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It is required to maintain an air conditioned hall

at 27°C DBT and 21°C WBT. It has a sensible heat load of 46.5 KW and latent heat load of 17.5 kW. The air supplied from outside atmosphere at 38°C DBT and 27°C WBT is 25 m3/min directly into the room through ventilation and infiltration. Outside air to be conditioned is passed through the cooling coil whose ADP is 15°C, 60% of the recirculated air from the hall is mixed with the conditioned air after the cooling coil. Determine:

a) Condition of air after the coil and before the recirculated air mixes with it.

b) Condition of air entering the hall i.e. after mixing with recirculated air.

c) Mass of fresh air entering the cooler

d) BPF of the cooling coil

e) Refrigerating load on the cooling coil

Draw the neat line diagram for the processes involved in the air conditioning of the hall and represent all the processes on the Psychrometric chart.

Subject:- Refrigeration and Air Conditioning

Topic:- Design of air conditioning systems

Difficulty:- Low

1 Answer
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$t_d4=27℃,t_w4=21℃, t_d1=38℃, t_w1=27℃,ADP=15℃$,

SH=46.5 kW,LH=17.5 kW

Locate points 1 and 4 on psychrometric chart, and draw vertical from 1 and horizontal from 4 to obtain point A.

$V ̇_a1=25 \frac{m3}{min}$

Let $Q ̇_si$ be rate at which sensible heat is added by infiltrated air= $m ̇_i ×(hA-h2)$

$m ̇_i=\frac{V ̇_a1}{v_1} =\frac{25}{0.907}$=27.56 kg/min

∴$Q ̇_si=27.56×(72.8-61)=325.21 \frac{kJ}{min}$=5.42 kW

Let $Q ̇_li$ be rate at which latent heat is added by infiltrated air= $m ̇_i ×(h1-hA)$

∴$Q ̇_li=27.56×(85-72.8)=336.23 \frac{kJ}{min}$=5.6 kW

Now RSH=SH+$Q ̇_si$=51.92 kW

RLH=$LH+Q ̇_li$=23.1 kW

RSHF=${RSH}{(RSH+RLH)}$=0.692

Now mark the RSHF value on SHF scale and join with alignment circle. Draw a line parallel to this from point 4. This is RSHF line. Join point 1 with ADP point of 15°C on saturation curve. This is GSHF line and intersects RSHF line at point 2. To mark point 3, mixing condition is

0.6=$\frac{length(23)}{length(24)}$

Condition of air after the coil and before the recirculated air mixes with it

$t_d2=19℃,t_w2$=17.5℃

Condition of air entering the hall i.e. after mixing with recirculated air

$t_d3=24℃,t_w3=19.8℃$

Mass of fresh air entering the cooler

$m ̇_f=\frac{(RSH+RLH)}{(h4-h2)}=6.25 \frac{kg}{sec}$=375 kg/min

BPF of the cooling coil

BPF= $\frac{(t_d2-ADP)}{(t_d1-ADP)}=\frac{(19-15)}{(38-15)}$=0.174

Refrigerating load on the cooling coil =$m ̇_f (h1-h2)=13500 \frac{kj}{min}$=64.3 TR

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