• remove heat from process operation
– HVAC, industrial, power generation
• mostly by evaporation (80%)
– partially by sensible heat loss (20%)
• contact hot water with cooler air
Evaporation
Water Evaporated from the Circulating Water into the Atmosphere by the Cooling Tower
• need about 1000 Btu/lb to evaporate water at ambient conditions
• Latent Heat 潜热
– Latent Heat of Vaporization is the heat energy removed by evaporating a small percentage of recirculated tower water. Each pound of water evaporated removes approximately 1,000 Btu in the form of heat.
Cooling Tower Heat Transfer Mechanisms
• Sensible Heat 显热
– Sensible heat is heat energy transferred between the water droplets and air when there is a difference in temperature between them.
• also act as air scrubbers
– corrosion and fouling problems
Tower Design Considerations
• heat load, supply temperature needed • location, wet bulb • cost • water quality and availability • footprint, other towers • materials • fill type
D = F2 x R BD = BDc + D + L
R = water recirculation, gpm D = tower drift, gpm F2 = drift eliminator efficiency factor (0.005% to 0.1%, use 0.01%
if unknown) BD = total blowdown, gpm BDc = controlled blowdown, gpm L = leakage, gpm
Cooling System Calculation
March 2014
Content
Cooling Tower & Mass Balance Concentration Rate (CR) L.S.I. pH adjustment with Acid
Cooling Tower Type
Function of Cooling Towers
Review of Basic Cooling Tower Definition of Terms
Cooling Tower Water Balance
Cooling Tower Terms
•Range The Difference between the Hot Water Temperature and the Cold Water TemperatureR TERMS
HOT WATER TEMPERATURE COLD WATER TEMPERATURE
RANGE T
DRY BULB TEMPERATURE
Determined by Humidity
WET BULB TEMPERATURE
APPROACH
•Approach The Difference between the Cold Water Temperature and the Ambient or Inlet Wet Bulb Temperature
Wet Bulb
• temperature of air saturated with water
– measured by psychrometer – calculated from psychrometric chart – at 100% RH, wet bulb = dry bulb
• wet bulb is lowest possible cooling temperature
• at low RH, cold water temperature can be less than dry bulb temperature
• E = R x delta T x Cp x f/1000
f = evaporation factor
• normally use 0.8 or 80%
Tower Drift
Water lost from the Tower as Liquid Droplets Entrained in the Exhaust Air. Drift is Independent of Water Lost by Evaporation
Cycles of Concentration
• most cooling from evaporation • as water evaporates, salts remain in system • salt concentration (ppm) increases in
recirculating water • cycles (of concentration) refers to increased