Pumps

In chilled water plants, centrifugal pumps provide the required differential pressure to circulate water through the chilled and condenser water distribution system.

In the centrifugal pump, an electric motor rotates an impeller that adds energy to the water after it is directed into the center of the rotating impeller.

The combination of centrifugal and rotational force imparts velocity to the water.

The pump casing is then designed to maximize the conversion of velocity energy of the fluid into pressure energy (Fig. 90).




	Fig. Nº90. Pump casing schematic section.
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Pump Types:
The most of the pumps used in the HVAC industry are single stage (one impeller) volute-type pumps that have either a single inlet or a double inlet (double suction). Double suction pumps are used in high volume applications; however either a single inlet or double inlet pump is available with similar performance characteristics and efficiencies.
Close-Coupled, Single Stage, End-Suction Pump: The closed-coupled pump has the impeller mounted on a motor shaft extension. The pump is mounted on a horizontal motor supported by the motor foot mountings.




	Fig. Nº91. Closed-Coupled End-Suction Pump
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A solid concrete pad is required for mounting. This compact pump has a single horizontal inlet and vertical discharge . The motor and pump can not be misaligned and they take up less floor space than flexible-coupled pumps. However, to replace the special motor with an extended shaft that is used can be difficult to get after a breakdown.

Frame-Mounted, End-Suction Pump on Base Plate: The motor and pump are mounted on a common rigid base plate for horizontal mounting. Mounting requires a solid concrete pad. The motor is flexible-coupled to the pump shaft. For horizontal mounting, the piping is horizontal on the suction side and vertical on the discharge side . The flexible-coupled shape allows the motor or pump to be removed without disturbing the other. However, the flexible coupling requires very careful alignment and a special guard. The flexible-coupled pump is usually less expensive than the close-coupled pump.

Double-Suction pumps: The water is introduced on each side of the impeller and the pump is flexibly connected to the motor. Typically, motor and pump are mounted on a common rigid base plate for horizontal mounting . Double-suction pumps are preferred in application over 1000 GPM because its very high efficiency and can be opened, inspected and serviced without disturbing the motor, impeller or the piping connections. The pump case can be split axially (parallel to the shaft) or vertically for servicing. This pump takes more floor space than end suction pumps and is more expensive.

Vertical In-Line Pumps: It is also a closed-coupled pump that has the motor mounted on the pump casing . These pumps have the suction and discharge connections arranged so they can be inserted directly into a pipe. Mounting requires adequately spaced pipe hangers and, sometimes, a vertical casing support. In the past, in-line pumps were used almost exclusively for small loads with low heads but now the widest rage of sizes is available. Considerable space saving can be achieved using in-line pumps but extra care must be taken to assure that pipe stress are not transferred to the pump casing.



	Fig. Nº92. Frame Mounted End-Suction Pump on Base Plate .
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	Fig. Nº93. Double-Suction Pumps
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	Fig. Nº94. Vertical In-Line Pumps
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Pump Performance Curves

The performance curve is the easiest and most satisfactory way to show graphically the relationship between head, capacity, horsepower, etc., of any pump (Fig. 95). For a given rotational speed and impeller size, the performance of a pump can be represented on a head-capacity curve of total developed head in feet of water versus flow in gallons per minute.




	Fig. Nº95. Pump Curve for Different Impeller Sizes
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Total dynamic head (TDH) is the difference between suction and discharge pressure and includes the difference between the velocity head at the suction and discharge connection. The lines sloping downward from left to right represent the varying quantities of water delivered by the pump with variations in head or pressure for a given impeller size.

The intersection of this line with zero delivery line shows the “shut-off head”, which is the pressure developed by the pump when the discharge valve is shut.

Starting from the shut-off head, as the pump delivers more water, the mechanical efficiency of the pump increases until a “best efficiency point” (BEP) is reached. Increasing the flow further decreases the efficiency until a point known as “end of curve” where the manufacturer no longer publishes the performance. As the impeller gets smaller, the pump efficiency also decreases.

The power requirements are also shown on the performance curve. The horsepower line that does not cross the pump curve is called “non-overloading” horsepower because operation at any point on the published pump curve will not overload the motor.

Parallel Operation

The primary purpose of operating pumps in parallel is to allow a wider range of flow than would be possible with a single fixed speed pump for systems with widely flow demand.

Usually there are no more than three or four pumps operating in parallel.

The combined parallel pump curve can be drawn holding the head constant and adding the flow. Fig. 96 shows a combined pump curve of a system with three identical pumps operating in parallel.




	Fig. Nº96. Combined Performance Curves for Three Identical Pumps Operating in Parallel
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Curve A represents the head-flow curve for any one of the pumps. Curve B is the combined pump curve for two pumps operating at the same time in parallel and curve C represents the combined performance for the three pumps. Notice that at any head value, the flow on curve B is twice the flow at the same head on curve A. Likewise, flow on curve C is three times the flow on curve A for the same head value. Curve X represents the system head curve. Points a, b, and c represents the flow that is delivered by the pumps at the three operating conditions which are: a single pump, two pumps and all (three) pumps operating at the same time.

Series Operation
In series operation, the discharge of one pump feeds the suction of a second pump (Fig 97). Unlike parallel operation, series pump curve can be drawn holding the flow constant and adding the head. Series operation allows that commercially available equipment can be used in a particular system because sometimes a single pump operation would result in a pump with an extremely high head and thus an equally high horsepower. For example, distributing pumping schemes applied in chilled water plants avoid using to big pumps for chilled water circulation that create unnecessary overpressure at the buildings close to the plant. Small pumps situated just at the building they feed mitigate the overpressure problem and at the same time save considerable pumping energy. Such schemes are based on the series pumps operation principles.




	Fig. Nº97. Combined Performance Curves for Three Identical Pumps Operating in Series
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System Curves
The system curve defines the system head required to produce a given flow rate for a liquid and its characteristics in a piping system design. To produce a given flow, the system head most overcome pipe friction, inside pipe surface roughness, actual fitting losses, actual valve losses, resistance to flow due to fluid viscosity, and possible system effect losses. The general shape of this curve is parabolic since the head loss is proportional to the square of the flow. In closed systems like chilled water distribution loops, the head varies only with flow variations, so when there is no flow, the head value is also nil

However, in an open system like piping circuit between a refrigerant plant condenser and its cooling tower, the elevation difference between the water level in the tower pan and the spray distribution pipe creates a fixed (static) head loss. That fixed loss occurs at all flow rates, so it is, therefore, and independent head loss (Fig. 99).

Variable Speed Pumping
The reasons why variable speed pumping saves energy are illustrated in Fig. 100. The pump (curve A) must operate between the flows marked as 1 and 2 in the figure. The higher flow rate is achieved where the pump curve intersects the system (curve X) at point ax. With a constant speed pump, the lower flow rate is obtained by throttling the pump discharge to steepen the system curve until the new system curve intersects the pump curve at the lower required flow rate, at point ay.



	Fig. Nº98. Typical Closed System Head Curve .
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	Fig. Nº99. Typical Open System Head Curve
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	Fig. Nº100. Capacity-Head and Efficiency Curves for Two Flow Rates Using Throttling and Variable Speed
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If instead of throttling, the pump were slowed down it would produce a new head-flow and efficiency curves, show n on Fig. 100 as light solid lines. The intersection of this slower speed head-flow curve (curve B) with the original system head curve X (point bx), represents the flow rate delivered by the pump operating at reduced speed. The difference in head between points ay and bx represents the head wasted across the control valve when the constant speed pump is throttled instead of slowed down.
The affinity laws are rules that govern the performance of a pump under varying conditions of flow and pressure. The laws can be roughly stated as follows:

- Flow varies with rotating speed
- Head varies with the square of the rotating speed
- Brake horsepower varies as the cube of the rotating speed

According to the last affinity law, if the slower flow rate in the Fig. 100 (marked as 1) were half the higher flow rate (marked as 2) then the power necessary to provide the slower flow rate would be (0.5)3 = 0.125 or 12.5 % of the power required for the higher flow rate. It means an energy saving of 87.5 %. Such savings justify the applications of variable speed pumping because of its low pay-back period. Note also that, at lower flow rate using a constant speed the efficiency lowers, as can be seen in the Fig. 100. On the contrary, when using variable speed pumping the pump efficiency curve shifts to the left at lower flow rate and the pump runs at least as efficient as at higher flow.

In addition to the energy savings associated with using variable speed pumping, there are other important benefits related to the health of the pump. Operating at reduced speed to obtain a lower flow rate produces lower radial bearing loads, which should give longer bearing life and less chance of bearing or seal failure. Besides, when the pump operates closer to its BEP less internal circulation of the fluid is produced. Thus, erosive damage to the pump is also mitigated.

Variable speed pumping allows the continued tuning of the system so that pump is performing optimally, despite of changes in either the system or the pump. If head requirements change over time, due to build-up of corrosion in pipe lines, the pump can simply be speeded up to account for the higher head required. Likewise, if the pump impeller wears and clearances become wider, the speed can be adjusted up to keep the pump from delivering less flow. The fudge factors often used in pump head calculations which may result in pumps over-sizing are essentially eliminated when variable speed pumping is applied.

The variable frequency drive (VFD) is by far the most commonly device to achieve variable speed pumping. A VFD work as a controller for a standard A.C. motor, adjusting the frequency and voltage to achieve a variable output speed. VFDs represent the state of the art for speed variation; they can be adapted to an existing standard motor allowing it to operate in a wide speed range without excessive energy losses. It is worth also pointing that VFDs are considerably more efficient than the other methods of achieving variable speed used in the past as fluid couplings and hydro-viscous (slip-clutch) arrangements, among others.

However, VFDs have some disadvantages as possible negative impact on power quality, motor noise, electromagnetic interference (EMI), radio frequency interference (RFI), and nuisance tripping. The efficient of the motor and drive combined should be considered as well as the type of motor being connected and the distance between the drive and the motor, and numerous accessories. Therefore, in each variable speed application several issues must be carefully followed to ensure successful results.

In summary, variable speed pumping is a very desirable alternative to reduce pumping costs for systems with a wide range of pump flow requirement. It could be used in retrofitting projects without buying new motors. Lower bearing loads and overall better health of the pump is achieved while fine-tune speed in response to change in the system or pump.