Chiller Pumps & Pumps selection

The performance of a centrifugal pump can be shown graphically on a characteristic curve. A typical characteristic curve shows the total dynamic head, brake horsepower, efficiency, and net positive Suction head all plotted over the capacity range of the pump. Performance of a centrifugal pump

Q-H curveEfficiency curveShaft power curveNPSH How to read the PerformanceCharacteristic of a water centrifugal pump

Combined performance curves

1- Head capacity curveThe available head produced by the pump decreases as the discharge increases. At Q= 0, the corresponding head is called shut off head point (1) Point (2) is called run out point below which the pump cannot operate. &should be shut down end-of curve

2- Efficiency curveThe efficiency of a centrifugal pump is the ratio of water power to brake power.

The highest efficiency of a pump occurs at the flow where the incidence angle of the fluid entering the hydraulic passages best matches with the blade angle. The operating condition where a pump design has its highest efficiency is referred to as the best efficiency point B.E.P.

3- Power curveThe shaft power is determined in order to select a motor for the pump. The shaft power can be determined directly from the manufacturers catalogue plot or calculated from the following formula:

From the equation, it is clear that the main parameter affecting the shaft power is the discharge and not the head. This is because of the increase in the discharge for the same pipe diameter leading to additional losses which need more power to drive the pump.

4- NPSH required curve , The NPSHR required increases with an increase in discharge.Operating the pump near the run-out point should be avoidedas the NPSHR value is high It may lead to cavitation problem.The Net Positive Suction Head Required is the minimum energy required at the suction flange for the pump to operate satisfactorily away from cavitation problem.

Motor size selection for centrifugal pump Most small pumps are monoblock; Since the manufacturer does not known at which point the pump will be operated. The end-of curve horsepower frequently used as basis for motor selection. in this case the motor power is determined by the manufacturer itself . However for large pump, we buy a pump without its motor; in this case, we have to determine its motor power. To do so, we plot the pipe system curve and then the operating condition can be determined i.e. discharge efficiency & power required. Due to discharge fluctuations, a shift in the operating point is expected which means an increase in pump power input. To avoid this, the motor power should be sized a bit higher than the calculated one.

The hydraulic power or water power is given by:Pump efficiency & motor power is selected from the manufacturer catalogues.Transmition efficiency is taken as follows:1- Case of shaft coupling = 1 , 2- Case of flat belt Transmition = 0.9 to 0.933- Case of V-belt Transmition = 0.93- 0.95.

UP to 7.5 kW add 20% From 7.5 - 40 kW add approximately 15%From 40 kW and above add approximately 10%.There is no simple rule of thumb in motor selection. The person selecting the motor must have a through knowledge of the pumping system and its characteristics, and the limitation of performance curves provided by the manufacturer. The motor may overload when the pump operates at a higher flow than anticipated. However ,after determining the shaft power the motor power can be selected with safety factor to assure proper motor operation . KSB pump catalogue presents the follows estimation values :Motor Power selection

Pumps in parallel & seriesRefer to the Fundamental of pumps by Dr. Hammoud

Static (hs)Friction (hf)FlowSystem Hydraulic Losseshf = kQ2

Bernoulli told us that[V2/2g + p/ + Z]2 [V2/2g + p/ + Z]1 = Pump Work (from 1 to 2)231 hloss (between 2 and 3) = kV2/2gTherefore, the Pump must give what the system (included Valve) takes!

Pipe system curveThe system curve represents the pressure needed at different flow rates to move water though the systemThe system curve consists of three main parameters:1- Static head2- Friction head loss3- Velocity head ( small and can be neglected)

How to draw the pipe resistance curves

Different values for the static head NO STATIC HEAD ALL FRICTION As the levels in the suction and discharge are the same there is no static head and, therefore, the system curve starts at zero flow and zero head and its shape is determined solely from pipeline losses. The point of operation is at the intersection of the system head curve and the pump curve. The flow rate may be reduced by throttling valve.

Different values for the static head POSITIVE STATIC HEAD In this case there is a positive static head involved. This static head does not affect the shape of the system curve or its "steepness", but it does dictate the head of the system curve at zero flow rate. The operating point is at the intersection of the system curve and pump curve. Again, the flow rate can be reduced by throttling the discharge valve.

Different values for the static head NEGATIVE STATIC HEAD (GRAVITY ASSIST FLOW( A certain flow rate will occur by gravity head alone. But to obtain higher flows, a pump Is required to overcome the pipe friction losses in excess of "H" the head of the suction above the level of the discharge. In other words, the system curve is plotted exactly as for any other case involving a static head and friction head, except the static head is now negative. The system curve begins at a negative value and shows the limited flow rate obtained by gravity alone.

Summary of static heads Allfriction

Operating point or duty point At this point the head required from the pump = the head given by the pump .Also At this point the pump would deliver the maximum discharge Qmax .

A centrifugal pump operating in a given system will deliver a flow rate corresponding to the intersection of its head-capacity curve with the pipe system curve. The intersection point is called Duty point or operating point

Pumps in parallel & series

What happens to Flow, Head and Power with Speed?So: Flow changes DIRECTLY (linear) with RPMHead changes as a SQUARE of RPMPower is proportional to Flow times Head it changes as CUBE of RPM Q ~ RPMH ~ RPM2SP ~ RPM3

Affinity laws (For the same pump)

Doubling the pump rotational speed leads to:1- Double the discharge.2- Increase the total head value by a factor of 4.3- Increase the power by a factor of 8.Affinity laws

Two Speed pumpingMultiple-speed motors can be used to reduce system over-pressure at reduced flow. In this example standard two-speed motors are available in models with speeds of 1750/1150 rpm, 1750/850 rpm, 1150/850rpm and 3500/1750 rpm. The corresponding figure shows the performance of a system with a 1750/1150 rpm multiple speed pump. In the figure, curve A shows the system's response when the pump runs at 1750rpm. When the pump runs at 1150rpm, operation is at point 1 and not at point 2 .If the system was designed to operate as shown in curve B, the pump would operate at or above the shut-off , it will be damaged if it runs at 1150 rpm. For that reason the designer must analyze the system carefully to determine the pump's limitations and the effect of lower speed on performance.

Two multi-speed pumps in parallel

3nd duty pointHH1QQ11st duty pointHesystem curvepump curve2nd duty pointIt can Replace the use of Multi-speed (3 speeds) manual adjusted circulated pump by VFD operation in heating system. (heating system circulated pump)Radiators On-off !!!Refer to the VFD by Dr. Hammoud

Speed reduction Pumps Shaft power

Pump & System curves

Application of chiller pumps

4 pipe systems

It is always good practice to consider a back up pump of equal capacity and proper valves to permit operation when the normal pump is inoperable. Usually this is an application for a parallel pump. Failure can occur in extremely cold weather for heating or in the middle of a hot spell during the cooling season, and the original investment cost of a by pass will be trivial compared to the inconvenience for the building occupants or the operator.Depending on the system curve and the pump curve and how many pumps are in the full system, a standby pump can provide up to 80% of design flow.Standby pump

Types of Pumps neededPump performance must be considered not only at the design point but across its entire characteristic curve. Centrifugal pumps are available with steep curves that drop from high head at low flow to low head at high flow versus those with flat curves that show a small change in head between shutoff to design flow

Some designers like to limit this to a 15% to 25% rise-to-shut off curve. These flat curve pumps are always recommended where two-way valves are applied to unit terminals. At part-loads, the valves will be operating at lower flows and this will move the system operating differential pressure up the pump curve.

When selecting a pump for chiller application, the following questions & factors should be considered:

Pump s selection

Pumps selection

1-What is the nature of the liquid to be pumped? (Water or antifreeze liquids , cold or hot etc.. ).2-What is the required volume flow rate (discharge)? What is the diversity factor?, what is the maximum and minimum amount of liquid to be pumped?3- What is the head required from the pump and the pipe system curve ?. For circulating pump the head required from the pump is to overcome the total head loss only.4-What are the conditions on the suction (inlet) side of the pump and on the discharge (outlet) side of the pump? check the NPSH. That is to say Cavitation must be avoided.

5- Check the specific speed. Read the corresponding catalogue. Select the range at which your pump may exist.6-Flat curve pumps rather than steep curve pumps is selected7-Plot the pipe system curve on each selected pump characteristic curve and then compare the head - discharge relationship, efficiency and power of the different pumps (from catalogues). The pump, which is operating at or near the point of B.E.P, ( maximum efficiency) should be selected. Afterwards, you should be able to identify one or two pumps that are suitable.8- Choose the lowest initial and running cost of the pump i.e. (power requirements). 9- What is the type of power source (electric motor, diesel engine, etc.)?10- Check the space, weight, and location ( indoors or outdoors ) of the pump.11- Refer to the governing codes and standards.

Single pump - Selected for a simple application. Single pump with trimmed impeller Optimizing pump capacity for a specific application. . Single pump with backup pump- In addition to a selected application, provides I00% backup. Two-speed pump - Provides limited variable flow steps with an added investment. Parallel pumps- Flexible capacity control without increasing system head; good for two-way valve control. Series pumps - Steep head change with limited flow change; two-way valves would require high differential pressure operation and capability. Primary-secondary pumping- Flexible zoning approach, with minimum pumping energy. Distributed pumps - Special application of primary-secondary pumping.Variable speed pumps- Applied to pumping systems to reduce power by lowering pump speed to meet control differential pressure in selected locations; usually applied to parallel pumping distribution systems employing primary-secondary or distributed pumping, with two-way control valves.

Remember this

The last group of booster sets is variable sped drivesThe last group of booster sets is variable sped drivesThe last group of booster sets is variable sped drivesIn many systems, particularly heating systems the operation means that the pump will not need to operate at fixed points on the system curve. An example as shown is heating systems, it can be shown that due to different periods of the year more or less heating (or air conditioning is needed), the Hydrovar by inputting the minimum and maximum frequency will then operate between these two points.