冷却水系统计算

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 concentration (ppm) of salts in recirculating water
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CR 的计算方法
Chlorides - The relative chloride levels can be a good indicator of the cycles except where:
The makeup chloride level is low (<10 ppm). Oxidizing biocides are used.
• 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.
Cycles of Concentration
4L 4L 4L
2L 1L
2L 1L
2L 1L
100 mg/4 L = 25 mg/L (makeup)
100 mg/2 L = 50 mg/L 2X
100 mg/1 L = 100 mg/L 4X
Increasing the Cycles
Q. How does increasing cycles in the tower affect the Evaporation Rate? A. It doesn’t. The heat load has not changed. It affects only BD and MU rates.
Calcium Cycles « True Cycles
Calcium is precipitating. Relative silica and chloride levels are the best indicators of cycles. The lower than expected conductivity cycles are probably due to gross precipitation of dissolved solids.
Cycles of Concentration
• Typically, magnesium is the best ion to use when calculating cycles of concentration • However, under conditions of high silica (>40 mg/L) and high pH (>7.8), there is a high probability of magnesium silicate precipitation • Under these conditions, cycles of concentration can best be determined by averaging the concentration ratio calculated based on several ions in both the makeup and the concentrated water.
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 • E = R x delta T x Cp x f/1000
Function of Cooling Towers
• remove heat from process operation
– HVAC, industrial, power generation
• mostly by evaporation (80%)
– partially by sensible heat loss (20%)
Calcium Cycles » True Cycles
Calcium is being re-dissolved. Relative conductivity and silica levels are the best indicators of cycles. Makeup chlorides level too low for accurate measurement. If an oxidizing biocide is being used as this example shows, the relative chloride level is even more unreliable for monitoring cycles.
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CR 的计算方法
Silica - The relative silica levels can be a good indicator except where:
The makeup silica level is low (<4 ppm). The makeup silica level is high
•Approach The Difference between the Cold Water Temperature and the Ambient or Inlet Wet Bulb Temperature
Wet Bulb
• temperature of air saturated with water
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CoC 的计算实例
In the water analysis above, all three of the key test parameters used to monitor cycles (in blue) indicate that about 5 cycles are being maintained.
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
CR 的计算方法
根据水量
根据水质
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CR 的计算方法
Conductivity - The relative conductivity levels are usually a good indicator of the cycles except where:
Low makeup conductivity is low (<100 μmhos). (< 50 μmhos, severe) significant likelihood of calcium carbonate scale forming (MA>Hardness, cond.<300umhos) large amounts of sulfuric acid being used for pH control
Cooling System Calculation
March 2014
Content
Cooling Tower & Mass Balance Concentration Rate (CR) L.S.I. pH adjustment with Acid
Cooling Tower Βιβλιοθήκη Baiduype
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 Temperature
Psychrometers
COOLING TOWER TERMS
HOT WATER TEMPERATURE
COLD WATER TEMPERATURE
RANGE T
DRY BULB TEMPERATURE
Determined by Humidity
APPROACH
WET BULB TEMPERATURE
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.
• contact hot water with cooler air
• 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
– 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
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Calcium Cycles ≈ Actual Cycles
Calcium is not precipitating. Note good agreement between cycles as determined by the relative conductivity, chloride, and silica levels.
浓缩倍数CR
• 定义：浓缩倍数是指循环水中的盐离子浓度与 补充水中的盐离子浓度的 比值。也代表了水 在系统中的循环次数。循环水处理的主要目的 之一是维持尽可能高的浓缩倍数，以提升水的 再利用率 • cycles of concentration refer to the degree that the total dissolved solids in the makeup water have been concentrated in the recirculating water of an evaporative heating or cooling water system.
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