Workers in the petrochemical industry often wear flame-retardant jumpsuits. Unfortunately, the material from which they are made hinders evaporation of perspiration, and consequently heat stress can be a hazard in plants where it is hot and humid.

A control room in a petrochemical facility with dimensions 20 ft × 40 ft × 10 ft is used by workers to relax periodically in a cool environment. One summer day, the ambient air temperature is 33°C and the relative humidity is 96%. The air conditioning (AC) unit in the control room takes in ambient air and cools it to 11°C, which causes condensation of water that is discharged to a drain, and delivers the cooled air to the control room. The room air is maintained at 22°C, and there are 15 changes of room air per hour. The process may be shown schematically as follows.

(a) Use the psychrometric chart to estimate the following properties of the ambient air, cooled air, and exhaust air: moisture content, dew point, humid volume, and enthalpy at the process condition.

Also estimate the volumetric flow rate (m³/h) and relative humidity of the exhaust air. What

can you say about the moisture content of the cooled air and the exhaust air? How could you know simply by inspection of the ambient air properties that some of the water entering the air conditioner would be condensed?

(b) Draw and fully label a flowchart of the process. (Suggestion: Label the molar flow rates of each stream component.) Then calculate the rate of condensation of water (kg/h).

(c) Prepare an inlet–outlet enthalpy table for an energy balance on the air conditioner, taking the same reference conditions used to prepare Figure 8.4-1 (given in the figure caption). Calculate the cooling load on the air conditioner [-Q̇_ac(kW)].

(d) Calculate the net rate of heat transfer to the air in the room, Q̇_r(kW). List probable sources of this heat.

(e) In practice, about 80% of the exhaust air would be recycled to the air conditioner inlet. State at least two benefits of doing so.

Transcribed Image Text:

Qac (kW) é, (kW) Ambient air 33°C, h, = 96% Exhaust air V = 15 room volumes/h AC Cooled air CONTROL UNIT 11°C ROOM 22°C Condensate 11°C