Balancing of a Water and Air System (PDF

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108 The enthalpy (total heat see table 11) of the air at the leaving WB temperature minus the enthalpy of the air at the entering WB temperature determines the quantity of heat in BTU/LB. that the air picks up while passing through the tower. If this heat gain (enthalpy difference), is divided by the theoretical enthalpy difference that would result if the air left the tower at a wet bulb temperature equal to the temperature of the water entering the tower, the result is the Performance Factor. This theoretical enthalpy difference occurs at zero approach and represents a tower performance of 100%: Measure Enthalpy Difference / Theoretical Enthalpy Difference = Performance Factor (PF) If the water flow GPM and the airflow through the tower remain constant, then the P.F. will remain the same as the heat load and WB temperature change. With this as a basis, a cooling tower can be tested at a reasonable operating condition and from the data of this test calculate the performance of the tower at the specified full load conditions. It is possible to prepare a performance curve for the test tower at a given P.F. It has been found that the most useful curve is to plot approach vs. heat rejection at specified entering WB temperature, test water flow GPM and test P.F. From this curve, the engineer can determine if the condenser water system will satisfy the requirements of the operation of the tower at a wide range of operating conditions. See Example A in Appendix 13.2. Cooling Tower Terms EWT = Entering water temperature ºF (ºC) LWT = Leaving water temperature ºF (ºC) EWB =Air entering tower wet bulb ºF (ºC) LWB =Air leaving tower wet bulbº F (ºC) GPM = Gallons of water per minute over d = Air density in lbs. per cubic ft. leaving (see tables 9) R = Range across tower = EWT - LWT A =Approach to wet bulb = LWT - EWB h = Enthalpy of air at a wet bulb temperature of EWT (see table 11) h1 = Enthalpy of air at LWB (see table 11) h2 = Enthalpy of air at EWB (see table 11) P.F. = Tower performance capacity factor. This is a percentage of the heat picked up by the air in passing through the tower to the amount it would have picked up if it had left the tower saturated at the EWT tower at test tower = (h2 – h3)/ (h-h3). THR = Total heat rejected by tower in BTU per minute = GPM x 8.337 x (EWT - LWT) 108
109 Lam = lbs. of air per minute through tower = THR / (h.2 - h3) CFM = Tower CFM = Lam / d (of leaving air) Water Temperature Water temperatures are to be measured using calibrated mercury-in-glass or resistance thermometers, accurate to 0.1ºF (0.1º"C) with indicators or recorders graduated in increments no greater than 0.2º"F (0.2º"C). The temperature-sensitive element must be carefully located so that the temperature being sensed is representative of the true average temperature of the bulk fluid. The hot water temperature entering the tower should be measured in the riser (s) leading to the tower inlet (s), as close as practical to the tower, or at the discharge of the inlet riser(s) into the flume or distribution box (es) on the tower (Figures 13.1 & 13.2). The cold-water temperatures should be measured on the discharge side of the system circulating pump (Figures 13.1 8z 13.2). If the temperature of the cold water leaving the pump is being measured through a bleed test valve (as shown in Figure 13.3), the average temperature must be corrected for the heat added by the pump and the valve. The test valve must be sufficiently opened to provide adequate quantities to establish accurate temperature measurements. 109
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Balancing of a water and air system
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3 8 TBA (Testing, Balancing and Adj
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5 Air Balancing Multiple Hood syste
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7 87 Series Loop 88 One pipe main 9
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9 Testing and Balancing HVAC A syst
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11 13. Filter frame properly instal
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13 13. Measure and record all requi
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15 15 Example 3: Find the nearest s
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17 For traverses taken in round duc
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19 Flat Oval Duct Traverses For fie
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21 Balancing Devices Volume Dampers
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23 Turning Vanes Turning vanes (Fig
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25 Balancing the VAV System The gen
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27 maximum out-side air damper for
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29 determines how much energy is in
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31 the water removing capacity of a
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33 Table 8A Dry Bulb on left, Wet B
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35 60 58 56 54 52 50 48 46 44 42 40
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37 Foot³ per 1lb Altitude 0 Feet A
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39 Table 9 C Air Density at 29.29
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41 41 Wet-Bulb (F) .0 .1 .2 .3 .4 .
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43 Table 11A Enthalpy at saturation
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45 Table 11 C Enthalpy at saturatio
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47 Wet bulb temperature____________
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49 and replacement should be shut o
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51 duct may be great; the balancing
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53 Design below the 250°F limit (A
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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
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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
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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: 107 BTUH = GPM X 60 X 8.337 X ∆T
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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