The goal of this experiment is to determine heat and mas balance for countercurrent air-water system in a Packed Cooling Tower. To find the Characteristic equation, Number of Transfer Units N toG and Number of Heights Transfer Units H toG Murphree gas phase stage efficiency and the Overall cooling tower effectiveness efficiency The Goal of the Experiment
Base unit components include: 1.Air distribution chamber. 2.A tank with heaters to simulate cooling loads of 0.5, 1.0 and 1.5kW. 3.A makeup tank with gauge mark and float operated control valve. 4.A centrifugal fan with intake damper to give 0.06kg s-1 max. air flow. 5.A water collecting basin. 6.An electrical panel Note Use distilled water to fill the makeup tank. Monitor and record the amount of water evaporated during all of the test operations of the cooling tower. Check wet bulb thermocouple reservoir for water. Add if necessary. This can be done by measuring the time needs to spend by added amount of water to the make-up tank. After the system reach to study sate, Record all temperatures, dry and wet bulb temperatures of the air and water temperature of all sections, mass flow-rate of ware and air.
The basic function of a cooling tower is to cool water by intimately mixing it with air. This cooling is accomplished by a combination of: Sensible heat transfer between the air and the water (Conduction and Convection) and it controlled by temperature differences and area of the contact between air and water. And the evaporation of a small portion of the water. In the cooling towers, the evaporation is the most effective part in the cooling process Some background theory
Mass Balance and Enthalpy Balance on Cooling Tower * Please see page 12 equations (1,2,&3) *Because the latent heat of water is a big value, so a small amount of water evaporation will produce large cooling effect. Therefor we can assume the mass velocity of the water falling down through the tower is constant with out large consequences error Please see equation (4) on page 12 Take mass balance over a differential section (see the fig.) *Mass velocity of dry air remain constant through the cooling tower Take enthalpy balance over the same differential section (1) (2) (3) (4) dz T2t2H2h2mwma T2t2H2h2mwma Air outlet Water Inlet T1t1H1h1mwma T1t1H1h1mwma Air Inlet Water Outlet z 1 2 1
Equation (4) can be rewritten in term of heat balance as in equation (5) (5) Take the integral of eq (5) over entire Column (6) Eq (6) represent Air Bulk Operating Line by plotting air enthalpies versus water temperatures.
Cooling Tower Operating line (Air bulk operating line) h1h1 h2h2 T1T1 T2T2 Water Temperature Enthalpy of Air O N
Saturated Air Operating line Water bulk at temp T Saturated Air water vapor Film Air bulk at temp t Heat movement If you assume that the drops of water falling through the tower are surrounding by a thin air film, * This film must be saturated with water vapor. * The heat and mass transfer take place between the film and the upstream air bulk Where there is no resistance to heat flow in the interface between the saturated air film and water. In other words, the interface temperature can be assumed to be equal to the bulk water temperature (Merkel assumption) T (wart temperature) t i (interface temperature) By plotting the enthalpies of the saturated air–water vapor mixture (film) and water bulk temperatures will produce a curve, please see the Figure. This carve represent Saturated Air Operating line or can be called Water Operating line
H1H1 H3H3 H2H2 h1h1 h3h3 h2h2 T1T1 T3T3 T2T2 Water Temperature Enthalpy Driving Force Diagram The relation between the temperature and enthalpy of the saturated air This curve applies to the air film surrounding the water It called Water Operating Line And limited for hot and cold water temp (T 2 and T 1 ) Air Operating Line or Tower Operating Line Represent Air condition through the column Enthalpy Driving Force H 2 -h 2 Cooling Range T 2 -T 1
Mass Balance and Enthalpy balance on Cooling Tower In terms of mass and heat transfer coefficients. * Please see page (5) (7) By rearrange eq 7 pleas see eq 11&12 on page 17 Take integral over entire Tower (8) (9)
= H toG Heights of Transfer Units N toG = Number of Air Enthalpy Transfer Units
Merkels Equation This equation is commonly referred to as the Merkel equation. The left-hand side of this equation is called the Tower Characteristic, which basically indicates the 'degree of difficulty to cool' the water or the 'performance demand' of the tower. The tower characteristic and the cooling process can be explained on a Psychrometric Chart By combing eqs (5 &9)
Please note that V=Z =Volume occupied by packing per unit plan area To obtain mean driving force (h m) Carey and Williamson method can be used. This depends upon the application of correction factor f to the observed value of H m - h 3 (at the arithmetic mean of inlet and out let water temps T 1 & T 2 )
Characteristic Cooling Tower Equation
The cooling tower effectiveness.ε. is defined as the ratio of the actual energy transfer to the maximum possible energy transfer Murphree gas phase stage efficiency Y as Y 2 Y 1 t as t 2 t 1 Air Temps Saturation line