# Return air from a special air-conditioned space is at 32oC dry bulb temperature… 1 answer below »

Part 1: Answer the following questions (Expected time to spend, 2 hours)

Note: I highly recommend you to do these two questions without looking at the lecture notes, at

least in your attempt.

Q1.

(a) Return air from a special air-conditioned space is at 32oC dry bulb temperature, and 50%

relative humidity (state 1). To satisfy the design requirements of the space, air has to be supplied

at 16oC dry bulb temperature, and 65% relative humidity (state 2). The pressure is constant at

101.3 kPa. There is no outdoor air supply to the space.

(a1) An engineer quickly identified that a cooling and dehumidifying coil cannot be used as the

only processing unit of the system. One solution (Solution 1) is to entire air flow to the saturated

state 4 (9.5oC), and then reheat sensibly from state 4 to state 2. Propose an alternative solution

(Solution 2) that may be more energy efficient, and illustrate your proposed solution on a

schematic psychrometric chart.

(a2) The mass flow rate of dry air to the space is 0.8 kg/s. Calculate the refrigeration capacity of

the cooling coil for both Solution 1 and Solution 2.

(a3) Calculate the heating capacity of the heating coil for both Solution 1 and Solution 2.

Data given

ASHRAE Psychrometric chart; see lecture notes.

(b)

(b) Briefly describe the basic principles of the variable air volume (VAV) systems in air

conditioning (less than 100 words).

Q2.

(a) Consider an exterior wall of a building, which has a 10cm thick layer of face brick on the

outside, followed by a layer of 10 cm thick concrete. A 15 cm thick layer of mineral wool

insulation is sandwiched between the concrete and a layer of plywood of thickness 10 mm. The

wall is 15 m long and 3 m high.

? The outside and inside heat transfer coefficients are 30 W/m2K and 9 W/m2K respectively.

? The outside and inside air temperature are -5

oC and 22oC respectively.

Calculate

(a1) The total thermal resistance (m2K/W);

(a2) The overall heat transfer coefficient (W/m2K),

(a3) The total heat transfer rate through the wall at steady state, and analyze

2

(a4) If the interior surface temperature would experience condensation when the indoor relative

humidity is as high as 90%.

Data given

ASHRAE Psychrometric chart; see lecture notes