1.0
Objectives The objectives of this experiment was to investigate the effect of air velocity on wet bulb approach and to check the pressure drop through packing.
2.0
Abstract
The processes of cooling water are among the oldest known. Usually water is cooled by exposing its surface to air. These processes all involve the exposure of water surface to air in varying degrees. The heat-transfer process involves latent heat transfer owing to vaporization of a small portion of the water and sensible heat transfer owing to the difference in temperature of water and air. In this experiment, we have to study of the effect of air velocity on wet bulb approach and pressure drop through the packing. The experiment was done with four different sets of orifice pressure drop values which are 100%, 75%, 50% and 25%. Based on the result, we had to calculate the value of nominal velocity and wet bulb approach. The values of nominal velocity air keep decreasing while the values of wet bulb not remain persistent. The average value of wet bulb is 280.00K. Then, the pressure drop through packing is decreased when the nominal velocity decreases. Therefore, it was showed the cooling tower was worked in optimum condition based on theoretical. Basically, common applications for cooling towers are providing cooled water for air-conditioning, manufacturing and electric power generation.
3.0
Data
Table A
Description
Unit
Air Flow 100%
75%
50%
25%
m-1
110
110
110
110
Air inlet wet bulb, T1 Air inlet dry bulb, T2 Air outlet wet bulb, T3 Air outlet wet bulb, T4 Water inlet temperature, T5
̊C
22.7
23.0
23.1
23.2
̊C
30.1
30.4
30.5
30.4
̊C
31.0
29.4
29.3
29.9
̊C
29.6
28.6
28.6
29.7
̊C
41.7
37.3
36.3
36.5
Water outlet temperature, T6
̊C
30.5
28.9
28.9
29.7
Orifice differential, DP1
Pa
87.0
63.0
43.5
22.0
Water flow rate, FT1
LPM
2
2
2
2
Heater power, Q1
Watt
1000
1000
1000
1000
Pa
38.0
23.6
17.5
11.0
Packing density
Pressure drop across packing, DP2
Table B
Air flow
100%
75%
50%
25%
Nominal Velocity of Air, m/s Wet Bulb Approach, K
0.4298
0.3640
0.3027
0.2158
7.80
5.85
5.80
6.50
Pressure, mm H2O
3.8749
2.4065
1.7845
1.1217
9
8
y = 5.0889x + 4.818 R² = 0.2477
Wet bulb approach, K and Packaging pressure drop, mmH2O
7
6
5
wet bulb approach vs. nominal air velocity
4 y = 12.432x - 1.7817 R² = 0.9278
packing pressure drop vs. nominal air velocity
3
2
1
0 0
0.1
0.2
0.3
0.4
0.5
Nominal air velocity, m/s
Fig. 3.1: Wet bulb temperature and Packing pressure drops versus Nominal air velocity
4.0
Discussion
When warm liquid is brought into contact with unsaturated gas, part of the liquid evaporates and the liquid temperature drops. The most important application of this principle is in the use of cooling tower to lower the temperature of recirculated water used for condensers and heat exchangers in chemical plants, power plants and air conditioning. Cooling towers are large-diameter column with special type of packing designed to give good gas-liquid contact with low pressure drop. Warm water is distributed over the packing by spray nozzles or a grid of notched fans, or in some design it is drawn through by natural convection. In counter-flow tower air enter below the layer of fill and passes upward counter current to the flow of descending water. This is a more efficient arrangement for heat transfer and permits a closer temperature approach. The most common type of packing for new installation is cellular fill or film-type packing, which consist of corrugated sheets of plastic similar to those used in plate type heat exchanger. The reduction in water temperature in cooling tower comes mainly from evaporation, although when air temperature is low, there is also some sensible heat transfer to the air. However, even when the air is warmer than the water, water can be cooled by evaporation if the wet-bulb temperature is below that of the water. In practice, the discharge temperature of water is 5 to 15°F (3 to 8°C) above the wet-bulb temperature, and this difference is known as the approach. The change in water temperature from inlet to exit is known as the range, and the range is generally 10 to 30°F (6 to 17°C). The performance of a cooling tower depends on the approach of the cold-water to the wet-bulb temperature of air, packing pressure drop and the velocity of entering air. From the tabulated result, the nominal velocity of air for orifice pressure drop at 100%, 75%, 50% and 25% are 0.4298, 0.3640, 0.3027 and 0.2158 m/s respectively. The nominal velocity is observed to decreases along with decreasing pressure drop.
When the wet-bulb temperature is measured, heat transfer and mass transfer take place at steady state with gradients. The flow of heat the interface just matches that needed for evaporation of the water that diffuses as vapour into the bulk gas. There is no significant gradient in the liquid, which remains at a constant temperature. By contrast in a cooling tower, the water temperature changes as droplets pass through the tower, and it is necessary to consider heat flow in the liquid phase as well as heat and mass transfer in the gas. Typical gradients at the bottom and top of the cooling tower are sketched in Fig. 4.1. At the bottom, the air temperature can be greater than the water temperature (Fig. 4.1) (a), but the water is being cooled because the interface temperature Ti is lower than bulk water temperature Tx. The humidity at the interface is greater than that in the bulk gas, which provides a driving force for mass transfer of water vapour.
If the inlet air temperature is less than the exit water temperature, as in Fig. 4.1 (b), the gradients are similar in shape, but there is less sensible heat transfer through the gas film. In all cases, the interface temperature must be above the wet-bulb temperature, since if Tx = Tw all the heat for vaporization would come from the gas, and there would be no temperature gradient in the water and no cooling of the water. As the air passes up through the tower, the air temperature might decrease for a short distance, but it will eventually increase as the air contacts warmer and warmer water. At the top, the gradient might be shown in Fig. 4.1(c).
Fig 4.1: Condition in cooling tower: (a), (b) at bottom of tower, (c) at the of tower
Heat transfer from the water to the interface is used to warm the air as well as to provide heat of vaporization, although the water cooling due to evaporation is much greater than that from sensible-heat transfer to the air. The exit gas temperature is usually within a few degrees Fahrenheit of the inlet water temperature. Wet bulb temperature were almost same for all four pressure drops, 280.00K. Theoretically, if the experiment were run under adiabatic conditions, where by no heat is transferred from the tower to its surroundings, with adequate air flow, the temperature of exiting liquid reached at steady state would be equal to the wet bulb temperature of incoming air. Therefore, the cooling tower is working optimally because the temperature of the exiting liquid is close to the wet bulb temperature of outside air. Figure 3.1 shows the relationship between wet bulb approach and packing pressure drops versus nominal air velocity. Theoretically the two lines in the figure 3.1 must be intersect. But for our cases it doesn’t. This may cause of the some fault in the cooling tower unit. As the nominal air velocity increases, wet bulb temperature remain constant but a sudden increase in the pressure drop curve is observed. This changes occur when the air entering cooling tower is lower, hence it delay the increase in heat transfer.
5.0
Conclusion & Recommendation
From the experiment, it shows that as the nominal air velocity decreases the pressure drop is also decreases. It also shows that nominal air velocity does not have effect on wet bulb temperature as the temperature remain constant. It can be concluded that nominal air velocity have effect on pressure drop through packing but not on the wet bulb temperature. As a recommendation several factors were identified as a key factors which can deviate the results of this experiment. One of those was the water may contained some impurities because the experiment was conducted with tap water, not distilled water and there must be the impurities deposited in cooling tower which will reduce the efficiency of the fan to blow out the air into the cooling system and it will let the water not properly cooled down.
6.0
References
McCabe, W., & Smith, J. (2005). Humidification Operation. In Unit Operations of Chemical Engineering (7th ed., p. 628). Boston: McGraw-Hill.
Bedekar, S., Nithiarasu, P., & Seetharamu, K. (n.d.). Experimental investigation of the performance of a counter-flow, packed-bed mechanical cooling tower. Energy, 943-947.
J.Benitez (1998), Principles and Modern Applications of Mass Transfer Operations.2nd ed.A John Wiley & Sons, Inc, Publication.
C.J.Geankoplis
(1997), Transport
Processes
and
Separation
Process
Principles.4th ed.Pearson Education: Prantice Hall.
C.E Thomas, (2009), Introduction to Process Technology: 3rd edition, Chengage Learning.
7.0
Appendices
For Air flowrate 100% Finding approach to wet bulb, Approach to wet bulb = Outlet water temp. – Inlet air wet bulb temp. = 30.5 0C – 22.7 0C = 7.8 0C
Finding specific volume, Based on air outlet wet bulb temp., T3 and air outlet dry bulb temp., T4 =
31.0 0C
=
304.15 K
=
29.6 0C
=
302.75 K
H
=
0.03
v
=
(2.83 x 10-3 + 4.56 x 10-3H) Tdb
=
0.8982 m3/kg
Twb
Tdb
Finding air mass flowrate v
=
0.8982 m3/kg
h
=
38 Pa x
=
3.8749 mm H2O
=
0.0137 √
=
0.0137 √
=
0.02846 kg/s
ṁ
0.10197162 mmH2O 1Pa
h v 3.8749 mm H2O 3 0.8982 m ⁄kg
Finding volumetric flowrate vͦ
=
mͦv
=
0.02846 kg/s x 0.8982 m3/kg
=
0.02556 m3/s
Finding nominal air velocity A
V
=
wh
=
15 cm x 60 cm
=
900 cm2
=
0.09 m2
=
vͦ A 3
=
0.02556 m ⁄s 0.09 m2
=
0.284 m/s
