Heating Ventilating and Air Conditioning: Analysis and Design
6th Edition
ISBN: 9780471470151
Author: Faye C. McQuiston, Jeffrey D. Spitler, Jerald D. Parker
Publisher: Wiley, John & Sons, Incorporated
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Chapter 3, Problem 3.53P
An interior zone of a large building is designed to have a supply air-flow rate of 5000 cfm (2.4 m3/s). The cooling load is constant at 10 tons (35 kW) with a SHF of 0.8 year-round. Indoor conditions are 75 F (24 C) db and 50 percent relative humidity (RH). (a) What is the maximum air dry bulb temperature and humidity ratio that would satisfy the load condition using all outdoor air? (b) Consider a different time when the outdoor air has a temperature of 40 F (4 C) db and 20 percent relative humidity. Return air and outdoor air may be mixed to cool the space. but humidification ill be required. Assume that saturated water vapor at 14.7 psia (101 kPa) is used to humidify the mixed air, and compute the amounts of outdoor and return air in cfm (m3/s). (c) At another time, outdoor air is at 65 F (18 C) db with a relative humidity of 90 percent. The cooling coil is estimated to have a minimum apparatus dew point of 45 F (7.2 C). What amount of outdoor and return air should be mixed before entering the coil to satisfy the given load condition? (d) What is the refrigeration load for the coil of part (c) above?
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Q2) Air is supplied to an automobile cabin from the outside, where the temperature is 20F
and the relative humidity is 70%. It is desired to keep the cabin at 70F and Ø = 60%.
Hoe many pounds of water must be supplied to each pound of air entering the car
cabin if these conditions are to be met? The barometric pressure is 29.5 in.Hg.
%3D
Ans. (0.00797 lbs/Ibdry air)
Given that Initial Temperature = 28.5 deg C, Dew Point = 9 deg C, Dry Bulb = 30 deg C, Wet Bulb = 28 deg C, Saturation Pressure = 1.148 kPa
Calculate the following:
Humidity Ratio w = 0.622PsPt-Ps=0.622PsPa kg water/kg air
Partial pressure of water vapour
where: Ps = partial pressure of water vapour (Psat at dewpoint temperature )
w = humidity ratio
Pt = total pressure (standard atmospheric pressure)
Pt = Pa + Ps
Pa = partial pressure of dry air = Pt - Ps
3. Relative Humidity
RH = PsPd
4. v = RaTPa m3/kg
5. ω = 1vkg/m3
where: Pd = saturation pressure at room temperature (from steam table)
ω = density of air
v = specific volume of dry air
T = absolute temperature of room temperature
Ra = gas constant of air =0.28706 kJ/kg-K
Q.1 An air conditioning system supplying air to a space with a sensible heat load of 14 kW and a
latent heat load of 9 kW has a cooling coil and a bypass path as shown schematically in Fig. The db-
temperature of the space is maintained at 26°C. The dry air mass flow rate of supply air is 1.2kg/s.
Outdoor ventilation air at 34°C db-temperature and 50% relative humidity is introduced into the
system with a dry air mass flow rate of 0.26 kg/s. The air leaving the cooling coil is fully
saturated at a db-temperature of 6°C. The pressure is constant at 101.3 kPa. Determine
db-temperature and relative humidity of the supply air to the space,
(ii)
(i)
(ii)
(iv)
wb-temperature of the space,
temperature of the air entering the cooling coil, and
refrigeration capacity of the cooling coil.
Chapter 3 Solutions
Heating Ventilating and Air Conditioning: Analysis and Design
Ch. 3 - A space is at a temperature of 75 F (24 C), and...Ch. 3 - Determine the humidity ratio, enthalpy, and...Ch. 3 - Suppose the air of Problem 3-2 is at a pressure...Ch. 3 - What is the enthalpy of moist air at 70 F (20 C)...Ch. 3 - The inside surface temperature of a window in a...Ch. 3 - What is the mass flow rate of dry air flowing at a...Ch. 3 - Determine the dew point of moist air at 80 F (27...Ch. 3 - A room is to be maintained at 72 F (22 C) db. It...Ch. 3 - Air is cooled from 80 F db and 67 F wb until it is...Ch. 3 - Conditions in a room are measured to be 80 F db...
Ch. 3 - The environmental conditions in a room are to be...Ch. 3 - Air enters a cooling coil at the rate of 5000 cfm...Ch. 3 - Air flowing in a duct has dry and wet bulb...Ch. 3 - Air is humidified with the dry bulb temperature...Ch. 3 - Air at 38 C db and 20 C wb is humidified...Ch. 3 - Two thousand cfm (1.0 m3/s) of air at an initial...Ch. 3 - Air at 40 F (5 C) db and 35 F (2 C) wb is mixed...Ch. 3 - Rework Problem 3-25, using Chart 1a, with the...Ch. 3 - The design cooling load for a zone in a building...Ch. 3 - Assume that the air in Problem 3-22 is supplied to...Ch. 3 - The sensible heat loss from a space is 500,000...Ch. 3 - Air enters a refrigeration coil at 90 Fdb and 75...Ch. 3 - A building has a total heating load of 200,000...Ch. 3 - Reconsider Problem 3-36 for an elevation of 5000...Ch. 3 - The system of Problem 3-34 has a supply air fan...Ch. 3 - An evaporative cooling system is to be used to...Ch. 3 - A cooling system is being designed for use at high...Ch. 3 - Consider a space heating system designed as shown...Ch. 3 - A variable-air-volume VAV cooling system is a type...Ch. 3 - Rework Problem 3-43 for an elevation of 5000 feet...Ch. 3 - The design condition for a space is 77 F (25 C) db...Ch. 3 - Rework Problem 3-45 for an elevation of 5000 feet...Ch. 3 - It is necessary to cool and dehumidify air from 80...Ch. 3 - Conditions in one zone of a dual-duct conditioning...Ch. 3 - Rework Problem 3-48 for an elevation of 5000 ft...Ch. 3 - A water coil in Problem 3-48 cools return air to...Ch. 3 - A multizone air handler provides air to several...Ch. 3 - Under normal operating conditions a zone has a...Ch. 3 - An interior zone of a large building is designed...Ch. 3 - Outdoor air is mixed with room return air to...Ch. 3 - Consider an enclosed swimming pool. The pool area...Ch. 3 - One particular zone served by a multizone air...Ch. 3 - A research building requires 100 percent outdoor...Ch. 3 - A space requires cooling in the amount of 120,000...
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Need a deep-dive on the concept behind this application? Look no further. Learn more about this topic, mechanical-engineering and related others by exploring similar questions and additional content below.Similar questions
- In a place where a summer air-conditioning plant will be installed, 40% of the air entering the air-conditioning plant is the fresh air, and 60% is the return (bypass) air. The sensible heat gain of the place is 10 kW. and the latent heat gain is 2.5 kW.> Outdoor conditions: 32 °C DB, %50 RH.> Indoor conditions: 20 °C DB. %50 RH.> Take the relative humidity of the air passing through the cooling coil 90% at the coil outlet. > The total air flow rate in the mixing chamber is 3000 m3/h> Take the density of the air as 1.2 kg/m3Based on the above. Calculate the amount of fresh air. What is the value of enthalpy of blowing air. Calculate the ratio of by-pass air at cooling coil. Calculate the flow rate of by-passed air.arrow_forwardIn a place where a summer air-conditioning plant will be installed, 40% of the air entering the air-conditioning plant is the fresh air, and 60% is the return (bypass) air. The sensible heat gain of the place is 10 kW. and the latent heat gain is 2.5 kW.> Outdoor conditions: 32 °C DB, %50 RH.> Indoor conditions: 20 °C DB. %50 RH.> Take the relative humidity of the air passing through the cooling coil 90% at the coil outlet. > The total air flow rate in the mixing chamber is 3000 m3/h> Take the density of the air as 1.2 kg/m3Based on the above.a) Show the process on a psychrometric diagram. b) Calculate the amount of fresh air. c) What is the value of enthalpy of blowing air. d) Calculate the ratio of by-pass air at cooling coil. e) Calculate the flow rate of by-passed air.arrow_forwardASHRAE standard 55, Thermal Environmental Conditions for Human Occupancy, notes that for thermal comfort purposes, the indoor temperature should range from approximately 67ºF to 82°F (19.4ºC to 27.8ºC), and systems designed to control humidity must be able to maintain a dew-point temperature of 16.8°C (62.2°F). ASHRAE standard 62.1, Ventilation and Acceptable Indoor Air Quality, recommends that relative humidity in occupied spaces be controlled to less than 65% to reduce the likelihood of conditions that can lead to microbial growth. Determine the minimum allowable indoor temperature that will ensure a humidity control system designed according to ASHRAE standard 55 will also satisfy the humidity requirements of standard 62.1arrow_forward
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