Plant Utilities and Energy EfficiencyCH505

Teaching Scheme

Coursecode

Course Name Teaching scheme

L T P Credit

CH505 Plant Utilities and Energy Efficiency

3 0 0 3

Process

• Process is simply a method by which products can be manufactured from raw material.

Input and Output from Process Plant

Utility

• Steam• Cooling water• Air• Refrigerant• Brine• Hot oil• Hydrogen• Inert gas (Nitrogen, Helium, Argon)• Electricity• Etc….

• Utility is inseparable part of process plant.

• It is not counted directly as a product, which leaves company premises. But ignoring them may have significant effect on overall profit.

Cooling water versus Air as cooling utility Cooling water

• Higher HTC compared air.

• Highest temperature of cooling water is fixed by process requirement.

• Smaller size of heat transfer equipment.

• Higher operating cost.

• Relatively lower temperature can be achieved.

Air

• Lower HTC compared water.

• There is no upper limit for highest temperature.

• Larger size of heat transfer equipment.

• Lower operating cost.

• Freely available.

Steam versus Hot oil as heating utility

Steam

• Saturated steam has higher heat transfer coefficient compared to hot oil.

• No pumping required for transportation.

• Smaller size of heat transfer equipment.

• Not recommended for application having temperature above 180°C.

Hot oil

• Lower heat transfer coefficient compared to saturated steam.

• High pumping cost.

• Large size of heat transfer equipment.

• Recommended for application having temperature between 180-400°C.

Cooling Tower• Cooling towers are a very important

part of many chemical plants.

• The primary task of a cooling tower is toreject heat into the atmosphere.

• They represent a relatively inexpensiveand dependable means of removinglow-grade heat from cooling water.

• The make-up water source is used toreplenish water lost to evaporation.

• Hot water from heat exchangers is sentto the cooling tower. The water exits thecooling tower and is sent back to theexchangers or to other units for furthercooling.

Working Principle • Humidification Operation: This

operation is concerned with theinterphase transfer of mass and ofenergy which result when a gas isbrought into contact with a pureliquid in which it is essentiallyinsoluble.

• The matter transferred betweenphases in such cases is the substanceconstituting the liquid phase, whicheither vaporizes or condenses.

• Mass transfer is only feasible whengas is unsaturated with vapor.

Components of Cooling Tower

• Frame and casing

• Fill (Splash/Film)

• Cold water basin

• Drift Eliminator

• Nozzles

• Fans

Splash Versus Film Fill

Important Definitions• Saturated vapor-gas mixture: If an insoluble gas B is brought into contact

with sufficient liquid A, the liquid will evaporate into the gas until ultimately,at equilibrium, the partial pressure of A in the gas-vapor mixture reaches toits saturation value, the vapor pressure pA at the prevailing temperature.

• Unsaturated vapor-gas mixture: If the partial pressure of the vapor in a gas isfor any reason less the equilibrium vapor pressure of the liquid at the sametemperature, the mixture is unsaturated.

• Absolute Humidity: It is the ratio of mass of vapor/mass of gas (Y’). If thequantities are expressed in moles, the ratio is the molal absolute humidity(Y).

𝑌 =𝑦𝐴𝑦𝐵

=𝑝𝐴𝑝𝐵

=𝑝𝐴

𝑝𝑡 − 𝑝𝐴

𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝐴

𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝐵

𝑌′ = 𝑌𝑀𝐴

𝑀𝐵=

𝑝𝐴

𝑝𝑡−𝑝𝐴

𝑀𝐴

𝑀𝐵

𝑚𝑎𝑠𝑠 𝑜𝑓 𝐴

𝑚𝑎𝑠𝑠 𝑜𝑓 𝐵

Definitions (Cont…)• Dry-bulb Temperature: This is the temperature of a vapor-gas mixture

as ordinarily determined by immersion of a thermometer in themixture.

• Wet-bulb Temperature: This is the steady state temperature reachedby a small amount of liquid evaporating into a large amount ofunsaturated vapor-gas mixture. To determine wet-bulb temperature,a thermometer whose bulb has been covered with a wick kept withthe liquid is immersed in a rapidly moving stream of the gas mixture.

• Wet-bulb temperature is going to limit the design of cooling tower.

Types of Cooling Towers

Cooling Tower

Natural Circulation

Atmospheric tower

Natural-draft

Mechanical-draft

Forced-draft

Induced-draft

Natural Circulation TowersAtmospheric Tower

• Depend on prevailingwinds for air movement.

Natural Draft

• Ensures more positive movement even incalm weather by depending upon thedisplacement of the warm air inside thetower by the cooler outside air.

Mechanical Draft TowersForced Draft

• Air is blown into the tower by fan at thebottom.

• Recirculation of hot and humid dischargedair into the fan is possible.

• Easy for inspection.

Mechanical Draft TowersInduced Draft

• Air is sucked from the towerfrom the fan mounted on top.

• More uniform internaldistribution of air.

Comparison of Mechanical-draft and Natural Circulation Cooling Towers

Feature Mechanical-draft Natural Circulation

Fan power Yes No

Draft is created by Fan Natural air media

Advantages Reduced tower height, low pump head, and water temperature control facility

Minimum operating cost

Fill surface (Packing) Plastic, glass fibre wood Chimney-type concrete shell

Water flow rates and capacity 2.5 m3/hr to several thousand m3/hr

Higher than 45,000 m3/hr

Cooling rate depends on Wet bulb temperature, fan diameter, speed of fan

Wet bulb temperature and relative humidity

Application Small and medium scale Power plant, heavy load, low wet bulb temperature, and high inlet and outlet water temperature

Performance of Cooling Tower• Range: Difference between

cooling tower inlet and outlettemperature.

• Approach: Difference betweencooling water outlettemperature and wet-bulbtemperature.

• Higher the approach, smaller thesize of cooling tower.

• Cycles of concentration (C.O.C) isthe ratio of dissolved solids incirculating water to the dissolvedsolids in make up water.Generally concentration ofdissolved solid is measured interms of its chlorideconcentration.

Material balance around cooling tower

M = Make-up water in m³/hC = Circulating water in m³/hE = Evaporated water in m³/hW = Windage loss of water in m³/hD = Drawoff (or blowdown) water in m³/hXM = Concentration of chlorides in make-up water (M)XC = Concentration of chlorides in circulating water (C)

Overall Water balance,𝑀 = 𝐸 +𝑊 + 𝐷

Chloride balance,𝑀𝑋𝑀 = 𝐷𝑋𝐶 +𝑊𝑋𝐶

𝑋𝐶𝑋𝑀

= 𝐶. 𝑂. 𝐶 =𝑀

𝐷 +𝑊=

𝐸

𝐷 +𝑊+ 1

𝐷 +𝑊 =𝐸

𝐶. 𝑂. 𝐶 − 1

Ignoring windage (or drift) loss as it is only 0.1-0.2% of circulating water. Latest design of drift eliminator reduces these losses below 0.1%.

𝐷 =𝐸

𝐶. 𝑂. 𝐶 − 1

• Blow down losses: Depends upon cycle of concentration (C.O.C.) and could be given by following equation,

𝐵𝑙𝑜𝑤𝑑𝑜𝑤𝑛 =𝐸𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑖𝑜𝑛 𝑙𝑜𝑠𝑠

𝐶. 𝑂. 𝐶. −1

• Cooling tower effectiveness (%) =𝑅𝑎𝑛𝑔𝑒

(𝑅𝑎𝑛𝑔𝑒+𝐴𝑝𝑝𝑟𝑜𝑎𝑐ℎ)∗ 100

• Evaporation rate can be estimated by following empirical expression,𝐸 = 0.00085 ∗ 𝐶 ∗ 𝑇1 − 𝑇2

E = Evaporation rate in m3/hr

C = circulation rate in m3/hr

T1-T2 = Difference between inlet and outlet temperature, °F

Performance of Cooling Tower

Example

• Estimate the cooling tower range, capacity, approach and effectiveness with the following parameters

Water flow rate through CT = 130 m3/hr

Specific heat of water = 1 kcal/kg °C

Inlet water temperature = 42 °C

Outlet water temperature = 37 °C

Ambient WBT = 31 °C

Examples• In a cooling tower, the Cycle of Concentration (C.O.C.) is 3 and

evaporation losses are 1%. The circulation rate is 1200 m3 /hr. Findout the blow down quantity required for maintaining the desired levelof dissolved solids in the cooling water.

• Answer = 6 m3 /hr

• Determine the amount of makeup required for a cooling tower with the following conditions:

• Answer: Evaporation losses: 28.9 m3/hr, Drift losses: 4.54 m3/hr,Blowdown losses: 7.24 m3/hr, Makeup water: 40.7 m3/hr

Factors affecting cooling tower performanceCharacteristics Factors affecting

Capacity Heat dissipation (kJ/hr), Circulated flow rate with range

Range Process side dictates its determination, function of the heat load and flow circulated

Approach and Wet-bulb temperature Closer the approach to the wet-bulb, more expensive the cooling tower due to increased size

Size of tower Ranking parameters to be considered in sizing of tower is in the order of approach is first, then flow rate, followed by range, finally to wet-bulb temperature.

Fill media Types: Splash fill or film fill

Cooling tower fan Sized to move a specified quantity of air through the system at a specified site

Energy saving opportunities in cooling tower• Follow manufacturer’s recommended clearances around cooling towers and

relocate or modify structures that interfere with the air intake or exhaust.

• Optimize cooling tower fan blade angle on a seasonal and/or load basis.

• On old counter-flow cooling towers, replace old spray type nozzles with new non-clogging nozzles.

• Periodically clean plugged cooling tower distribution nozzles.

• Balance flow to cooling tower hot water basins.

• Cover hot water basins to minimize algae growth that contributes to fouling.

• Optimize blow down flow rate, as per COC limit.

• Segregate high heat loads like furnaces, air compressors etc and isolate cooling towers for sensitive applications like A/C plants, condensers of captive power plant etc. A 1oC cooling water temperature increase may increase A/C compressor kW by 2.7%.

• Monitor approach, effectiveness and cooling capacity for continuous optimization efforts, as per seasonal variations as well as load side variations.

• Consider COC improvement measures for water savings.

• Consider energy efficient blade adoption for fan energy savings.

• Consider possible improvements on CW pumps w.r.t. efficiency improvement.

• Control cooling tower fans based on leaving water temperatures especially in case of small units.

Energy saving opportunities in cooling tower

Case study: Approach vs Flow rate• Suppose a cooling tower is installed that is 21.65 m wide × 36.9 m

long × 15.24m high, has three 7.32 m diameter fans and each powered by 25 kW motors.

Assignment-I• An induced draft-cooling tower is designed for a range of 8° C. The energy

auditor finds the operating range is 3 °C. In your opinion what could be thereasons for this situation. Also suggest solution so that cooling towerfunctions properly.

• Is wet bulb temperature an important factor in performance of coolingtower? Explain briefly.

• Find out the blow down rate of a cooling tower from the following data:Cooling water flow rate is 600 m3/hr. The operating range is 8 oC. The TDSconcentration in circulating water is 1500 ppm and TDS in make up water is300 ppm.

• In a cooling tower, the cooling water circulation rate is 1200 m3/hr. Theoperating range is 8oC. If the blowdown rate of the cooling tower is 1 % ofthe circulation rate, calculate the evaporation loss and COC.

• Two innovative questions and their answers.