Water Cooling Tower Experiment

Water Cooling Tower Experiment

• The aim is to measure pressure drop data and tower characteristics (KaV/L) for different liquid and air flow rates (L/G) in a counter-current mechanical draft cooling tower
• The experiment also assesses the effect of L/G on (KaV/L) and estimates the mass transfer coefficient Ka for varied L/G values
• Data will be gathered to correlate the pressure drop (∆H/Z) with either L/G or G

Theory

• Water is typically cooled using an air stream in spray ponds or cooling towers
• The Markel's Theory states that the enthalpy potential difference drives the cooling process
• Each water particle is surrounded by an air film, where the enthalpy difference between the film and surrounding air drives the cooling
• Merkel's integrated equation: KaV/L = ∫(T1 to T2) dT / (h' - h)

Key Variables and Parameters

• K = Mass transfer coefficient (lb of water/h-ft² or kg of water/h-m²)
• a = contact area (ft²/ft³ or m²/m³)
• V = Active cooling volume (ft³/ft² or m³/m²)
• L = Water flow rate (lb/h-ft² or kg/h-m²)
• h' = Enthalpy of saturated air (Btu/lb or kJ/kg)
• h = Enthalpy of air stream (Btu/lb or kJ/kg)
• T = Entering water temperature (°F or °C)
• KaV/L = tower characteristic varying with L/G ratio

Numerical Evaluation of Tower Characteristics

• The tower characteristic can be evaluated numerically by: (KaV/L) = (T1-T2)/4 [(1/(hw - ha)]
• Where, hw = Enthalpy of air-water vapour mixture at bulk water temperature, Btu/lb or kJ/kg of dry air
• ha = Enthalpy of air-water vapour mixture at its wet bulb temperature, Btu/lb or kJ/kg of dry air
• Δh₁ = Value of (hw – ha) at temp = T2 + 0.1(T1 - T2)
• Δh₂ = Value of (hw – ha) at temp = T2 + 0.4(T₁ – T2)
• Δh3 = Value of (hw – ha) at temp = T₁ - 0.4(T1 - T2)
• Δh4 = Value of (hw – ha) at temp = T₁ - 0.1(T1 - T2)

Cooling Tower Components

• The cooling tower is mounted on a water tank
• Water is heated, then pumped from the top throughout the the column
• Air enters from the bottom, passes through the packing, and is cooled by the water

Key Components and Sensors

• RTD sensors calculate the temperature of the water, and the air
• The packing material is SS mesh

Utilities

• Water supply and drain
• Electricity: 1 Phase, 220 V AC and 3 Kw
• Floor area of 1.2 m×1 m

Experimental Procedure

• Maintain a flow rate ratio (L/G) between 0.75 and 1.5

Procedure

• Set the heater to 45°C in the water storage tank after filling from the water make up tank
• Start the water pump to circulate water through the column with some minimum flow rate
• Activate the air fan at a low velocity and run the system for 30 minutes to achieve steady state, making sure there is enough water flow to wet the packing
• Readings are taken to achieve steady state by using a range of gas flow rates, while maintaining a constant water flow rate for each gas velocity
• A series is also taken with constant gas velocity and different liquid flow rates

Measurements

• Pressure drop readings, water inlet and outlet temperatures (T1, T2), air inlet and outlet dry bulb (DB) and wet bulb (WB) temperatures

Operational Parameters

• Liquid rate (L): 0.7 to 3.5 kg/m²-s
• Air rate (G): 1.6 to 2.8 kg/m²-s
• Pressure drop: <250 N/m² = 25 mm H2O

Standard Data

• Tower Material: Stainless Steel 304 Grade
• Tower Size: Cross-section 6" × 6", Height 30"
• Packing: Expanded wire mesh
• Blower: FD type with 1/2 HP Crompton motor
• Air Flow Measurement: Orifice meter with U-tube manometer
• Water Flow Measurement: Rotameter
• Dry & Wet Bulb Temp. Measurement: Hygrometer 2 set
• Hot Water Tank Material: Stainless Steel 304 Grade, double wall, insulated with ceramic wool
• Hot water circulation: Magnetic pump (Polypropylene), maximum working temperature 85°C
• Heater: 1.5 kW Nichrome wire heater
• Temperature sensors: RTD PT-100 type
• Control panel: 0-200°C, RTD PT-100 type
• Digital Temperature Indicator: 0-199.9°C, RTD PT-100 type, On/Off switch, Mains Indicator & fuse
• Rigid MS Structure for support

Formulae

• Pressure drop across orifice: ∆p = (pa - pb) = R1(pm - Pf) g/gc
• Mass flow rate of dry air = G = [m/(1 + Y₁)]/ cross-section area of column

Variables for Observation and Calculation

• Chamber Volume: V (m³)
• Packing Height: Z (m)
• Packing Surface Area: av (m²/m³)
• Air Flow Rate: G kg/m²-s (dry air)
• Water Flow Rate: W or L (kg water/m²-s)
• Water Temperature (Entering): T2 (°C)
• Water Temperature (Leaving): T1 (°C)
• Air Entering: Dry bulb= t2, wet bulb temp= tw2
• Air Leaving: Dry bulb= t1, wet bulb temp= tw1
• Orifice Manometer Reading: R1, m of water
• Pressure Drop: R2, m of water
• Y1: (corresponding to tdb1 and twb1)
• h1: (corresponding to T1)
• Y2: (corresponding to tdb2 and twb2)
• h2: (corresponding to T2)

Calculations

• SI system: gc=1
• ∆H0= R₁[(pm/pf) – 1], m
• Manometer fluid density (water): pm = 1000 kg/m³
• Air density: pf = 1.128 kg/m³

Constants

• Orifice diameter (do): 34 mm
• Pipe diameter (Dpipe): 68 mm
• R₁ = 3.5 cm of water= 0.035 m
• ΔΗο= 30.99 m
• Vo= 15.28 m/s
• Αο= (π/4) do² = 9.079 × 10−4 m²
• Mass flow rate of air, m= 15.28 × 9.079 × 10-4 × 1.128 = 0.015648 kg/s = 56.33 kg/h

Additional Equations

• Pressure drop across the packed bed: ∆P = R2(pm - Pf) g/gc
• The pressure drop per unit height of packing: ΔP/Z or ΔΗ/Ζ

Maintenance and Precautions

• Don't switch on the heater before filling the water
• Maintain constant air flow
• Avoid low voltage operation of the pump

Troubleshooting

• Remove and clean the rotameter if suspended particles enter
• Seal leakage with Teflon tape
• Tighten control knob if the rotameter fluctuates
• Correct sensor connections if D.T.C displays '1'
• Replace the bath heater if the panel LED is ON, but the temperature doesn't rise