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Pump and cooling tower energy performance
- 1. Energy Performance of Pumps and Cooling Towers Prepared By: Maulik Bhagat 1
- 2. Pumps 2
- 3. Outline: Introduction Type of pumps Assessment of pumps Energy efficiency opportunities 3
- 4. Introduction Pump have two main purposes • Transfer liquid from source to destination • Circulate liquid around a system Used for: •Domestic, commercial, industrial, agricultural services •Municipal water and wastewater services 4
- 6. Main pump components • Prime movers: electric motors, diesel engines, air system • Piping to carry fluid • Valves to control flow in system • Other fittings, control, instrumentation End-use equipment • Heat exchangers, tanks, hydraulic machines 6
- 7. Pumping System Characteristics • Head destination • Resistance of the system Static head • Two types: static and friction source 7
- 8. Static Head • Difference in height between source and destination • Independent of flow Static head Flow 8
- 9. Friction head • Resistance to flow in pipe and fittings • Depends on size, pipes, pipe fittings, flow rate, nature of liquid • Proportional to square of flow rate Friction head Flow 9
- 10. Type of Pumps Pump Classification 10
- 11. Positive Displacement Pumps • Reciprocating pump • Displacement by reciprocation of piston plunger • Used only for viscous fluids and oil wells • Rotary pump • Displacement by rotary action of gear, cam or vanes • Several sub-types • Used for special services in industry 11
- 14. Rotodynamic pumps • Mode of operation • Rotating impeller converts kinetic energy into pressure to pump the fluid • Two types • Centrifugal pumps: pumping water in industry – 75% of pumps installed • Special effect pumps: specialized conditions 14
- 15. Centrifugal Pumps How do it work? 15
- 16. Assessment of pumps How to Calculate Pump Performance • Pump shaft power (Ps) is actual horsepower delivered to the pump shaft Pump shaft power (Ps): Ps = Hydraulic power Hp / pump efficiency ηPump Pump Efficiency (ηPump): ηPump = Hydraulic Power / Pump Shaft Power • Pump output/Hydraulic/Water horsepower (Hp) is the liquid horsepower delivered by the pump Hydraulic power (Hp): Hp = Q (m3/s) x Total head, hd - hs (m) x ρ (kg/m3) x g (m/s2) / 1000 16
- 17. Difficulties in Pump Assessment • Absence of pump specification data to assess pump performance • Difficulties in flow measurement and flows are often estimated • Improper calibration of pressure gauges & measuring instruments 17
- 18. Energy Efficiency Opportunities 1. Selecting the right pump 2. Controlling the flow rate by speed variation 3. Pumps in parallel to meet varying demand 4. Eliminating flow control valve 5. Eliminating by-pass control 6. Start/stop control of pump 7. Impeller trimming 18
- 19. Cooling Tower 19
- 20. Outline: Introduction Types of cooling towers Assessment of cooling towers Energy efficiency opportunities 20
- 21. Introduction A cooling tower is an equipment used to reduce the temperature of a water stream by extracting heat from water and emitting it to the atmosphere. 21
- 22. Component of Cooling Tower 22
- 23. Components of a cooling tower • Frame and casing: support exterior enclosures • Fill: facilitate heat transfer by maximizing water / air contact • Splash fill • Film fill • Cold water basin: receives water at bottom of tower 23
- 24. Components of a cooling tower • Drift eliminators: capture droplets in air stream • Air inlet: entry point of air • Nozzles: spray water to wet the fill • Fans: deliver air flow in the tower 24
- 25. Types of Cooling Towers • Natural Draft Cooling Tower • Mechanical Draft Cooling Tower • Forced Draft Cooling Tower • Induced Draft Cooling Tower 25
- 26. Assessment of Cooling Towers Performance Parameters 1. Range 2. Approach 3. Effectiveness 4. Cooling capacity 5. Evaporation loss 6. Cycles of concentration 7. Blow down losses 8. Liquid / Gas ratio 26
- 27. 1. Range Hot Water Temperature (In) High range = good performance (In) to the Tower (Out) from the Tower Cold Water Temperature (Out) Approach Range (°C) = CW inlet temp – CW outlet temp Range Difference between cooling water inlet and outlet temperature: Wet Bulb Temperature (Ambient) 27
- 28. 2. Approach Approach (°C) = CW outlet temp – Wet bulb temp Low approach = good performance Range Hot Water Temperature (In) Approach Difference between cooling tower outlet cold water temperature and ambient wet bulb temperature: (In) to the Tower (Out) from the Tower Cold Water Temperature (Out) Wet Bulb Temperature (Ambient) 28
- 29. 3. Effectiveness Hot Water Temperature (In) = 100 x (CW temp – CW out temp) / (CW in temp – Wet bulb temp) High effectiveness = good performance Approach = Range / (Range + Approach) Range Effectiveness in % (In) to the Tower (Out) from the Tower Cold Water Temperature (Out) Wet Bulb Temperature (Ambient) 29
- 30. 4. Cooling Capacity Hot Water Temperature (In) High cooling capacity = good performance Approach = mass flow rate of water X specific heat X temperature difference Range Heat rejected in kCal/hr or tons of refrigeration (TR) (In) to the Tower (Out) from the Tower Cold Water Temperature (Out) Wet Bulb Temperature (Ambient) 30
- 31. 5. Evaporation Loss Hot Water Temperature (In) = 0.00085 x 1.8 x circulation rate (m3/hr) x (T1-T2) T1-T2 = Temp. difference between inlet and outlet water Approach = theoretically, 1.8 m3 for every 10,000,000 kCal heat rejected Range Water quantity (m3/hr) evaporated for cooling duty (In) to the Tower (Out) from the Tower Cold Water Temperature (Out) Wet Bulb Temperature (Ambient) 31
- 32. 6. Cycles of concentration (C.O.C.) Ratio of dissolved solids in circulating water to the dissolved solids in make up water 7. Blow Down Losses Depend on cycles of concentration and the evaporation losses Blow Down = Evaporation Loss / (C.O.C. – 1) 32
- 33. 8. Liquid Gas (L/G) Ratio Ratio between water and air mass flow rates Heat removed from the water must be equal to the heat absorbed by the surrounding air L(T1 – T2) = G(h2 – h1) L/G = (h2 – h1) / (T1 – T2) T1 = hot water temp (oC) T2 = cold water temp (oC) Enthalpy of air water vapor mixture at inlet wet bulb temp (h1) and outlet wet bulb temp (h2) 33
- 34. Energy Efficiency Opportunities 1. Selecting a cooling tower 2. Fills 3. Pumps and water distribution 4. Fans and motors 34
- 35. 35
Notes de l'éditeur
- TO THE TRAINER This PowerPoint presentation can be used to train people about the basics of pumps and pumping systems. The information on the slides is the minimum information that should be explained. The trainer notes for each slide provide more detailed information, but it is up to the trainer to decide if and how much of this information is presented also. Additional materials that can be used for the training session are available on www.energyefficiencyasia.org under “Energy Equipment” and include: Textbook chapter on this energy equipment that forms the basis of this PowerPoint presentation but has more detailed information Quiz – ten multiple choice questions that trainees can answer after the training session Workshop exercise – a practical calculation related to this equipment Option checklist – a list of the most important options to improve energy efficiency of this equipment Company case studies – participants of past courses have given the feedback that they would like to hear about options implemented at companies for each energy equipment. More than 200 examples are available from 44 companies in the cement, steel, chemicals, ceramics and pulp & paper sectors
- Pumps have two main purposes: Transfer of liquid from one place to another place (e.g. water from an underground aquifer into a water storage tank) Circulate liquid around a system (e.g. cooling water or lubricants through machines and equipment)
- The main components of a pumping system are: Pumps (different types of pumps are explained in section 2) Prime movers: electric motors, diesel engines or air system Piping, used to carry the fluid Valves, used to control the flow in the system Other fittings, controls and instrumentation End-use equipment, which have different requirements (e.g. pressure, flow) and therefore determine the pumping system components and configuration. Examples include heat exchangers, tanks and hydraulic machines
- Pressure is needed to pump the liquid through the system at a certain rate. This pressure has to be high enough to overcome the resistance of the system, which is also called “head”. The total head is the sum of static head and friction head. Static head Static head is the difference in height between the source and destination of the pumped liquid (see top figure) Static head is independent of the flow (see bottom figure)
- Friction head: This is the loss needed to overcome that is caused by the resistance to flow in the pipe and fittings. It is dependent on size, condition and type of pipe, number and type of pipe fittings, flow rate, and nature of the liquid. The friction head is proportional to the square of the flow rate as shown in the figure. A closed loop circulating system only exhibits friction head (i.e. not static head).
- Pumps come in a variety of sizes for a wide range of applications. They can be classified according to their basic operating principle as dynamic or positive displacement pumps In principle, any liquid can be handled by any of the pump designs. Centrifugal pump is generally the most economical but less efficient. Positive displacement pumps are generally more efficient than centrifugal pumps, but higher maintenance costs.
- Positive displacement pumps are further classified based upon the mode of displacement: Reciprocating pump if the displacement is by reciprocation of a piston plunger. Reciprocating pumps are used only for pumping viscous liquids and oil wells. Rotary pumps if the displacement is by rotary action of a gear, cam or vanes in a chamber of diaphragm in a fixed casing. Rotary pumps are further classified such as internal gear, external gear, lobe and slide vane etc. These pumps are used for special services with particular conditions existing in industrial sites.
- Dynamic pumps are also characterized by their mode of operation: a rotating impeller converts kinetic energy into pressure or velocity that is needed to pump the fluid. There are two types of dynamic pumps: Centrifugal pumps are the most common pumps used for pumping water in industrial applications. Typically, more than 75% of the pumps installed in an industry are centrifugal pumps. For this reason, this pump is further described on the next slides. Special effect pumps are particularly used for specialized conditions at an industrial site
- A centrifugal pump is one of the simplest pieces of equipment in any process plant. The figure shows how this type of pump operates: Liquid is forced into an impeller either by atmospheric pressure, or in case of a jet pump by artificial pressure. The vanes of impeller pass kinetic energy to the liquid, thereby causing the liquid to rotate. The liquid leaves the impeller at high velocity. The impeller is surrounded by a volute casing or in case of a turbine pump a stationary diffuser ring. The volute or stationary diffuser ring converts the kinetic energy into pressure energy.
- The work performed by a pump is a function of the total head and of the weight of the liquid pumped in a given time period. Pump shaft power (Ps) is the actual horsepower delivered to the pump shaft, and can be calculated as follows: Pump shaft power Ps = Hydraulic power hp / Pump efficiency ηpump or Pump efficiency ηpump = Hydraulic power / Pump shaft power Pump output, water horsepower or hydraulic horsepower (hp) is the liquid horsepower delivered by the pump, and can be calculated as follows: Hydraulic power hp = Q (m3/s) x (hd - hs in m) x ρ (kg/m3) x g (m/s2) / 1000 Where: Q = flow rate hd = discharge head hs = suction head ρ = density of the fluid g = acceleration due to gravity
- This section includes the factors affecting pump performance and areas of energy conservation. The main areas for energy conservation include: Selecting the right pump Controlling the flow rate by speed variation Pumps in parallel to meet varying demand Eliminating flow control valve Eliminating by-pass control Start/stop control of pump Impeller trimming
- Cooled water is needed for, for example, air conditioners, manufacturing processes or power generation. A cooling tower is an equipment used to reduce the temperature of a water stream by extracting heat from water and emitting it to the atmosphere. This figure shows a cooling tower. Cooling towers make use of evaporation whereby some of the water is evaporated into a moving air stream and subsequently discharged into the atmosphere. As a result, the remainder of the water is cooled down significantly (Figure 1). Cooling towers are able to lower the water temperatures more than devices that use only air to reject heat, like the radiator in a car, and are therefore more cost-effective and energy efficient.
- Frame and casing. Most towers have structural frames that support the exterior enclosures (casings), motors, fans, and other components. With some smaller designs, such as some glass fiber units, the casing may essentially be the frame. Fill. Most towers employ fills (made of plastic or wood) to facilitate heat transfer by maximizing water and air contact. There are two types of fill: Splash fill: water falls over successive layers of horizontal splash bars, continuously breaking into smaller droplets, while also wetting the fill surface. Plastic splash fills promote better heat transfer than wood splash fills. Film fill: consists of thin, closely spaced plastic surfaces over which the water spreads, forming a thin film in contact with the air. These surfaces may be flat, corrugated, honeycombed, or other patterns. The film type of fill is the more efficient and provides same heat transfer in a smaller volume than the splash fill. Cold-water basin. The cold-water basin is located at or near the bottom of the tower, and it receives the cooled water that flows down through the tower and fill. The basin usually has a sump or low point for the cold-water discharge connection. In many tower designs, the cold-water basin is beneath the entire fill. In some forced draft counter flow design, however, the water at the bottom of the fill is channeled to a perimeter trough that functions as the cold-water basin. Propeller fans are mounted beneath the fill to blow the air up through the tower. With this design, the tower is mounted on legs, providing easy access to the fans and their motors.
- Drift eliminators. These capture water droplets entrapped in the air stream that otherwise would be lost to the atmosphere. Air inlet. This is the point of entry for the air entering a tower. The inlet may take up an entire side of a tower (cross-flow design) or be located low on the side or the bottom of the tower (counter-flow design). Louvers. Generally, cross-flow towers have inlet louvers. The purpose of louvers is to equalize air flow into the fill and retain the water within the tower. Many counter flow tower designs do not require louvers. Nozzles. These spray water to wet the fill. Uniform water distribution at the top of the fill is essential to achieve proper wetting of the entire fill surface. Nozzles can either be fixed and spray in a round or square patterns, or they can be part of a rotating assembly as found in some circular cross-section towers. Fans. Both axial (propeller type) and centrifugal fans are used in towers. Generally, propeller fans are used in induced draft towers and both propeller and centrifugal fans are found in forced draft towers. Depending upon their size, the type of propeller fans used is either fixed or variable pitch. A fan with non-automatic adjustable pitch blades can be used over a wide kW range because the fan can be adjusted to deliver the desired air flow at the lowest power consumption. Automatic variable pitch blades can vary air flow in response to changing load conditions.
- The natural draft or hyperbolic cooling tower makes use of the difference in temperature between the ambient air and the hotter air inside the tower. It works as follows: Hot air moves upwards through the tower (because hot air rises) Fresh cool air is drawn into the tower through an air inlet at the bottom. Due to the layout of the tower, no fan is required and there is almost no circulation of hot air that could affect the performance. Concrete is used for the tower shell with a height of up to 200 m. These cooling towers are mostly only for large heat duties because large concrete structures are expensive.
- These measured parameters and then used to determine the cooling tower performance in several ways. These are: Range Approach Effectiveness Cooling capacity Evaporation loss Cycles of concentration Blow down losses Liquid / Gas ratio We will now go through each of these parameters on the next slides
- The range is the difference between the cooling tower water inlet and outlet temperature. The formula for cooling tower range in degrees Celcius is cooling water inlet temperature minus cooling water outlet temperature A high CT Range means that the cooling tower has been able to reduce the water temperature effectively, and is thus performing well.
- Approach is the difference between the cooling tower outlet cold-water temperature and ambient wet bulb temperature. The formula for approach is CT Approach in degrees Celcius is cold water outlet temperature minus the wet bulb temperature The lower the approach the better the cooling tower performance. Although, both range and approach should be monitored, the `Approach’ is a better indicator of cooling tower performance.
- Cooling tower effectiveness is the ratio between the range and the ideal range (in percentage), i.e. difference between cooling water inlet temperature and ambient wet bulb temperature Effectiveness = Range / (Range + Approach) The formula for cooling tower effectiveness is: CT Effectiveness (%) = 100 x (CW temp – CW out temp) / (CW in temp – WB temp) The higher this ratio, the higher the cooling tower effectiveness.
- Cooling capacity is the heat rejected in kCal/hr or tons of refrigeration (TR), given as product of mass flow rate of water, specific heat and temperature difference.
- Evaporation loss is the water quantity evaporated for cooling duty. Theoretically the evaporation quantity works out to 1.8 m3 for every 1,000,000 kCal heat rejected. The following formula can be used (Perry): Evaporation loss (m3/hr) = 0.00085 x 1.8 x circulation rate (m3/hr) x (T1-T2) T1 - T2 = temperature difference between inlet and outlet water
- Cycles of concentration, C.O.C, is the ratio of dissolved solids in circulating water to the dissolved solids in make up water. Blow down losses depend upon cycles of concentration and the evaporation losses. Blow down losses is given by the following relation: Blow down = evaporation loss / (cycles of concentration – 1)
- The L/G ratio of a cooling tower is the ratio between the water and the air mass flow rates. Cooling towers have certain design values, but seasonal variations require adjustment and tuning of water and air flow rates to get the best cooling tower effectiveness. Adjustments can be made by water box loading changes or blade angle adjustments. Thermodynamic rules also dictate that the heat removed from the water must be equal to the heat absorbed by the surrounding air. Therefore the following formulae can be used (mention that these are the same formula, just written differently): L(T1 – T2) = G(h2 – h1) L/G = (h2 – h1) / (T1 – T2) Where: L/G = liquid to gas mass flow ratio (kg/kg) T1 = hot water temperature (0C) T2 = cold-water temperature (0C) h2 = enthalpy of air-water vapor mixture at exhaust wet-bulb temperature (same units as above) h1 = enthalpy of air-water vapor mixture at inlet wet-bulb temperature (same units as above)
- The main areas for improving the energy efficiency of cooling towers are: Selecting the right cooling tower (because the structural aspects of the cooling tower cannot be changed after it is installed) Fills Pumps and water distribution system Fans and motors We will go through these one by one on the next slides.