heated makeup air that is totally safe," said Kaler. "This version of the standard is in place now, and engineers should be sure that they are specifying units that comply with the newest version of Z83.4." (Although not strictly "makeup air" units, door heaters have been added to this standard and must comply with all of the same requirements as dedicated makeup air units.)
The biggest change that is coming is the new version of ANSI Standard Z83.18, "Recirculating direct gas-fired industrial air heaters." This standard covers any unit that might use recirculated air for part or all of its operating range. "In the past, engineers could specify a direct-fired recirculation unit as long as they provided a minimum of 4 cfm of outside air for every 1,000 Btuh of heat provided. This will no longer be the case effective January 1, 2003," said Kaler.
On that date, it will be necessary for the unit to operate in compliance with a sliding scale of outside air vs. "equivalent temperature rise." Kaler noted that the greater the temperature rise, the greater the amount of outside air required by the standard. To complicate the issue further, the unit must be self-regulating or "adaptive." "It must be able to vary the amount of outside air, or the temperature rise, or both, in order to keep the unit in compliance at all times. The required amount of outside air could fluctuate from 100% down to as little as 10% or 15%, depending upon the temperature rise through the burner section," noted Kaler.
Manufacturers of direct gas-fired makeup air equipment must prove that their equipment will "adapt" through its entire operating range to within 5% of laboratory measured outside airflow. This has been a significant challenge. Kaler stated that many manufacturers have attempted to use some form of outside air damper position indication as a way to prove the amount of outside airflow.
"However, conventional damper systems have enough hysterisis built in that they cannot meet the 5% criteria when cycled through their full range in both directions. In addition, airflow through dampers is well known to be nonlinear. A small damper opening results in a large airflow change. As of this date, damper position has been rejected by ETL (ITS) as a repeatable and accurate method for complying with the new standard."
The most promising method, and one that has already passed the ETL tests in the laboratory, is a "patent applied for" technique that actually measures airflow and provides adaptive control of damper position using a digital controller.
