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Q1:Moist air exists under conditions of 24°C db temperature and relative humidity 50 percent. The pressure is 101.3 kPa. Using the psychrometric chart, determine (i) the wet-bulb temperature, (ii) the dew-point, (iii) the humidity ratio, (iv) the enthalpy, and (v) the specific volume. Please show all the steps on the psychrometric chart.See Answer
Q2: Problem 2 (50 points) Consider the secondary loop of a chilled water pumping system, such as the one in the diagram below. Suppose one of the secondary pumps is active and the other is an identical backup. The performance data for each pump is provided on the next page. The provided operating conditions are the design conditions for this system. a) What is the approximate pump efficiency degradation (expressed in percentage points) due to trimming the impeller from 14" to 11.6"? Assume the 14" pump impeller serves the same system curve. b) If the 14" impeller were kept in place and a throttle valve were used at the pump discharge to regulate flow to the design condition flow, what would be the brake horsepower requirement of the active pump? What percentage increase in shaft power (relative to the 11.6" impeller pump) would this represent? c) For the scenario in part (b), how much head is lost across the throttle valve? Provide a calculation and explanation. d) If the system has zero control static head, and pump speed for the 11.6" impeller is adjusted from 100% to 70%, what would be the new pump flow, head, and brake horsepower? What percentage pump power reduction does this represent? e) Suppose the system controls pump speed to maintain a differential pressure (DP) setpoint and the DP sensor is located between the supply and return lines at one of the air handlers. Also suppose that an operator adjusts pump speed to produce 70% of the design flow. How would this scenario change your answers to part (d)? Provide qualitative descriptions (e.g., up, down, or stay the same) and explanations for each aspect. TWO POSITION ISOLATION VALVE 5 5 CHILLER #1 CHILLER #2 VARIABLE SPEED DRIVE (OPTIONAL) COMMON LEG SUPPLY AIR TEMPERATURE HIH PRIMARY LOOP VSD COIL VSD SECONDARY LOOP Page 2 of 3 KP - Horizontal Split Case Pump Head - ft NPSHr - ft 250 225 200 175 150 125 100 75 50 25 0 30 15 14.00 in 11.60 in 10.00 in 50 100 Operating Conditions Flow, rated Differential head / pressure, rated (requested) Differential head / pressure, rated (actual) Efficiency Speed, rated NPSH required Stages Impeller diameter, rated 150 600.0 USgpm 120.0 ft 119.9 ft 69.85 % 1780 rpm 13.88 ft 1 11.60 in 200 250 50 300 58 350 400 Liquid Liquid type Temperature, max Fluid density, rated / max Viscosity, rated 64 PACO KP is a single-stage, between bearings, split case pump. The axially split design allows easy removal of the top casing and access to the pump components without disturbing the motor or pipe work. (PC29) Benefits • Double suction minimizes axial load, which extends the life of the wear rings, shaft seals and bearings • Double Volute Design for increased efficiency, lower life cycle costs, & prolonged seal and bearing life • Independent bearing housing design allows access to the pump components without removing the top half of the casing • Suction baffles reduce losses and improve NPSH-R by directing flow into the eye of the impeller • High energy efficiency and low life cycle costs 68 450 500 550 Flow-USgpm Cold Water 68.00 deg F 1.000 SG 1.00 CP A 600 70 650 71 NPSHr 700 71 System Curve #1 750 800 Driver & Power Data 70 850 Motor sizing specification Site Supply Frequency Nameplate motor rating Rated power (based on duty point) Max power (non-overloading) Frame Size 900 MCSF 60 Hz 950 Max power (non- overloading) 30.00 hp/22.37 kW 26.02 hp 27.44 hp 286T Page 3 of 3 1,000See Answer
Q3:(20 pts). For comfort, the temperature of classrooms are maintained no higher than 30 % RH at 25 C. A classroom contains approximately 1000 kg of air (a metric ton). The windows are open and the indoor air is in equilibrium with the outdoor air. Your boss wants to know what the energy bill is going to be after you close the windows and turn on the HVAC system; that is, how much energy (Joules) has to be moved: (1) into the room if class were held in Boulder on the first snow day last month; and (2), out of the room if the class were held in New Orleans?See Answer
Q4:Take Home Exam Q1: The following data refer to public hall AC system: 100% Outdoor conditions: tab= ?; o = ? Comfort conditions: tab = 20°C; = 50% Seating capacity of the hall = z Outdoor air supplied = 0.3 m³/min/person If the required comfort condition is achieved first by adiabatic humidification and then sensible cooling process find: 1. The capacity of the cooling coil in (TR) 2. The coil surface temperature if the by-pass factor is 0.25 3. The mass of water required in the humidifier, in kg/hr 4. The efficiency of the humidifier Q2: Air enters a cooling coil (of By-Pass factor BF) at 24°C dry-bulb-temperature and 50% relative humidity with a dry air mass flow rate of 0.9 kg/s. The air leaving the cooling coil at 9°C is reheated to 13°C. The total pressure is constant at P kPa. Determine (i) the Apparatus dew-point temperature (ADP)of the coil, (ii) the rate of moisture removal in the cooling coil, in kg/hr (iii) the refrigeration capacity of the cooling coil, in kW (iv) The heat input rate of the heating coil, in kW Enthalpy kikgSee Answer
Q5:1. When design a constant air volume (CAV) HVAC system for a commercial office building, the following design conditions were considered: The design ambient conditions are 92 F dry bulb and 78 F wet bulb. The building can be treated as a single zone with a total zone load of 500,000 Btu/hr and a sensible heat ratio of 0.8. The design occupancy is 80 people and the corresponding ventilation air flow rate is 1600 cfm, which is based on 20 cfm of outdoor air per person. The zone temperature thermostat setting is 77 F. which is within the summer comfort zone. The bypass factor for the coil is 0.175 and the chilled water temperature is 50 F. Reheat is zero at design condition. (1) Determine circulation and ventilation airflow rates, and thermodynamic states at design conditions using iterative methods. a. Guess the unknown values needed in the calculations shown in page 3. For example guess 47% relative humidity in the zone and estimate the circulation air flow rate by using the saturated air enthalpy at 50F for supply air with the following equation: LT-ms (hz - hs) b. Follow the psychrometric processes (page 3) to calculate the thermodynamic states and circulation air flow rate. c. Use the new zone state properties and circulation flow rate to repeat the calculations in b. Until further the results are converged, i.e. the changes in two consecutive calculation results are small. Take the results in the final iteration as the solutions. Record the results in Table 1./nc. Use the new zone state properties and circulation flow rate to repeat the calculations in b. Until further the results are converged, i.e. the changes in two consecutive calculation results are small. Take the results in the final iteration as the solutions. Record the results in Table 1. (2) Calculate the zone relative humidity, coil load, and the circulation flow rate in cfm after finding the converged solutions in (1). List the results in Table 2. Iteration Ambient (A) Entering (E) Coil exit (C) Supply (S) Water (w) x-state (x) Zone (Z) Table 1 Properties of thermodynamics states of two iterations Temperature (F) Humidity Ratio (1bmw/1bma) 92 50 77 77 Relative Humidity 1 1 2 Enthalpy (Btu/lbm) 1 2See Answer
Q6:2. Consider the designed CAV HVAC system in 1 is operating at part load with ambient conditions of 86 F dry-bulb and 70 F wet-bulb. The total load is now 350,000 Btu/hr with the same sensible heat ratio of 0.8 and the same zone temperature of 77 F. The circulation flow rate is the same as design conditions found in Problem 1 and ventilation flow rate is the same at 1600 cfm. The bypass factor for the coil is 0.175 and the chilled water temperature is 50 F, which is the same as design conditions. Use the same method in problem 1 to calculate the zone relative humidity, coil load, and reheat heat transfer rate. List the results in Table 2.See Answer
Q7:3. Variable air volume (VAV) HVAC system can vary circulation flow rate for different operating conditions. Consider a VAV HAVC system operates at the same part load as in Problem 2 with ambient conditions of 86 F dry-bulb and 70 F wet-bulb. The total load is 350,000 Btu/hr with the same sensible heat ratio of 0.8 and the same zone temperature of 77 F. The ventilation flow rate is the same at 1600 cfm. The bypass factor for the coil is 0.175 and the chilled water temperature is 50 F, which is the same as design conditions. Reheat is zero. Use the same method in problem 1 to calculate the zone relative humidity, coil load, and reheat heat transfer rate. List the results in Table 2. CAV at design condition CAV in part load VAV in part load Table 2 Properties of thermodynamics states of two iterations Circulation flow rate lbm/hr cfm Zone relative humidity Total load Btu/hr 500000 350000 350000 tons Coil load Btu/hr tons Reheat Btu/hr 0 0See Answer
Q8:5.5 A flow of 2000 cfm of moist outdoor air at 90 F and 70% RH mixes with a 1000 cfm stream of return air at 65 F and 20% RH. Determine the temperature, relative humidity, humidity ratio, and enthalpy of the mixed stream at sea level location.See Answer
Q9:5.7 For air-conditioning system operation in cold weather, it is possible to introduce enough outdoor air to eliminate cooling by the air conditioner. This is called an economizer, and the control strategy is to vary the amount of outdoor air entering a plenum so that the outlet state is at the set point. For zone conditions of 75 F and 50% RH and a constant circulating air flow rate of 25,000 cfm, determine the outdoor and return air flow rates necessary to maintain a 55 F supply air leaving the plenum. The outdoor air temperature varies between 10 F and 50 F and the relative humidity is always 30%.See Answer
Q10:5.10 SI A cooling coil with a bypass factor of 0.11 and a chilled water temperature of 12 C processes a flow of 3500 L/s. Determine the outlet state and the total, sensible and latent energy terms for entering conditions of 32 C and relative humidity between 10 and 90%.See Answer
Q11:5.17 A flow of 4000 cfm of air at 53 F and 95% RH enters the reheat coil of a building zone. The zone set temperature is 78 F and the zone sensible heat ratio (SHR) is 0.8. 1. Determine the maximum sensible and total load that can be met by the air flow and the room RH at the maximum load. 2. Determine the reheat energy that must be supplied to keep the zone at 78 as the sensible load is decreased from the maximum value to zero with the SHR remaining at 0.8. The flow rate, temperature, and humidity of the air supplied to the reheat coil remain the same.See Answer
Q12:Instructions by student: Design a vapor-compression refrigeration system using r134a capable of cooling a space to a temperature of - 25 °C with a lifting capacity of 10 kW. The system must be able to reject heat to surroundings with a maximum temperature of 40 °C. The maximum pressure ratio across a compressor is 5 with an isentropic efficiency of 90%. Determine a) the heat rejected at the condenser and b) the refrigeration system's coefficient of performance (COP). Bonus, demonstrate you have developed a design which maximizes performance under the prescribed conditions (10 pts). In EEsSee Answer
Q13:7.5 An HVAC system is being designed to serve the interior and exterior zones of a small office building. The design occupancy is 25 for the interior zone and 40 for the exterior zone. The ambient design condition is 95 F and 70% relative humidity. The circulation flow rates are 4,000 cfm and 8,000 cfm for the interior and exterior zones, respectively. a. Specify the minimum and maximum zone setpoint temperatures that you would use in designing the system. Assume that the zone humidity will be about 50%. b. Determine the necessary ventilation airflow rates for the system and for each zone. C. Determine the ventilation load for the two levels of zone set temperature selected in b. d. Draw some conclusions from your analysis.See Answer
Q14:7.7 Area The air distribution system for a restaurant is being designed. The occupancy and cooling loads for the three main areas are given in the table. The cooling design loads are based on a design ambient of 95 F db and 78 F wb and a SLR of 0.7. These loads do not include the ventilation load. Occupancy Cooling load (Btu/hr) 120,000 40,000 30,000 Dining Bar Kitchen 150 20 4 a. Specify the set points, flow rates, and capacity of the air- conditioning systems. b. Draw some conclusions from your analysis.See Answer
Q15:Table 2: Material properties, resistance, UA, and capacitance for each roof layer U (1/R) W/m2- с 25 F01 Outer Surface Resistance F13 Build-Up Roofing 4 cm Roofing Insulation 10 cm Concrete R5 Combined 10 cm Steel Joints 10 cm Airspace 20 mm Plaster Ceiling F03 Inside Horizontal Surface Resistance Total R R1 R2 R3 L k mm 9.5 0.16 28 W/m- с 40 rho Cp R C- kg/m³ kJ/kg- с m²/W 0.04 0.06 16.84 0.75 1,120 0.03 43 1.46 1.21 0.84 R4 R5 R5a 100 45.4 7,800 0.5 R5b 100 0.0251 1.2 R6 20 0.16 800 R7 100 0.53 1,280 1.004 1.05 1.33 0.19 5.30 0.18 5.56 0.12 8.33 0.18 5.56 0.13 8.00 0.16 6.25 XXX XXX с (tho*cp*L) kJ/m2-C 15.53 2.08 107.52 11.82 390.00 0.12 16.80 a. Determine the overall conductance of the roof. b. Use the thermal network approach to represent the transient response. Assume that all of the thermal storage is in the concrete and that the other roof elements have only a thermal resistance. Verify that the capacitance of the steel joists is negligible. c. Determine and plot the heat gain as a function of time over the course of the day d. Discuss how the HVAC system design is affected by energy storage in the concrete.See Answer
Q16:9.12 A zone of a commercial building in Miami is maintained at 24 C and 50% RH. It is occupied by 12 people from 8 am to 5 pm, each with a computer (100 W), 4 printers (200 W) and a copier (1000 W). The required ventilation flow rate is 7.5 l/s per person. The exterior walls are Wall type 31 with 100 m² of west-facing wall area. The windows are double pane reflective coated windows with 20 m² of area. The design day dry-bulb and wet-bulb temperatures and solar radiation are given in the table below. Determine and plot the components of the total heat gain for the zone. Determine the maximum heat gain, the SHR at this time, and the time it occurs. Draw some conclusions from your analysis. Table 3 shows the hourly ambient dry-bulb and wet- bulb temperature and solar radiation and Table 4 shows the material properties, thermal resistance, UA-value, and capacitance for each wall layer. Please complete Table 4 in the by inputting the correct thermal resistance, UA-value and capacitance of the gray boxes. Table 3: Hour ambient data Time T₁ 1 2 3 4 5 6 7 8 9 10 11 12 F01 surface M15 concrete 104 insulation G01 Gypsum board 27 26 26 26 26 26 26 27 28 29 30 31 L E k Tub 24 24 24 23 23 23 24 24 24 24 25 25 W/m- K rho G₁ W/m² 0 Kg/m³ 0 0 0 0 12 83 139 184 219 243 262 23 24 Table 4: Wall Data Time kJ/kg- K 13 14 15 16 17 18 19 20 21 22 Cp R (L/K) m². K/W 0.044 0.17 1.95 2242.58 0.92 0.07 0.05 19.22 0.96 0.01 0.16 800.92 1.09 T₂ (C) 32 32 32 32 32 31 30 29 29 28 27 27 R m². K/W U (1/R) W/m². K Tub 25 25 25 25 25 25 25 24 24 24 24 24 Gt W/m² 406 606 758 834 796 562 6 0 0 0 0 0 C (rho x Cp x L) kJ/m²-k N/ASee Answer
Q17:Consider a building located in Flagstaff, Arizona for problems 1-4. Its wall is constructed as ASHRARE wall type 3. It has 20 reflective double pane windows with dimensions of 6 × 3 ft. 1. The inside temperature is 77 F. The outside air temperature is the 99.6 % heating dry bulb temperature. Find the transmission heat loss from the windows. (20)See Answer
Q18:2. One zone of the building is designed as office for 10 people occupancy. The building inside is maintained at 77 F. The outside air temperature is the 99.6% heating dry bulb temperature. Find the ventilation sensible heat loss. (20) Some constants and unit conversions: air heat capacity C 0.2396 Btu/lb-F, air gas constant R= 0.06856 Btu/lb-R, 1 Btu = 778 lbf-ft, 1 psia = 144 lbf/ft²See Answer
Q19:4. The building has a zone that has a total cooling load of 300,000 Btu/hr with a sensible load ratio of 0.75. The zone set point is 77 F. The supply air is at 55 F and 90% relative humidity. Determine the required supply air volume flow rate and the zone relative humidity. (20)See Answer
Q20:5. Which ASHRAE standard is used for specifying thermal comfort in buildings? Determine the optimal air temperature for comfort in a classroom. Include your assumptions and calculations. Explain why it is impossible have an optimal temperature that pleases everyone. (20)See Answer

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