Plant Utilities and Energy Efficiency CH505

Plant Utilities and Energy Efficiency CH505

Teaching Scheme Course code CH505 Course Name Plant Utilities and Energy Efficiency Teaching scheme L T P Credit 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 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. Steam Cooling water Air Refrigerant Brine Hot oil Hydrogen Inert gas (Nitrogen, Helium, Argon) Electricity Etc.

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 to reject heat into the atmosphere. They represent a relatively inexpensive and dependable means of removing low-grade heat from cooling water. The make-up water source is used to replenish water lost to evaporation. Hot water from heat exchangers is sent to the cooling tower. The water exits the cooling tower and is sent back to the exchangers or to other units for further cooling.

Working Principle Humidification Operation: This operation is concerned with the interphase transfer of mass and of energy which result when a gas is brought into contact with a pure liquid in which it is essentially insoluble. The matter transferred between phases in such cases is the substance constituting the liquid phase, which either vaporizes or condenses. Mass transfer is only feasible when gas 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 to its saturation value, the vapor pressure p A at the prevailing temperature. Unsaturated vapor-gas mixture: If the partial pressure of the vapor in a gas is for any reason less the equilibrium vapor pressure of the liquid at the same temperature, the mixture is unsaturated. Absolute Humidity: It is the ratio of mass of vapor/mass of gas (Y ). If the quantities are expressed in moles, the ratio is the molal absolute humidity (Y). Y = y A y B = p A p B = Y = Y M A M B = p A moles of A p t p A moles of B p A M A mass of A p t p A M B 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 the mixture. Wet-bulb Temperature: This is the steady state temperature reached by a small amount of liquid evaporating into a large amount of unsaturated vapor-gas mixture. To determine wet-bulb temperature, a thermometer whose bulb has been covered with a wick kept with the 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 Forced-draft Naturaldraft Mechanicaldraft Induceddraft

Natural Circulation Towers Atmospheric Tower Depend on prevailing winds for air movement. Natural Draft Ensures more positive movement even in calm weather by depending upon the displacement of the warm air inside the tower by the cooler outside air.

Mechanical Draft Towers Forced Draft Air is blown into the tower by fan at the bottom. Recirculation of hot and humid discharged air into the fan is possible. Easy for inspection.

Mechanical Draft Towers Induced Draft Air is sucked from the tower from the fan mounted on top. More uniform internal distribution 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 Cooling rate depends on 2.5 m 3 /hr to several thousand m 3 /hr Wet bulb temperature, fan diameter, speed of fan Higher than 45,000 m 3 /hr 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 outlet temperature. Approach: Difference between cooling water outlet temperature and wet-bulb temperature. Higher the approach, smaller the size of cooling tower. Cycles of concentration (C.O.C) is the ratio of dissolved solids in circulating water to the dissolved solids in make up water. Generally concentration of dissolved solid is measured in terms of its chloride concentration.

Material balance around cooling tower Overall Water balance, M = E + W + D Chloride balance, MX M = DX C + WX C X C X M = C. O. C = D + W = M D + W = E C. O. C 1 E D + W + 1 M = Make-up water in m³/h C = Circulating water in m³/h E = Evaporated water in m³/h W = Windage loss of water in m³/h D = Drawoff (or blowdown) water in m³/h X M = Concentration of chlorides in make-up water (M) X C = Concentration of chlorides in circulating water (C) 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

Performance of Cooling Tower Blow down losses: Depends upon cycle of concentration (C.O.C.) and could be given by following equation, Evaporation loss Blowdown = C. O. C. 1 Range Cooling tower effectiveness (%) = 100 (Range+Approach) Evaporation rate can be estimated by following empirical expression, E = 0.00085 C T1 T2 E = Evaporation rate in m 3 /hr C = circulation rate in m 3 /hr T1-T2 = Difference between inlet and outlet temperature, F

Example Estimate the cooling tower range, capacity, approach and effectiveness with the following parameters Water flow rate through CT = 130 m 3 /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 m 3 /hr. Find out the blow down quantity required for maintaining the desired level of dissolved solids in the cooling water. Answer = 6 m 3 /hr Determine the amount of makeup required for a cooling tower with the following conditions: Answer: Evaporation losses: 28.9 m 3 /hr, Drift losses: 4.54 m 3 /hr, Blowdown losses: 7.24 m 3 /hr, Makeup water: 40.7 m 3 /hr

Factors affecting cooling tower performance Characteristics Capacity Range Approach and Wet-bulb temperature Size of tower Fill media Cooling tower fan Factors affecting Heat dissipation (kj/hr), Circulated flow rate with range Process side dictates its determination, function of the heat load and flow circulated Closer the approach to the wet-bulb, more expensive the cooling tower due to increased size 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 wetbulb temperature. Types: Splash fill or film fill 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 nonclogging 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 1 o C cooling water temperature increase may increase A/C compressor kw by 2.7%.

Energy saving opportunities in cooling tower 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.

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 the reasons for this situation. Also suggest solution so that cooling tower functions properly. Is wet bulb temperature an important factor in performance of cooling tower? Explain briefly. Find out the blow down rate of a cooling tower from the following data: Cooling water flow rate is 600 m 3 /hr. The operating range is 8 o C. The TDS concentration in circulating water is 1500 ppm and TDS in make up water is 300 ppm. In a cooling tower, the cooling water circulation rate is 1200 m 3 /hr. The operating range is 8 o C. If the blowdown rate of the cooling tower is 1 % of the circulation rate, calculate the evaporation loss and COC. Two innovative questions and their answers.