PUMPS and FANS - concept and explanation.ppt

Editor's Notes

• #1: Ask the participants to list some pump applications.
• #2: Centrifugal pumps account for the vast majority of pumps applications in industry. Centrifugal pumps give energy to the fluid by centrifugal action. They rely on the flow of fluid to create a seal to prevent fluid flowing backward through the pump. The volute is the most common centrifugal. The impeller vanes generally curve backwards, but radial and forward vanes are used.The velocity head of the fluid is converted into pressure head.
• #3: Pumps are components of pumping systems, which also include motors, drives, piping and valves. Typically much less than half the electricity input to a pumping system is converted into useful movement of fluid. The rest is dissipated by the various components that make up the system. Energy losses are still greater when the system is not operating at its design point. Thus, there appears to be to be a considerable potential for saving electricity, both by improving component efficiencies and through better system design. 
• #4: The system curve is a plot of system resistance vs flow. Once the operating point ie flow and pressure are determined, the pump has to be selected by superimposing the pump curve to intersect with the operating point on the system curve. The characteristic curve of a centrifugal pump is shown in the figure. The pump has to be selected so that it will operate at its best efficiency point. Oversizing of flow during initial selection creates can cause efficiency point to shift resulting reduced operational efficiency of the pump. Also the oversized pump needs to be throttled to meet the reduced flow condition.
• #5: Selection of a pump is vital to energy efficiency in operations later. Allowances and liberal factors of safety enhances the capacity and head requirement. In actual operation the pump will be oversized leading to throttling.
• #6: Use the white board and do the following Assume that we need to pump 68 m3/hr. to a 47 meter head with a pump that is 60% efficient at that point. Liquid Power - 68 x 47 / 360 = 8.9 Kw Where ‘360’ is a constant Shaft Power - 8.9 / 0.60 = 14.8 Kw Where 0.6 is the efficiency at that point Motor Power - 14.8 / 0.9 = 16.4 Kw Where 0.9 is the motor efficiency  As shown in the drawing, we should be using impeller "E" to do this, but we have an oversized pump so we are using the larger impeller "A" with the pump discharge valve throttled back to 68 cubic meters per hour, giving us an actual head of 76 meters. Now our Kilowatts look like this: 68 x 76 / 360 = 14.3 Kw being produced by the pump, and 14.3 / 0.50 = 28.6 Kw required to do this. Subtracting the amount of kilowatts we should have been using gives us: 28.6 - 14.8 = 13.8 extra kilowatts being used to pump against the throttled discharge valve. Extra energy used - 8760 hrs/yr x 13.8 = 120,880 kw. = $ 10,000/annum In this example the extra cost of the electricity could almost equal the cost of purchasing two or three pumps.
• #7: This is just similar to fans that we have seen in the last session. The normal operating point A is what the pump had been designed for. But to accommodate lower flow, the flow is throttled leading to an incremental pressure drop. The flow at B is less than flow at A but has been achieved at the cost of additional head which involves waste of energy.
• #9: For centrifugal pumps the laws that are applicable to fans are also applicable for pumps. Flow is proportional to speed, Head proportional to square of head and power to the cube of speed.
• #10: Varying flow requirements can be met by conventional and low cost options such as by pass control or throttle control. But both these methods are highly energy inefficient. There are occasions when you might want to permanently change the amount of fluid you are pumping, or change the discharge head of a centrifugal pump. This can be economically achieved by trimming the impeller or replacing the pump with a reduced size of the impeller or at the worst replacing the pump itself. The most efficient way to deal with varying flows is by means of a variable speed drive. This ensures that the pump always operates at the best efficiency point and eliminates the need for any throttling. The virtue of this method is that it reduces the energy input to the system instead of dumping the excess. With decreasing costs in power electronics the variable speed drives are becoming more popular today.
• #11: The normal mistakes done in selecting a pump is given above. Remember that the cost of operating an inefficient pump is many times more than the pump cost itself.
• #12: As a first step, a water balance has to be made out quantifying requirements for various uses. Once this is done analysis needs to be made to reduce or recycle water. Reducing water consumption is the simplest way to save energy. Also look for standby equipment and equipment which are not in operation, but utilising water. For example a DG set which operates only during emergency might have water getting circulated for cooling. Such uses can be curtailed. Just to meet one or two applications where a higher pressure is required, the entire pumping system may be operating at a higher head. Reduce the pressure and provide booster pumps for such applications.
• #13: First you should be sure of the flow and head requirements and then select the pump to operate in this point at maximum efficiency. The motor has to be selected so that the loading is maintained high. Energy efficient motor can be considered during new installations. Provide appropriate meters and log data so that analysis becomes easy. Where large variations are expected, multiple pumps have to be judiciously selected and operated for maximum efficiency.
• #14: Variable speed drives can be considered only where continuous fluctuations are expected. Each pump casing has provision for replacement with lower or higher size impellers up to a certain limit and varies with manufacturer. Flow changes can be met by changing the impeller or trimming the existing impeller. In multistage pumps such as boiler feed pumps, too high a pressure than requirement can be dealt by removing a few stages of the pump.
• #33: If fans rated for a higher flow rate, but a lesser flow is actually required: then flow reduction is effected by the following methods Recirculation: Venting the high-pressure air, or recirculating it to the inlet, is often used with positive-displacement blowers. It is sometimes used with fan systems, but is the least efficient method as there is no reduction in the air being moved. Damper: Restricting the airflow is accomplished with dampers or valves which close off the airflow at the inlet or outlet. Inlet vanes, which swirl the air entering the centrifugal fan or blower, are more efficient than dampers or butterfly valves. VFC: Variable speed fluid coupling. Motor speed is constant but the fluid coupling speed changes thus changing the speed of fan. VFD: Variable frequency drive which varies the speed of the motor itself Changing the blade angle is a method used with some vane-axial fans Changing the rotational speed is the most efficient. If the volume requirement is constant, it can be achieved by selecting appropriate pulley sizes. If the volume varies with the process, adjustable-speed drives can be used.
• #37: Existing condition This is a coconut shell fired boiler. Boiler is operating with damper 100 % opening. Excess air is around 200 % with an oxygen level of 14 %. This means too much air is being supplied to the boiler. Improved Condition The excess air was reduced by closing the damper by 50 % and excess air was reduced to 75 %. Now the fuel consumption reduced by 5%.    Not only that. We have reduced the air by closing the damper which has incurred a unnecessary pressure loss. Now how can we reduce the power of the fan ?                    
• #38: Existing condition   The ID fan is in oversized condition for the existing operating load.   So the minimum cost option is to reduce the motor pulley and reduce the fan speed. The motor pulley was reduced from 8” to 6”. Energy consumption in existing Condition (Damper control) 26 kW After reducing the motor pulley To 6” (damper fully open) 14 kW Annual saving 12 x 8000 96,000 kWH Speed reduction by pulley change can give only one fixed speed. Since the boiler is operating at constant load speed reduction through pulley change is the appropriate option. Where boiler load fluctuates a variable speed drive (inverter) may be installed. The speed can be varied according to load. The speed control can be effected in relation to fuel feeder RPM. The savings with variable speed drive will be marginally more because The motor and fan can be directly coupled thus eliminating the v-belt losses