Return Fans Or Relief Fans
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1 The following article was published in ASHRAE Journal, April Copyright 2004 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. Return Fans Or Relief Fans How to Choose? By John Kettler, P.E., Life ember ASHRAE A re return fans necessary? Some designers cannot imagine a system of any substantial size operating without them. Others condemn them and maintain ASHRAE should advocate banning them completely. Opponents of return fans favor power exhaust (relief) fans. This article compares the two systems in terms of energy consumption and control requirements. Supply-exhaust systems save energy (horsepower) except in cases with both high system pressure losses on the return air side and a high percentage of outdoor air. However, other factors such as control complexities and system configuration need to be considered and are discussed in this article. An earlier ASHRAE Journal article 1 covered many of the pros and cons of relief fans. Placement of the exhaust fan (either as part of a packaged air-handling unit or remotely mounted) can have significant bearing on the design decision, as well as on the pressure losses in the return air system. This article specifically refers to relief fan as an exhaust fan because they also can act as smoke exhaust fans. Fan Energy Use Analysis Figures 1 and 2 show a supply and return/exhaust (SR) system and a supply/exhaust (SE) system operating under similar conditions. For the following analy- About the Author John Kettler, P.E., is a controls program manager at York International, Controls Group, Kansas City, o. 28 ASHRAE Journal ashrae.org April 2004
2 SR System B A Return Air SE System B A Return Air Exhaust Air Damper OA 0.5 in. P C Return/Exhaust Fan 18,000 Exhaust Fan 18,000 Return Air Damper 0.25 in. P Return Air Damper D F Filter Coil E Supply C D E ± Filter Coil Fan 20,000 OA ± 0.75 in in. Supply P P Fan Figure 1: Supply and return/exhaust fan system in. P 0.75 in. P Figure 2: Supply and exhaust fan system. F 20,000 ses of these systems, these assumptions have been made: Fan efficiency for supply fans is assumed to be 75%; Fan efficiency for return or exhaust fan (smaller fans) is 65%; Pressure drop across the return air damper is higher for the return/exhaust system to allow outdoor air to be properly controlled; 2 No consideration is given to possible turbulence losses caused by the return fan, which can be significant depending on air-handling unit design. For this comparison, the amount of supply air (Point F) is always 20,000 (9438 L/s), and the return/exhaust air (Point A) is always 18,000 (8494 L/s). The 2,000- (944 L/s) difference is for building pressurization and incidental exhaust. This could be a constant or variable volume system. The control system is discussed later. Comparing SR & SE Systems System Airflow () Pressure (in. w.g.) HP Required (HP= P/6356/Eff.) SR-1 20,000 2, SE-1 20,000 2, Case 1 inimum Outdoor Air Flow In Case 1 the systems are operating at minimum outdoor air flow, and the difference between the supply volume and return volume (2,000 [944 L/s]) is being exhausted through local exhausts or building leaks. Therefore, zero exhaust is required. However, the supply fan for the SE system must have slightly larger static pressure capability than the supply fan for the SR fan system to handle the loss through the return duct. In Figure 2 the supply fan must handle the pressure loss from Point A. In Figure 1 it only has to handle the pressure loss from Point C. The return damper for the SR system is sized for 0.5-in. (124 Nine Examples Pa) pressure drop to ensure negative pressure in the mixed air section. The return damper for the SE system can be larger to minimize this pressure drop. However, the total system horsepower required is less for the SE system than the power required for the SR fan system, in part due to fan efficiencies. If the system were operating in a cooling mode, the added heat of the return fan would have to be added to the cooling load (as would the added heat of the larger supply fan for the SE system). April 2004 ASHRAE Journal 29
3 System Airflow () Pressure (in. w.g.) HP Required (HP= P/6356/Eff.) SR-2 20,000 10,000 8, SE-2 20,000 10,000 8, Case 2 50% Outdoor Air Flow In Case 2, the SE and SR systems operate at 50% outdoor air. At this point, the horsepower required is nearly identical for both systems. As noted earlier, this does not take into account higher turbulence losses that typically occur on the discharge of return/exhaust fans (associated with the airstream being diverted into two paths). This system effect results in higher horsepower requirements for the return/exhaust fan. System Airflow () Pressure (in. w.g.) HP Required (HP= P/6356/Eff.) SR-3 20,000 20,000 18, SE-3 20,000 20,000 18, SE-3A 20,000 20,000 18, Case 3 100% Outdoor Air Flow In Case 3, the SE and SR systems operate at 100% outdoor air. The horsepower for the return fan and the exhaust fan are identical since they are both operating as 100% exhaust fans. However, the SE system requires slightly more horsepower because of the added restriction at the mixed air (outdoor air) section (to account for damper losses required for proper control when transitioning from 100% outdoor air to 90% outdoor air). As pointed out in an earlier article, 1 if the outdoor air damper is sized as for the SE system (SE-3A), when the system moves from 100% outdoor air and the return air damper opens slightly, the exhaust fan will pull air from the source of lower pressure differential (from the mixed air section rather than the building). The building becomes more positive, speeding up the exhaust fan and exacerbating the negative building pressure. Although the mixed air control loop ultimately compensates by opening the return damper more and closing the outdoor air damper, serious hunting and building pressure fluctuations can occur. Sizing (or linking) the outdoor air damper so that at full open position, the pressure drop from the return path to the mixed air plenum is the same as through the outdoor air damper eliminates this instability when the system transitions from 100% outdoor air to partial return air. However, this results in greater fan horsepower when the system is at 100% outdoor air. Locating the exhaust fan remote from the return and mixed air ducts allows full sizing of the outdoor air damper and eliminates the possibility of outdoor air being directly exhausted by the exhaust fan. When a relatively high resistance exists in the return path, a closely coupled exhaust fan with an outdoor air damper sized to eliminate the instability (when the system transitions from 100% outdoor air to partial return) results in more horsepower required for the SE system. A remotely mounted exhaust fan results in total system horsepower requirements similar to an SR fan system (System SE-3A and 9A). Conclusions Relating to Horsepower Requirements When considering fan horsepower alone, return fans should be used only on systems that continuously operate with high percentages of outdoor and exhaust air, and have high restrictions on the return path. From the previous example, this indicates systems with > 0.5 in. w.g. (124 Pa) return duct pressure loss and full economizer (100% outdoor air) capability require less fan horsepower if a return fan is used. However, system construction and control differences between the two types of systems must be considered. Construction Considerations The SR fan system requires that the return fan be near to, and directly connected to, the return ducting to the supply fan. The 30 ASHRAE Journal ashrae.org April 2004
4 in. OA 50% OA 100% OA System Airflow () Pressure (in. w.g.) HP Required (HP= P/6356/Eff.) SR-4 20,000 2, SE-4 20,000 2, SR-5 20,000 10,000 8, SE-5 20,000 10,000 8, * SR-6 20,000 20,000 18, SE-6 20,000 20,000 18, * The pressure is (0.25) (8,000/18,000) 2 = 0.05 due to the reduced exhaust airflow. Cases 4, 5 & 6 Low Return Air System Pressure Loss Cases 4 through 6 are similar to Cases 1 through 3 except with reduced restriction on the return air path. This may be more typical of open plenum return systems. Horsepower requirements follow a similar pattern that favor the use of power exhaust fans except when operating near 100% outdoor air (90% exhaust). in. OA 50% OA System Airflow () Pressure (in. w.g.) HP Required (HP= P/6356/Eff.) SR-7 20,000 2, % OA SE-7 20,000 2, SR-8 20,000 10,000 8, SE-8 20,000 10,000 8, * SR-9 20,000 20,000 18, SE-9 20,000 20,000 18, SE-9A** 20,000 20,000 18, * The pressure is (0.25) (8,000/18,000) 2 = 0.05 due to the reduced exhaust airflow. ** See comments on Case 3. Cases 7, 8 & 9 High Return Air System Pressure Loss Cases 7 through 9 show the effect of high-pressure loss (1 in. w.g. [249 Pa]) on the return side. This may be more typical of ducted return systems. While the pattern is similar, it indicates more horsepower consumed by the SE system when the amount of outdoor air rises above 50% and the exhaust air rises above 40% (provided the exhaust fan is close coupled to the outdoor air duct and the outdoor air damper sized to prevent outdoor air from being directly exhausted, as explained in Case 3). exhaust fan does not require that it be connected to the return duct. Its only function is to exhaust excess air from the building to prevent overpressurization, giving the designer more freedom in locating it. Depending on building construction, almost no ducting may be required. Control Requirements Control requirements for VAV systems can be complex. Interaction between control loops may be the deciding factor when specifying a system. The design engineer must make the final selection based on experience and comfort level with a particular system. In general, SR system control is more complex, requiring additional controls and tuning. Constant Volume Systems For constant volume systems, no additional volume or flow April 2004 ASHRAE Journal 31
5 controls are required on the SR fan system. However, damper sizing and characteristics are vital to maintain constant flow while the economizer controls modulate the outdoor, return and exhaust dampers. 2 For the most common mixing box configurations, ASHRAE tests 3 have shown that parallel blade dampers are required for nearly constant flow as the mixing damper positions vary. The specific damper characteristic must be matched to the mixing box and subsystem loss. Analysis of damper characteristics is not covered in detail here. Additional control is recom- L H 2 3 P mended on the exhaust fan Lobby L H system. A separate controller VSD measuring building pressure should be used to modulate the exhaust fan speed to maintain building pressure as additional outdoor air is intro- 4 duced by the supply fan. Thus, AFS ± for a constant volume system, return fans can use simpler 5 controls than supply/exhaust Figure 3: Interactive control loops. fan systems. Variable Volume Systems The controls for variable volume systems are more complex, particularly when a return/exhaust fan is used. Although unsubstantiated, it is the author s belief that improper control of the return fan is the basis of many VAV system complaints. Volumetric flow tracking of supply and returns has been documented as unreliable in various articles and ASHRAE research. 4,5,6 In addition to the safety controls, and terminal unit controls, there are five interacting control loops on supply and return fan VAV systems and four interacting control loops on a VAV system with power exhaust (Figure 3). Damper sizing remains an important factor for variable volume systems. Stability can be a problem even though the fan speed can be modulated to compensate for differences in damper characteristics. Supply/Return Fan Control 1. Loop 1. Supply fan volume control. Typically, this is accomplished with a static pressure sensor located two-thirds down the supply duct. However, location is not critical if the setpoint is reset based on the position of the VAV box dampers (required by ANSI/ASHRAE/IESNA Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, for systems with direct digital controls at VAV terminals). 2. Loop 2. Return fan static pressure control. This controller senses static pressure on the discharge of the return fan (Point B) and is set to maintain a fixed static pressure, typically about 0.25-in. w.g. as indicated in the case studies. 3. Loop 3. Building static pressure control. This is accomplished with a sensor monitoring building pressure, usually on the first floor, referenced to outdoor ambient. This controller modulates the exhaust air damper as required to maintain a slightly positive building pressure. 4. Loop 4. inimum outdoor air flow control. The simplest method is a minimum airflow station in the minimum air section of a split outdoor air damper. The lowest cost approach is to measure the pressure drop across the outdoor air damper when it is in the minimum position and modulate the return air damper as required to maintain this pressure drop. Either of these methods can be enhanced with demand-controlled ventilation based on CO 2 measurements. ore complex control systems can be used to measure and control total outdoor air flow by staging one-quarter, threequarter airflow stations in the outdoor air Loop 5. Although not a pressure/flow control loop, the temperature control of the economizer dampers also affects stability by changing the system pressure as the dampers modulate. The control loops in Figure 3 need careful tuning to minimize the effect of interaction between the loops. This is particularly important because of the interaction of control loops 1 and 2, and 2 and 3. Lesser interaction occurs with exhaust fan systems because the loops are not as closely coupled, and the time delay that exists between a change in exhaust volume and significant change in building pressurization. To further reduce the interaction of the control loops, it is possible to decouple the ductwork between the exhaust fan and the supply fan and have the exhaust fan draw directly from the space. Supply/Exhaust Fan Control 1. Loop 1. Supply fan control. This is the same as Loop Loop 2. Building static pressure control. Similar to Loop 3 except instead of modulating the position of the exhaust air damper the speed of the exhaust fan is modulated to maintain building pressure. The exhaust duct can draw directly from the space. 3. Loop 3. Same as Loop Loop 4. Same as Loop 5. Conclusions This article reviewed calculations and power consumption as well as control considerations for several select systems. 32 ASHRAE Journal April 2004 VSD 1 T P L
6 The system designer is encouraged to follow similar calculations for any system under consideration. Specific items for consideration when designing air-handling systems include: 1. Can an exhaust fan be decoupled from the return duct system? If so, this favors the exhaust fan approach. 2. Is the pressure loss in the return duct system greater than 0.5 in. w.g. (124 Pa)? This tends to favor the return fan system. 3. Will the system be operating a high percentage of the time near 100% outdoor air? If so, and a high loss exists in the return duct system, this favors the return fan. Supply fans with powered exhaust fans provide several distinct advantages over supply/return/exhaust systems. However, calculations similar to those in this article must be made for the particular system under consideration. Advantages of exhaust fans: 1. They offer reduced total fan horsepower in almost all cases if return dampers are correctly selected. The exception is systems with high-pressure drop in the return duct combined with a high percentage of the air being exhausted during most hours of operation. 2. They offer more choices on location of the powered exhaust fan as compared to a return/exhaust fan. 3. ore simple and more stable control loops on VAV systems. System designers should seriously consider all these items when selecting a system and ask whether any justification exists for using return fans. References 1. Taylor, S.T Comparing economizer relief systems. ASHRAE Journal 42(9) ASHRAE Guideline , Selecting Outdoor, Return, and Relief Dampers for Air-Side Economizer Systems. 3. Van Becelaere, R ixing box damper testing. ASHRAE Transactions 104(2). 4. Krarti,. Techniques for measuring and controlling outside air intake rates in variable air volume systems. ASHRAE RP-980 Final Report. 5. Kettler. J.P Controlling minimum ventilation volume in VAV systems. ASHRAE Journal 40(5): Felker, L inimum outside air damper control. ASHRAE Journal 44(4). 7. Kettler, J.P easuring and controlling outdoor airflow. ASHRAE IAQ Applications Winter. Advertisement in the print edition formerly in this space. April 2004 ASHRAE Journal 33
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