middle east technical university

MIDDLE EAST TECHNICAL UNIVERSITY
Department of Chemical Engineering
Ch.E 328,
Separation Processes
Prof.Dr.H.Önder Özbelge
May 1, 2014
HOMEWORK VIII
May 8, 2014
Problem 1
An air conditioning system has to be designed to change the air in a laboratory every 15 min. The
laboratory measures 25x80x4.5 meters, and is to be supplied with air at 25º C and 40 % saturation.
The air conditioner takes ambient air, refrigerates it, separates out the condensed water and then
reheats it in an exchanger with condensing steam at 2 Atm.
a) When the outside air is at 35º C and 95 % relative humidity, determine the volume of air
brought into the air conditioner and temperature to which it must be cooled.
b) Determine the tons of refrigeration required (1 ton refrigeration is equivalent to 3516.84 W)
and the rate of saturated steam necessary.
Problem 2
A countercurrent packed cooling tower is in operation and is processing 10000 kg/h of water. On a
typical day, the following data are collected:
Inlet temp. of hot water:
35°C
Outlet temp. of cooled water:
20°C
Enthalpy of inlet air (wet):
40 kJ/kg dry air
Enthalpy of outlet air (wet):
100 kJ/kg dry air
Calculate:
a) The flow rate of the inlet air?
b) Minimum amount of air that could be utilized?
c) The minimum temperature to which the liquid could be cooled down?
d) The temperature to which the liquid could be cooled down if the inlet enthalpy of air changes
to 48 kJ/kg dry air (T inlet,H2O= 35°C). Note that enthalpy of outlet air also changes. However air
and water flow rates remain the same. The solution requires a trial and error procedure. When
nd
solving part d, stop after 2 trial and make your comments on the solution.(ChE 328 MT I
2006 Question)
Problem 3.
A counter-current forced draft cooling tower packed with wood slats is to cool water from 44º C to 28º
C. The entering air is at 30º C and has a wet bulb temperature of 23º C. The water rate is 20000 kg
2
water/hr and a gas flux of Gv’ = 1.5 kg dry air/s m is required. The overall mass transfer coefficient
can be estimated by:
0.4
Kya = 0.026 GL
Gv’
0.5
2
2
In which Kya [=] kg mole/hr m , GL and Gv’ [=] kg/m hr
a) Calculate the minimum air rate
b) For 1.7 times the minimum air rate find the cross sectional area of the tower
c) Calculate the height of the tower
Question 4
A cooling tower cools 4000 lt/min of water from 40 ºC to 32 ºC using counter current forced draft air
entering at 43 ºC and 20 % saturation. Measurements indicate that the air leaves at 35 ºC and 34 ºC
wet bulb temperature.
The plant manager wishes to cool the water to as cold a temperature as possible. One possible means
would be to increase the air flow rate. It is found that air fan speed can be increased without
overloading the fan motors so that air flow is 1.5 times that previously used. Tower flooding will not
occur at this higher air rate.
What will be the outlet water temperature attained with the higher air rate, and what will be the air
outlet condition?
What will be the rate of heat (Q) and mass transfer (M)?
0.4
Kya = 0.0068 GL
’0.5
GV
Bir bilge kisi, çölde öğrencileriyle otururken demiş
ki;
- "Gece ile gündüzü nasıl ayırt edersiniz? Tam olarak
ne zaman
karanlık başlar, ne zaman ortalık aydınlanır?"
Öğrencilerden biri;
- "Uzaktaki sürüye bakarım," demiş, "Koyunu keçiden
ayıramadığım zaman
akşam olmuş demektir."
Diğeri demiş ki, "Beyaz ipliği siyah iplikten
ayırdedebildiğimde,
anlarım ki karanlık bitmiştir.
Başka bir öğrenci söz almış ve "Hocam" demiş,
"İncir ağacını, zeytin
ağacından ayırdığım zaman, anlarım ki sabah
başlamıştır."
Bilge kişi, uzun süre susmuş. Öğrenciler
meraklanmışlar ve "Siz ne
düşünüyorsunuz saygıdeğer hocam?" diye sormuşlar.
Bilge kişi şöyle demiş;
- "Yürürken karşıma bir kadın çıktığında, güzel
mi çirkin mi, siyah mı
beyaz mı diye ayırmadan ona "bacım" diyebildiğimde...
ve yine yürürken önüme çıkan erkeği, zengin mi
yoksul mu diye
bakmadan, milletine, ırkına, dinine aldırmadan,
"kardeşim"
sayabildiğimde anlarım ki bende
AYDINLIK başlamıştır..."