nd London South Bank University Division of Chemical and Petroleum Engineering pplying cooled water for a wide variety of processes brought int t into intimate contact with an air stream

o rise, producing ng a natural circulation. If the cooling quired air flow. This is normal with the cooling towers of ral landscapes er towers and as natural circulation does not provide towers with fans are classified as "Induced Draught" or outlet and draws the air through the packing, whereas in and drives the air through the packing Air flow is at the fan and help to desipate the humid air ets or thin films which presents a large surface area to the cooled largely by evaporation and in some cases, to a of some of the water into the air steam, a continuous small n. This make up is normally less than 5% of the cooling niet wet bulb temperature of the air. Their power ng and probably more important, their consumption of moling from natural sources. flow rate of water (0.04 1/1) and a fixed air flow rate water Hom 200m ms of eight similar "decks of inclined, wet table, laminated valve and water flow meter to the column cap After its er the tap packing deck and, as it spreads over the plates, a downward through the packing the water is cooled masin, past a thermometer and into the load tank where it ends to fall. This causes the float operated needle valve to mk. Under steady conditions, the rate at which the water y small airborne droplets in the air discharge a rate which is controlled by the intake damper setting swer and dry bulb thermometers before it enters the moisture content in ent increases and the water is cooled On rester which traps most of the entrained droplets and ere via the air measuring orifice and further wet mosphere via 3 3/11/23 water Cooling tower objective Fixed air flow rate 1) To determine the water tempe arep for a fined flow rate of water Co.04 LIS) and enthalpy balance between alr, watel) ji To carry out an in to determine the evaperation rate T: 16 W make wis 13.5 mm KYY MIL make up water 200 Pressure TA WO Com A Tara TAWI 2205 gel TAdi 22 flow rate of watson 4/5 14.9 Twinke 58 Twoudled 15 20.5 5h2 Cold 16.9 hot 21.5 2410 713 7 18 26 300 T 13 235.5 35.5 23-5 Colum B 8211 15.5 20 23-5 27 25 hot 22 220 pm T12 16 17-5 5.518 Colum C 320 TIS 22 36 8.5 20 10 сай 18 hot 21.5 260 TI Amoon Water Cooling Tower Effect of packing surface area on cooling range and approach London South Bank University 150 Division of Chemical and Petroleum Engineering Background Cooling towers provide an economical and acceptable method of supplying cooled water for a wide variety of processes. In a cooling tower, the warm process water which is to be cooled is brought into intimate contact with an air stream. Within the cooling tower the air density decreases causing the air to rise, producing a natural circulation. If the cooling tower is sufficiently tall, the "natural draught" will generate the required air flow. This is normal with the cooling towers of inland electricity generating stations which feature in many industrial landscapes. Typical industrial or commercial cooling duties feature much smaller towers and as natural circulation does not provide sufficient air flow, fans are employed to promote air flow. Cooling towers with fans are classified as "Induced Draught" or "Forced Draught" according to the location of the fan. In an Induced Draught Cooling Tower the fan is placed at the air outlet and draws the air through the packing, whereas in a Forced Draught, Cooling Tower the fan is placed at the air inlet and drives the air through the packing. Air flow is normally upwards, therefore the natural buoyancy of the air will assist the fan and help to dissipate the humid air. The packing in a cooling tower breaks the water into small droplets or thin films which presents a large surface area to the air stream and thereby assists the cooling process. The water is cooled largely by evaporation and in some cases, to a small extent, by direct contact cooling. Due to the evaporation of some of the water into the air steam, a continuous small supply of fresh water ("make up") must be added to the system. This make up is normally less than 5% of the cooling water flow rate. Correctly sized, towers can cool the water to within 5K of the inlet wet bulb temperature of the air. Their power requirement is only a fraction of that required by air blast cooling, and probably more important, their consumption of fresh water is considerably less than that required by water cooling from natural sources. Objective For each packing set, i) To determine the water temperature drop for a fixed flow rate of water (0.04 L/s) and a fixed air flow rate ii) To carry out an enthalpy balance (between air and water) iii) To determine the evaporation rate Description of apparatus (a) Packing Three sets of packing are available. Each set of packing consists of eight similar "decks" of inclined, wet table, laminated plastic plates. Set A: 7 plates per deck giving a packing area/volume ratio of 77 m¹¹ Set B: 10 plates per deck giving a packing area/volume ratio of 110 m² Set C: 18 plates per deck giving a packing area/volume ratio of 200 m²² (b) Water Circuit Warm water is pumped from the load tank through the control valve and water flow-meter to the column cap. After its temperature is measured, the water is uniformly distributed over the tap packing deck and, as it spreads over the plates, a large thin film of water is exposed to the air stream. As it flows downward through the packing the water is cooled, largely by the evaporation of a small portion of the total flow. The cooled water falls from the lowest packing deck_int9 the basin, past a thermometer and into the load tank where it is re-heated. 21 Due to the evaporation the level of the water in the load tank tends to fall. This causes the float operated needle valve to open and transfer water from the make-up tank into the load tank. Under steady conditions, the rate at which the water leaves the make-up tank is equal to the rate of evaporation plus any small airborne droplets in the air discharge. (c) Air Circuit Air from the atmosphere, pre-heated if desired, enters the fan at a rate which is controlled by the intake damper setting. The fan discharges into the distribution chamber and the air passes wet and dry bulb thermometers before it enters the packed column. As the air stream flows through the packings, its moisture content increases and the water is cooled. On leaving the top of the column the air passes through the droplet arrester which traps most of the entrained droplets and returns them to the packings. The air is then discharged to the atmosphere via the air measuring orifice and further wet 371 MA and dry bulb-thermometers. Procedure The desired packed column is fitted into the unit. Having filled the system with water to the normal working level, the fan is switched on and the intake damper adjusted to give the desired air flow rate as indicated by the orifice differential pressure. The pump is now switched on and the water flow rate set to the desired valve. As the packings become wetted, some water is retained by them, and the water level in the load tank will fall slightly, opening the float valve. Water will then flow from the make-up tank until the level in the load tank is restored. Having re-filled the make-up tank, the "process cooling load", i.e. the heaters in the load tank, may be switched to 0.5, 1.0 or 1.5 kW. Conditions may now be allowed to stabilize before temperature and flow rates are observed. At the commencement of a test the make-up tank is filled to the gauge mark. After about ten minutes the quantity of water required to refill the tank to the gauge mark is recorded and the make-up supply rate calculated. Tests may be repeated with other water and/or air flow rates, and with the air pre-heater switched on to give different air inlet conditions at the same specific humidity as the atmosphere. Note Always use distilled water. On full cooling load the rate of evaporation from this tower is about 2 L/h. The impurities in this, plus any dust washed from the air, will appear in the system. This will reduce the visibility of the processes in the cooling tower and will cause scaling problems. Results See the attached sheets 17:22,215 TYPICAL 24-83 AREA VOL 252 77m IPAT BEND C 36% OBSERVATIONS hot WATER FLOW RATE tempe croton ORIFICE DIFFERENTIAL" 16mm₂0 -42gms" 18-st MAIR UP 325gm 2008 YOURTH 041 kW a fired air E RESULTING ofween air, w BALANCES na 'S AIR FLOW RATE 10-871 pk AIR ENTHALPY CHANGE EVAPORATION RATE MAKE-UP MATE 205 25 WATER ENTHALPY CHANGE 1-0-0137 SPECIFIC PUMIDITY ky by -0.0195 DRY BILD/C -200 - 0:0137/15 -0-0587 kg -ma(ha-h₂) 200 h = ORIFILE DIFFERENTIAL (MMH₂0) V SPECIFIC VOLUME OF AIR (m²/kg) Mat MASS FLOWRATE OF MR, lgfs h -0.0287(75-5-46)-1-GIN AD TOTAL ENTHALPY OF AIR AT INLET (kJ/kg) ha TOTAL ENTHALEY OF AIR AT EXIT (431leg) My MASE FLOWRATE OF WATER (kg/s) te= TEMPERATE OF INLET WATER. 'C EXIT TEMP. OF WATER (*) to 0-012-418 (36-266)-1-65kw m (wo-w) -0-0587(0-cita-0-0104) 021-54-16