Cooling towers
AbstractThe objective of this experiment was to explore various
operating conditions of a Hilton Bench Top Cooling Tower H892 and
to analyse its performance associated at each condition. the
influence of varying cooling load, water flow rate and air flow
rate on the performance was investigated. The experiment involved
keeping two of the three variables constant while varying the third
during each run by isolating the influence of each mvariable in
operation. Data , such as the water temperature and air
pschrometric properties at various points, was collected and
manipulated into parameters useful in analyzing the operation of
the cooling tower. The most important parameter is the efficiency.
The steady flow equations ( energy and mass balances) were employed
inorder to investigate the forms of water lost from the system and
to provide an insight on the amount of energy transferred between
phases under different conditions. It was determined that the
maximum efficiency was achieved
at-----------------------------------------------------------------------at
a cooling load of ----------------for the case where ----------is
kept constant and the----------------is varied. It was therefore
concluded that optimum cooling is achieved
when--------------------
Table of contents Abstract Table of contents 1. Introduction
2. Theory Cooling tower fundamentals Cooling tower terminology
Mass balance analysis Energy balance analysis Psychrometry 3.
Experimental Apparatus Cooling tower preparation Experimental
procedure Precautions
4. Results Approach temperature Temperature range Efficiency
Mass balance Energy balance 5. Discussion 6. Conclusion 7.
References 8. Appendices
IntroductionA cooling tower is a rejection device, which emits
waste heat to the atmosphere through the cooling of a water stream
to a lower temperature. The type of heat rejection in a cooling
tower is termed evaporative in that it allows a small portion of
water to be cooled to evaporate into moving air stream to provide
significant cooling to the rest of that water stream. Cooling
occurs as a result of direct heat transfer by conduction and
convenction between two phases. The heat from the water stream is
transferred to the air stream raising the air temperature and its
relative humidity and this is discharged to the atmosphere.
Humidification processes are frequently carried out to control the
cooling and the recovery of water by contacting it with low
humidity air. It is therefore common practice for chemical
engineers to intergrate cooling towers into plant design to remove
all unwanted heat from process streams. the cooled water produced
by the cooling tower may then be distributed throughout the whole
plant for various cooling purposes using heat exchangers. The
conservation of water, for both economic and environmental reasons
is thus promoted. the Hilton Bench tower uses the counter- current
evaporating method ( also known as wet cooling method). This method
is widely used when cooling heated water in industries. When the
heated water comes into contactwith air, a large heat of water
allows a high heat transfer from the water to the air with little
water evaporating into the air , hence the water is conserved. An
advantage of this is that heated water can be cooled down to the
inlet wet-bulb temperature of the air. The purpose of conducting
the experiment was to investigate the influence of varying the
cooling load, water flow rate and air flow rate on the performance
of the cooling tower and to determine all end state properties of
the air and water from charts and tables. Experiments were
performed at different cooling loads of 0.5kW, 1.0kW and 1.5kW at
constant air and water flow rates. Also the influence of varying
the flow rate on the performance of the tower was analysed by
perfoming experiments at constant cooling loads and at orifice
differential of-------. The ssame approach was taken when analyzing
the tower performance at differe air flow rates, constant water
flow rate of 40g/s and cooling load.
TheoryCooling towers fall into two main sub-divisions: natural
draft and mechanical draft. ( coulson et.al . (1999)) Mechanical
draft cooling towers are much more widely used and these towers
utilize large fans to force air through circulated water. The water
falls downward over fill surfaces which help increase the contact
time between the water and the air. This helps maximize heat
transfer between the two.conversely, the natural draught cooling
tower uses buoyancy through a tall chimney. Warm , moist air
naturallyrises due to the density difference to that of dry, cooler
outside air.warm moist air is less dense than drier air at the same
pressure.the moist air buoyancy produces a current of air through
the tower, thus the natural draught cooling tower does not require
the aid of any mechanical devices to induce tha air flow. One of
the important mechanical draught cooling towers is the forced
draught cooling tower. This tower follows a simple operation,
atmospheric is forced vertically through the tower through the use
of a fan, while the water flows down through gravity.this produces
countercurrent flow in the cooling tower. Included in the coling
tower structure is packing which serves the purpose of increasing
the surface area of contact between the air and water allowing more
efficient cooling. The mechanicaldrught cooling tower produces
greater coolingthan the natural draught cooling tower and occupies
less space. Mechanical draught cooling towers are generally used
for achiving higher cooling or for capacity considerations, however
they are utilized due to economic considerations( such as costs)
when compared to natural draught cooling tower.
temperature range /Cooling Range This is the difference between
the temperature of the water entering and that leaving the cooling
tower.
T
range
= T water-inlet
T water- outlet
Changes in water and air flow-rates adveserly affect the
temperature range. According to Hill and Stanford, 1967,by
increasing the air flow rate in the cooling tower, an increase in
the temperature range is expected since a higher flow-rate of air
increases the contact time of water to less himidfied air which
increases the mass transfer driving force. Increasing the water
flowrate has the inverse effect, lowering the temperature range and
evaporative cooling by increasing the exposure of water to a more
humidified air. Therefore it can be seen that as temperature range
increases so does the performance of the cooling tower.
ApproachThe difference between the temperature of the cold water
leaving the tower and the wet-bulb temperature of the air is known
as the approach.
T
approach
= T water- outlet
T wet bulb air
The approach fixes the operating temperature of the tower and is
a most important parameter in determining both tower size and cost.
The smaller the approach temperature, the higher the performance of
the cooling tower under the operating conditions. For optimum
efficiency of the cooling tower, the approach temperature would
ideally be zero since it is not possible for the air to coolthe
water any further than the wet bulb temperatureof the incoming air.
However, in practice it is impossible to achieve a zero approach
temperature as inorder to achieve this, the tower should be
infinitelt tall. Cooling towers are thus not designed to operate
with an approach temperature below 2.80C and it is generally
accepted that an approach temperature of 5.50Cis attainable. (
Mccabe et al, 2005).
Wet and dry bulb temperatureWet-Bulb Temperature - The lowest
temperature that water theoretically can reach by evaporation.
Wet-Bulb Temperature is an extremely important parameter in tower
selection and design and should be measured by a psychrometer.
Temperature can be measured with two thermometers simultaneously
- one with a wet bulb and one with a dry bulb- in order to
determine the relative humidity of the surrounding air. This method
was used historically by meteorologists, but we will use it
extensively in this experiment to measure the water concentration
in the air. If the air is saturated with water vapor (100% relative
humidity), then the wet bulb and dry bulb temperatures be equal. If
the relative humidity is less than 100%, then the wet bulb
temperature will be lower due to evaporation of water from the
surrounding wrap. To obtain an accurate wet bulb temperature, it is
important to make sure that the wet bulb reaches steady state under
the conditions of air flow and relative humidity. The relative
humidity is determined by looking at wet-bulb and dry-bulb
temperatures on a psycrometric chart.
Water loss due to driftThe water entrained in the air flow as
fine droplets and discharged to the atmosphere is defined as drift
( Mckelvey and Brooke, 1959). Drift loss does not include water
lost by evaporation.
m waterdrift = m watermakeup
m waterevap
thus, the mass of water evaporated can be determined by taking
the difference between the mass flow rate of water in the air
entering the cooling tower and the mass flowrate of water in the
air leaving the tower. Using psychometry the following equation is
obtained
mwater
evap
= mdry air (Y1air
outlet
Y1air
inlet)
the psychometric chart is used to determine the absolute
humidity and the specific volume at the respective dry bulb and wet
bulb temperatures of the inlet and outlet air.
Heat Load The amount of heat to be removed from the circulating
water through the tower. Heat load is equal to water circulation
rate (gpm) times the cooling range times 500 and is expressed in
BTU/hr. Heat load is also an important parameter in determining
tower size and cost.
Pumping HeadThe pressure required to pump the water from the
tower basin, through the entire system and return to the top of the
tower.
Make-UpThe amount of water required to replace normal losses
caused by bleedoff, drift, and evaporation. It is the amount of
water required to compensate for the loss of water due to
evaporation and losses due to drift during the normal operation of
the cooling tower. This can be further expressed by the equation
given below
m make up = m evapmmake up
m drift
is the mass flow rate of make-up water (kg/s)
m m
evap drift
is the mass flow rate of evaporated water (kg/s) is the mass
flow rate of water lost due to drift (kg/s)
Bleed Off (Blowdown)The circulating water in the tower which is
discharged to waste to help keep the dissolved solids concentrating
in the water below a maximum allowable limit. As a result of
evaporation, dissolved solids concentration will continually
increase unless reduced by bleed off.
Efficiency of the towerThis is defined as the ratio betweenthe
range and the ideal range(i.e the difference between the cooling
water inlet temperature and the ambient wet bulb temperature). The
cooling tower efficiency can be expressed as
=where is cooling tower efficiency Ti is the inlet temperature
of water to the tower (oC , oF ) To is the outlet temperature of
water from the tower (oC , oF ) Twb is the wet bulb temperature of
air (oC , oF )
Energy Balance on the Cooling TowerHeat transfer is occurring
primarily in the load tank, where the water is brought up to the
feed temperature. A small amount of heat is also lost to the
surroundings by radiation/conduction/convection. Work is done on
the water by the pump. Energy is transferred along with mass loss,
because dry air enters, and humid air leaves.
Q = the rate of heating added to the system
P = rate of work done by the pump on the waterHexit = rate of
enthalpy loss in exiting vapor Hentry = rate of enthalpy gain due
to entering air and entering water from make-up tank Now let's set
up the energy equation for the portion of the process illustrated
at the right:
Q + P = H exit Hentrydefining some mass flow rates and
enthalpies:
at steady state
[work done on system] = [energy loss due to enthalpy change]
me the rate of (liquid) water addition from the "make-up" tank
ma the mass flow rate of air ms the mass flow rate of steamhda the
specific enthalpy of (dry) air hs the specific enthalpy of
steamsubscript A -------- entering the chamber subscript B --------
leaving the chamber The energy balance equation is now written:
Q + P = (mahda + mshs)B (mahda + mshs)A mE hEsteam/air leaving
steam/air entering water entering from make-up tank
Mass Balance on the Cooling TowerConservation of mass gives us
the following simple equations at steady state: Air in = air
out
( ma )A = ( ma )B
Water in = water out
(ms) A + mE = (ms )BThis equation simply says that the mass flow
rate of water entering the system (because the entering air is
humid) plus the mass flow rate of water entering from the make-up
tank equals the mass flow rate of water leaving the system as
vapor/steam. The ratio of water vapor to air can be determined if
the humidity is known.
