Ever since the advent of variable air volume (VAV) supply systems, ensuring adequate ventilation air or exhaust makeup air throughout the supply air volume range has been a problem.
Ever since the advent of variable air volume (VAV) supply systems, ensuring adequate ventilation air or exhaust makeup air throughout the supply air volume range has been a problem. The difficulty comes in providing sufficient outside air (OSA) throughout the range of supply air volumes. However, cost effective solutions to the problem are available for systems equipped with direct digital controls (DDC).
Setting the minimum OSA level for constant volume air supply systems is relatively easy, whether to meet building ventilation (people) requirements or exhaust makeup (plus air for building pressurization) needs. Although wind direction and speed can affect the level of air provided, a one-time setting is generally accepted as a reasonable way to ensure minimum OSA quantities.
With VAV systems, the supply airflow constantly changes to meet the cooling loads, which in turn changes the OSA quantity unless minimum damper positions are modified to keep the OSA volume constant. Fluctuating OSA quantities are a problem because OSA needs generally do not change while the building is occupied.
Considering the approaches
This long-recognized operating principle is often ignored because it is expensive to design an air supply system to compensate for variations in OSA volumes. One approach that has been used successfully is to install flow measuring stations on the supply and return ductwork for each fan system to maintain a constant differential equal to the desired OSA. However, this approach is fairly expensive ($5000-$10,000/fan system, depending on its size).
Another way to ensure minimum OSA quantities are provided through the range of a VAV system has been to set the minimum OSA when the fan is supplying the minimum amount of air. The drawback to this approach is that an excessive amount of OSA is drawn in as the supply air quantity increases during the summer. The undue load placed on the cooling coil wastes energy and likely overloads the capacity of the cooling coil unless allowances have been made.
Using the power of DDC
Through the power of DDC, data needed to calculate the OSA to a set minimum are available. A typical DDC system records the following data, which are needed to make the necessary calculations. They are:
– Outside air temperature, OSAT
– Mixed-air temperature, MAT
– Return air temperature, RAT
– Supply air volume (total of VAV box cfms).
Equations needed to perform the calculations are:
MAT(CFMs) = OSAT(CFMo) + RAT(CFMr)
and
CFMs = CFMo + CFMr
where:
MAT = mixed-air temperature, F
OSAT = outside air temperature, F
RAT = return air temperature, F
CFMs = supply quantity, cfm
CFMo = outside air quantity, cfm
CFMr = return air quantity, cfm
First, we solve for MAT. By substitution, we get:
MAT = [OSAT(CFMo) + RAT(CFMs – CFMo)] / CFMs
Next, we multiply both sides of the equation by CFMs and expand:
CFMs(MAT) = OSAT(CFMo) + RAT(CFMs) – RAT(CFMo)
Now, collect common terms and factor:
CFMs(MAT – RAT) = CFMo(OSAT – RAT)
Finally, divide both sides by (OSAT – RAT) to achieve:
CFMo = CFMs(MAT – RAT) / (OSAT – RAT)
Since the OSAT, CFMo, RAT, CFMs, and CFMo (desired) are known, the required MAT can be determined under various OSA conditions. Controlling to the required MAT lets the outside air volume remain constant, allowing, of course, for economizer override. (The table provides MATs for various OSATs and supply air CFMs, assuming a steady RAT of 74 F.)
Technicians performing air balancing use these kinds of equations when they cannot get a good measurement of an OSA quantity directly. In most commercial and some industrial buildings, the people ventilation load (generally 20 cfm/person) — instead of exhaust makeup and pressurization air — is the determining factor. However, each area served by an air supply must be looked at individually. If the system is driven by exhaust makeup air needs, an additional amount of OSA should be introduced to pressurize the building (for example, 5% of the total supply cfm or an air quantity based on the actual infiltration rate of the building).
DDC programming
DDC programming for the calculation and damper commands is reasonably simple and requires few lines of code. Most manufacturers’ VAV box cfm volume points can be calculated in the programming, negating the need for an OSA flow station on the air handler. An averaging-type, mixed-air sensor is recommended because a single point probe may give inaccurate control. These sensors are not expensive and are reasonably easy to install.
The result of the equation yields the mixed-air temperature setpoint needed to program the mixed-air proportional-integral-derivative (PID) loop. The programming can be done in one of several ways, depending on existing program architecture and individual programmer’s choice.
One way, for example, is to look at the output of the mixed-air PID loop and the economizer PID loop and set the damper position based on the higher of the two. (As the economizer cycle winds down, the dampers will be forced to stay open based on the MAT setpoint calculation.)
Adding the equation
CFMo = CFMs(MAT – RAT) / (OSAT – RAT)
to the program gives a real time OSA volume on the fan. The value can be alarmed, added to the fan system graphic, or used in programming. Trending the value yields hard numbers that can be presented to local jurisdictions or interested parties to prove code compliance.– Edited by Jeanine Katzel, Senior Editor, 630-320-7142, [email protected]
Key concepts
It is difficult to ensure adequate outside air throughout the range of supply air volumes in VAV systems.
DDC controls make available the data needed to calculate outside air to a set minimum.
Programming the DDC system to perform the necessary calculation and damper commands is reasonably simple and requires few lines of code.
Calculating mixed-airtemperature
Problem: A 20,000-cfm system (see illustration) serves an area occupied by 200 people and has two restrooms with a combined exhaust of 2000 cfm. ASHRAE Standard 62-1989R, “Ventilation for Acceptable IAQ,” requires an OSA quantity of 20 cfm/person. For this situation, then, 4000 cfm is needed to meet the people ventilation load.
This amount exceeds the exhaust plus pressurization quantity of 3000 cfm (2000 cfm plus 5% of 20,000 cfm). Therefore, 4000 cfm will be the OSA requirement. A return air temperature of 74 F and supply air volume of 18,500 cfm are assumed. (Both would be measured by the DDC system.) Using an OSAT of 85 F, we arrive at the following.
Solution: The mixed-air temperature can be determined in this way:
MAT = [OSAT(CFMo) + RAT(CFMs – CFMo)]
/ CFMs = 85(4000) + 74(18,500 – 4000)
/ 18,500 = 76.4 F
Desired MATs at various temperatures and flowrates using 4000 cfm OSA requirement*
Desired MAT, F OSAT, F Supply air, cfm OSA, % Notes
52 20 10,000 40 2
59 30 12,000 33 1
64 40 14,000 29 1
68 50 16,000 25 1
71 60 17,500 23 1
73 70 18,000 22 1
75 80 18,000 22 —
77 90 20,000 20 —
Notes: 1 = Economizer overrides; 2 = Heating required
* A steady 74-F RAT is assumed
More info
Copies of ASHRAE standards are available from the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, 1791 Tullie Circle, N.E., Atlanta, GA 30329-2305; 800-527-4723; fax 404-321-5478; e-mail [email protected]; or visit the web site at ashrae.org. Prices vary.
Questions about the technical content of this article should be directed to author Jerry Siemens at 503-627-2709.
