Balancing of a Water and Air System (PDF

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50 balance of the remaining system. In cases such as master kitchen-exhaust systems, which are sometimes used in shopping center food courts, no single group is responsible for the entire design. The base building designer typically lays out ductwork to (or through) each tenant space and each tenant selects a hood and lays out connecting ductwork. Often the base building designer has incomplete information on tenant exhaust requirements. Therefore, one engineer must be responsible for defining criteria for each tenant’s design and for evaluating proposed tenant work to ensure that tenant designs match the system’s capacity. The engineer should also evaluate any proposed changes to the system, such as changing tenancy. Rudimentary computer modeling of the exhaust system may be helpful (Elovitz 1992). Given the unpredictability and volatility of tenant requirements, it may not be possible to balance the entire system perfectly. However, without adequate supervision, the probability that at least part of the system will be badly out of balance is very high. For greatest success with multiple-hood exhaust systems, minimize pressure losses in the ducts by keeping velocities low, minimizing sharp transitions, and using hoods with relatively high-pressure drops. When pressure loss in the ducts is low compared to the loss through the hood, changes in pressure loss in the ductwork because of field conditions or changes in design airflow will have a smaller effect on total pressure loss and thus on actual airflow. Minimum code-required air velocity must be maintained in all parts of the exhaust ductwork at all times. If fewer or smaller hoods are installed than the design anticipated, resulting in low velocity in portions of the ductwork, the velocity must be brought up to the minimum. One way is to introduce replacement air, preferably untempered, directly into the exhaust duct where it is required (see Figure 13). The bypass duct should connect to the top or sides (at least 2 in. from the bottom) of the exhaust duct to prevent backflow of water or grease through the bypass duct when fans are off. This arrangement is shown in NFPA Standard 96 and should be discussed with the authority having jurisdiction. A fire damper should be provided in the bypass duct, located close to the exhaust duct. Bypass duct construction should be the same as the exhaust duct construction, including enclosure and clearance requirements, for at least several feet beyond the fire damper. Means to adjust the bypass airflow must be provided upstream of the fire damper. All dampers must be in the clean bypass air duct so they are not exposed to grease-laden exhaust air. The difference in pressure between replacement and exhaust air 50
51 duct may be great; the balancing device must be able to make a fine airflow adjustment against this pressure difference. It is best to provide two balancing devices in series, such as an orifice plate or blast gate for coarse adjustment followed by an opposedblade damper for fine adjustment. Directly measuring air velocities in the exhaust ductwork to assess exhaust system performance may be desirable. Velocity (pitot-tube) traverses may be performed in kitchen exhaust systems, but holes drilled for the pitot tube must be liquid tight to maintain the fire-safe integrity of the ductwork per NFPA Standard 96. Holes should never be drilled in the bottom of a duct, where they may collect grease. Velocity traverses should not be performed when cooking is in progress because grease collects on the instrumentation. PRINCIPLES AND PROCEDURES FOR BALANCING HYDRONIC SYSTEMS Both air- and water-side balance techniques must be performed with sufficient accuracy to ensure that the system operates economically, with minimum energy, and with proper distribution. Airside balance requires precise flow measuring because air, which is usually the prime heating or cooling transport medium, is more difficult to measure in the field. Reducing airflow below design airflow reduces heat transfer directly and linearly with respect to waterside flow. In contrast, the heat transfer rate for the waterside does not vary linearly with the water flow rate through a heat exchanger (coil), because of the characteristics of heat exchangers. As a result, waterside heat transfer is less sensitive to changes in flow, and the required accuracy of flow is lower when using traditional design criteria. The relatively high pressures associated with hydronic systems allow for easier measurement of pressure, although application of flow and head relationships should be thoroughly understood. 51
Page 1 and 2: Balancing of a water and air system
Page 3 and 4: 3 8 TBA (Testing, Balancing and Adj
Page 5 and 6: 5 Air Balancing Multiple Hood syste
Page 7 and 8: 7 87 Series Loop 88 One pipe main 9
Page 9 and 10: 9 Testing and Balancing HVAC A syst
Page 11 and 12: 11 13. Filter frame properly instal
Page 13 and 14: 13 13. Measure and record all requi
Page 15 and 16: 15 15 Example 3: Find the nearest s
Page 17 and 18: 17 For traverses taken in round duc
Page 19 and 20: 19 Flat Oval Duct Traverses For fie
Page 21 and 22: 21 Balancing Devices Volume Dampers
Page 23 and 24: 23 Turning Vanes Turning vanes (Fig
Page 25 and 26: 25 Balancing the VAV System The gen
Page 27 and 28: 27 maximum out-side air damper for
Page 29 and 30: 29 determines how much energy is in
Page 31 and 32: 31 the water removing capacity of a
Page 33 and 34: 33 Table 8A Dry Bulb on left, Wet B
Page 35 and 36: 35 60 58 56 54 52 50 48 46 44 42 40
Page 37 and 38: 37 Foot³ per 1lb Altitude 0 Feet A
Page 39 and 40: 39 Table 9 C Air Density at 29.29
Page 41 and 42: 41 41 Wet-Bulb (F) .0 .1 .2 .3 .4 .
Page 43 and 44: 43 Table 11A Enthalpy at saturation
Page 45 and 46: 45 Table 11 C Enthalpy at saturatio
Page 47 and 48: 47 Wet bulb temperature____________
Page 49: 49 and replacement should be shut o
Page 53 and 54: 53 Design below the 250°F limit (A
Page 55 and 56: 55 Differential Pressure Gauges The
Page 57 and 58: 57 Venturi The venturi is a device
Page 59 and 60: 59 Pressure Total pressure Velocity
Page 61 and 62: 61 Table 1 shows that if the coil i
Page 63 and 64: 63 adjustment of these devices is n
Page 65 and 66: 65 Grouting Motor and lubrication N
Page 67 and 68: 67 For every outside temperature ot
Page 69 and 70: 69 4. Identify the riser with the h
Page 71 and 72: 71 2. Turn the pump off 3. Close th
Page 73 and 74: 73 Net positive suction head availa
Page 75 and 76: 75 All automatic control valves mus
Page 77 and 78: 77 77
Page 79 and 80: 79 4. Read and record the actual pu
Page 81 and 82: 81 13. Record the coils design and
Page 83 and 84: 83 Air Control Air is always presen
Page 85 and 86: 85 • allow draining of all or par
Page 87 and 88: 87 Valves have linear throttling ch
Page 89 and 90: 89 to all the terminals down the li
Page 91 and 92: 91 Pump Circuits Primary-Secondary
Page 93 and 94: 93 • Evaporator: • the design a
Page 95 and 96: 95 95
Page 97 and 98: 97 Condenser • the design and act
Page 99 and 100: 99 • Each reading of liquid tempe
Page 101 and 102:
101 • Design and actual entering
Page 103 and 104:
103 differs from the average by mor
Page 105 and 106:
105 • Orifice plate • Turbine m
Page 107 and 108:
107 BTUH = GPM X 60 X 8.337 X ∆T
Page 109 and 110:
109 Lam = lbs. of air per minute th
Page 111 and 112:
111 When dry bulb temperature measu
Page 113 and 114:
113 GPM = FPM X Gallons per foot FP
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