2 The Goal of the Experiment 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 NtoG and Number of Heights Transfer Units HtoGMurphree gas phase stage efficiency and the Overall cooling tower effectiveness efficiency
4 Base unit components include: Air distribution chamber. A tank with heaters to simulate cooling loads of 0.5, 1.0 and 1.5kW.A makeup tank with gauge mark and float operated control valve.A centrifugal fan with intake damper to give 0.06kg s-1 max. air flow.A water collecting basin.An electrical panelNoteUse 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.
5 Some background theory 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
6 Mass Balance and Enthalpy Balance on Cooling Tower *Please see page 12 equations (1,2,&3)Take mass balance over a differential section (see the fig.)dzT2 t2H2 h2mw maAiroutletWaterInletT1 t1H1 h1Outletz1’(1)(2)2*Mass velocity of dry air remain constant through the cooling towerTake enthalpy balance over the same differential section(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 errorPlease see equation (4) on page 121(4)
7 Take the integral of eq (5) over entire Column 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.
8 Cooling Tower Operating line (Air bulk operating line) h1h2T1T2Water TemperatureEnthalpy of AirONCooling Tower Operating line (Air bulk operating line)
9 Saturated Air Operating line Saturated Air water vapor FilmSaturated Air Operating lineIf 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 bulkWater bulk at temp TWhere 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) ≈ ti (interface temperature)Heat movementAir bulk at temp tBy 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
10 The relation between the temperature and enthalpy of the saturated air H2Enthalpy Driving Force H2-h2EnthalpyH3h2H1This curve applies to the air film surrounding the waterIt called Water Operating LineAnd limited for hot and cold water temp (T2 and T1)h3h1Cooling RangeT2-T1T1T3T2Water TemperatureAir Operating Line or Tower Operating LineRepresent Air condition through the columnDriving Force Diagram
11 Mass Balance and Enthalpy balance on Cooling Tower In terms of mass and heat transfer coefficients.*Please see page 15-19(5)(7)By rearrange eq 7pleas see eq 11&12 on page 17(8)Take integral over entire Tower(9)
12 = HtoG Heights of Transfer Units NtoG = Number of Air Enthalpy Transfer Units
13 By combing eqs (5 &9)Merkel’s EquationThis 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
14 Please note that V=Z =Volume occupied by packing per unit plan area To obtain mean driving force (∆hm) Carey and Williamson method can be used. This depends upon the application of correction factor f to the observed value of Hm- h3 (at the arithmetic mean of inlet and out let water temps T1 & T2)
16 The cooling tower effectiveness. ε The cooling tower effectiveness .ε. is defined as the ratio of the actual energy transfer to the maximum possible energy transferYasY2Y1tas t2 t1Air TempsSaturation lineMurphree gas phase stage efficiency