17-04-2014, 02:22 PM
Cooling Tower Thermal Design
Tower Thermal Design.pdf (Size: 1.49 MB / Downloads: 268)
Psychrometrics
Psychrometrics deals with thermodynamic properties of moist air and uses these properties to
analyze conditions and process involving moist air. Atmospheric air contains many gases
components as well as water vapor and miscellaneous contaminants (e.g., smoke, pollen and
gaseous pollutants). Dry air exists when all water vapor and contaminants have been removed
from atmospheric air. The composition of dry air is relatively constant, but small variations in
the amounts of individual components occur with time, geographic location, and altitude. The
apparent molecular mass or weighted average molecular weight of all components, for dry air is
28.9645, based on the carbon-12 scale. The gas constant for dry air, based on the carbon-12
scale is 1545.32/28.9645 =53.352 ft lbf / lbm oR.
Moist air is a binary mixture of dry air and water vapor. The amount of water vapor in moist air
varies from zero (dry air) to a maximum that depends on temperature and pressure. The latter
condition refers to saturation, a state of neutral equilibrium between moist air and the condensed
water phase. Unless otherwise stated, saturation refers to a flat interface surface between the
moist air and the condensed phase.
Heat & Mass Transfer Fundamental
Many theories have been developed since the early 1900s describing the heat and mass transfer
phenomenon which takes place in several types of atmospheric water cooling devices. Most of
these theories are based on sound engineering principles. The cooling tower may be considered
as a heat exchanger in which water and air are in direct contact with one another. There is no
acceptable method for accurately calculating the total contact surface between water and air.
Therefore, a "K" factor, or heat transfer coefficient, cannot be determined directly from test data
or by known heat transfer theories. The process is further complicated by mass transfer.
Experimental tests conducted on the specified equipment designs can be evaluated using
accepted and proven theories which have been developed using dimensional analysis
techniques. These same basic methods and theories can be used for thermal design and to
predict performance at the operating conditions other than the design point.
Tower Demand
Liechtenstein introduced the "Cooling Tower" equation in 1943 and he used Merkel theory
in conjunction with differential and fundamental equations to define cooling tower boundary
conditions. The resulting dimensionless groups related the variables for heat and mass
transfer on the counter flow type tower. Liechtenstein determined by experimental testing
that his equation did not fully account for the air mass rate or velocity. He also implies in the
original paper that tests conducted at the University of California suggested a variation in the
tower characteristic due to the inlet water temperature. A method is given for adjusting the
tower characteristic for the effect. Several investigators have substantiated the effect of hot
water temperature and air velocity on the counter flow tower.
Consideration of By-Pass Wall Water
This factor accounts for the amount of water which unavoidably bypasses the fill along the
outside and partition walls, internal columns, internal risers etc. This water is not cooled as
much as the water passing through the fill. This effect is well known and recognized as the
WALL EFFECT but there is no precise theory on how to predict and account for it. This is not a
reason for neglecting it in the calculations. It may be very large particularly in a small tower
where it can be as big as 20%. Even large towers can have 2% to 5% on the walls. The approach
to this problem is very simple. The by-pass wall water is assumed to be only half cooled.
How to estimate the by-pass wall water? Through an example, the estimation can be discussed.
A 36 x 36 ft tower cell has 144 nozzles. 40 nozzles are near to the four walls each projecting 10
% of their water onto those walls. 40 x 10% / 144 = 2.78 %. 4 nozzles are in the corners and
project 20% of their water into the wall. 4 x 20% / 144 = 0.56 %. There are 25 internal columns.
Each column receives 5% of the water from 4 adjacent nozzles 25 x 4 x 5% / 144 = 3.47 %.
Then total by-pass is 6.81% and the water amount for being half cooled is 6.81 / 2 = 3.4%. This
means that 3.4% of total water flow is passing through the wall under not being cooled. This is
not an exaggerated number. Obviously this evaluation largely depends on water distribution
design and the type of nozzles used. A lot of precautions can be taken to minimize this value but
it must be kept in mind that that it is better to have a little water on the walls than leaving dry
spots with no water at all. Many cooling tower fills do not redistribute the water very well and
air will rush through a dry spot where there is less resistance.