Chilled Water Pump Design Guide: Section 1.0: Introduction
Chilled Water Pump Design Guide: Section 1.0: Introduction
Chilled Water Pump Design Guide: Section 1.0: Introduction
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DESIGN
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you more knowledge on how to complete the schedule for your specific chilled water pump
application.
The primary units that are used in the calculator are United States Customary System Units
(USCS). As such, this guide focuses exclusively on the USCS. However, an SI version will also be
provided in the future.
The chilled water pump must be sized properly to circulate the correct amount of flow at the
correct pressure in order to achieve proper cooling within the building. If there is insufficient
flow and pressure, then there will not be enough cooling and the building may get hot during
periods of high cooling load. If there is too much pressure and flow, then the electrical system
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will be unnecessarily burdened and if there is no speed control, then electricity may be wasted
due to excess pumping.
Figure 1: This figure shows an overall chilled water system. The chilled water pump circulates chilled
water through the system.
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Figure 2: The fluid information section is the first set of inputs required for an output from the
calculator.
Fluid Type: There are several fluid types of available for use in the calculator, including water,
propylene glycol mixtures and ethylene glycol mixtures. If you select a fluid type from the drop
down menu and a temperature, then the calculator will automatically fill in the properties for
density, dynamic viscosity and kinematic viscosity. If your fluid type is not available in the drop
down menu, then you must manually input all the values into the calculator, in order for the
calculator to work.
Temperature: The temperature is used by the calculator to automatically find the density,
dynamic viscosity and kinematic viscosity from the fluid properties built-in to the calculator. If
you are not using one of the drop-down fluid types, then the temperature is not necessary to
get an output from the calculator.
The typical chilled water temperatures are shown below. You should select the lowest supply
chilled water temperature.
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Density: The density is used in the calculations to find the Reynolds Number. The Reynolds
Number is used to find the friction loss coefficient.
Dynamic Viscosity: The dynamic viscosity is used in the calculator to find the Reynolds Number.
Pipe Information
The calculator requires you to insert the pipe section information which includes the flow rate,
pipe material, pipe type, pipe size and length of pipe. A different pipe section should be used if
any of these variables are changed.
Figure 3: The pipe information is used to find the inner area and relative roughness of the piping.
There are various pipes available for use in the calculator but you can also add your own pipe
information. The pipes built-in to the calculator include ASTM A53 Steel (Schedule 40 & 80),
ASTM B88 Copper (Type K, L & M), ASTM D2241 PVC (SDR 26) and ASTM F2389 Polypropylene
(DR 9). These are the most common pipes used in chilled water pipe application. If you have a
special case, then please use the references sheet to add in your pipe information or contact
Justin via email justin@engproguides.com.
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Figure 4: This figure is a sample of the pipe information built-in to the calculator, references tab.
Each pipe material and pipe type within that pipe material have its own standard pipe sizes. For
example, Schedule 40 Steel does not have a 5/8 inch pipe size. When you change pipe materials
and pipe types, please also change the pipe size to ensure the pipe size you want is available
within the standard. The calculator will give you an error if you select a non-standard pipe size
within the pipe material & type.
Figure 5: Inputting the valves and fittings on a piping section will help to determine the total loss
through the piping section.
The valve and fitting quantities are matched to 3-K values within the references tab. If you have
a special valve and fitting, you can add in the 3-K values to the references tab. If you don’t have
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the 3-K values you can either choose a similar valve/fitting or you can estimate the pressure
drop and add this to the equipment information section.
Figure 6: The pressure loss due to equipment is specific to the manufacturer of the equipment. You
should check the manufacturer product data for this information, but you should also remember that
the pressure loss is specific to a certain flow rate. If your flow rate is different, then you will need to
adjust the pressure drop to match your application.
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Figure 7: Pipe expansion and reductions are different from valves and fittings.
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The next equation calculates the Reynolds Number. This equation uses the velocity from the
previous equation along with the pipe inner diameter and the fluid properties (density &
viscosity).
The Reynolds Number classifies the fluid flow into either (1) Laminar, (2) Transition or (3)
Turbulent. The breakdown between these three classifications is defined below. The friction
calculations are most accurate with fluid flow in the turbulent region. For this reason, the
calculator will highlight in red any Reynolds Number that is below the turbulent region.
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Iterative Process: Since the friction coefficient is on both sides of the equation, you must use an
iterative process to find the friction coefficient. You must first choose a value for the friction
coefficient on the right side of the equation and then solve for the friction coefficient on the left
side. Then use the friction coefficient that you just computed and plug-in this value to the right
side of the equation and repeat the process. The process ends when the right and left side
friction coefficients converge to nearly the same number. The calculator completes this process
by running nine iterations.
Turbulent Flow: This equation only works for turbulent flow. A different equation is used for
laminar flow. Luckily in practical chilled water applications, flow is nearly always turbulent.
However, the calculator does incorporate conditional formatting to visually tell you if the flow is
not turbulent. You should use your knowledge of the turbulent range from the previous section
to ensure that your flow calculations are in the turbulent range.
Figure 8: The top of the calculator summarizes the pressure drop of the hydraulically remote run. This
includes the pressure drop due to piping, valves & fittings, equipment and expansions/reducers.
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and the length of piping to calculate the pressure drop. For more details, see the equation
below. The output for this equation is the pressure drop in units of feet head.
The pressure drop through valves and fittings is found through the 3-K method. The 3-K method
uses three K-values to characterize each type of valve and fitting. These three K-values are K1,
Kinf and Kd. These K-values are used with the Reynolds Number and nominal pipe diameter to
find the final K-value.
Since, the calculated K-value is a function of Reynolds Number and nominal pipe diameter, the
K-value is applicable for various pipe sizes, pipe materials, fluids and fluid velocities. Once you
have the K-value, the K-value is used to calculate the pressure drop through the valves and
fittings.
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the equipment. Typically, the manufacturer will provide a single value that indicates the
pressure drop at a specified flow rate (GPM). This is typical of chillers, fan coils and air handling
units. Other times, a manufacturer will provide a graph that shows the pressure drop at various
flow rates. This is typical of flow meters, control valves and strainers.
An example of the equations used in the calculator includes the square reduction equation.
First, you must solve for the K-value.
Figure 9: The tapered reduction type requires you to input the first and second diameters with respect
to the flow direction. This is similar for all pipe expansion/reduction types.
The calculator automatically ensures that you are meeting the requirements of the equation.
There are other equations for each pipe expansion/reduction and each set of requirements as
shown in the table below.
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Once, the K-value is calculated, then the calculator calculates the pressure drop with the below
equation.
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Figure 10: Once you have found the pressure drop, then you can begin to fill in the chilled water pump
schedule. This figure shows the first half of the schedule.
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Figure 11: The second half of the schedule is shown in this figure.
6.3 SERVICE
This column is used to clarify the fluid of the pump, since these schedules may often be used as
a general pump schedule. A general pump schedule could include chilled water pumps,
condenser water pumps, hot water pumps, etc. In this case, the service column could be
removed since the entire schedule is dedicated to chilled water pumps.
6.4 LOCATION
Chilled water pumps are typically located in a mechanical room with or near the chiller(s). The
chilled water pump should be located with sufficient net positive suction head to ensure proper
operation of the pump.
For more information on net positive suction head, please see the condenser water pump and
expansion tank guides and calculators.
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displacement and reciprocating pumps in detail because they are not typically used for chilled
water pumps.
Centrifugal pumps are the most common type of pumps used for chilled water applications. The
following information is tailored to centrifugal pumps and should not be applied freely to
positive displacement pumps.
Centrifugal pumps operate on the principle of "centrifugal force", which is the conversion of
rotational kinetic energy imparted by rotating impellers onto the liquid to produce a flow rate
(kinetic energy) at a certain pressure (pressure energy). Fluid enters the pump at the center or
eye of the impeller. The rotating impellers then push the fluid to the outer edges, imparting a
flow rate and pressure.
There are two main types of centrifugal pumps, in-line and end-suction. These two types are
discussed below.
Figure 12: This figure shows a cut-away of an end-suction centrifugal pump. In a centrifugal pump,
the impeller rotates, which pushes the fluid to the outer edges of the volute. The volute directs the
fluid to the discharge. This centrifugal pump is an end suction type, because the suction and
discharge are at 90 degrees to each other. (1) Fluid flows into the center of the impeller, (2) as the
impeller rotates, (3) the centrifugal force pushes fluid to the edges (4) until the fluid travels out the
pump discharge.
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Figure 13: (1) Fluid flows into the center of the impeller, (2) as the impeller rotates; (3) the centrifugal
force pushes the fluid to the edges (4) until the fluid travels out the pump discharge.
Within each centrifugal pump type (end suction & in-line) are horizontal and vertical type
pumps, which are characterized by the orientation of the pump shaft as either horizontal or
vertical. In addition, pumps can be further classified by the number of stages that the fluid
proceeds through. Finally the last classification is how the pump is connected to the motor.
Pumps can be long-coupled where the pump is connected to the motor by a flexible coupling or
they can be close-coupled where the connection between the pump and motor is through a
rigid coupling. The table below is a summary of the most common chilled water pump types.
The same four types of pumps also can be found with a vertical arrangement.
Another type of pump that is used for larger flow rates is the split case type. This centrifugal
pump type has two chambers (split case) as opposed to the single chamber for end suction and
in-line pumps.
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The typical pump efficiencies are within the range of 60% to 80%. You should select a pump that
has its best efficiency point near your design operating point (typically around 10%). For
example, if the best efficiency point of a pump is 72%, then you should choose this pump if your
operating point is greater than 62% efficiency.
Sometimes the pump manufacturer will indicate a speed slightly below 1,200, 1,800 or 3,600.
This is because the motor is an induction motor as opposed to a synchronous motor. This
means that electrically the rotation will be 1,200, 1,800 or 3,600 RPM, but the shaft will lag
slightly behind this rotation.
6.8 MOTOR
The motor provides the necessary mechanical power to rotate the impeller within the pump. A
motor takes in electrical power and converts the power to rotation. The motor must be able to
meet the brake horsepower requirements of the pump. This is based on the pump flow rate,
pump pressure and the efficiency of the pump.
With these inputs, the brake horsepower of the chilled water pump is calculated with the
following equation.
The pump needs a motor to provide the power to spin the pump. The motor will need to
provide more than the pump power because there will be losses due to motor inefficiencies.
Typically motor efficiencies range from 90% – 95%.
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The calculated motor horsepower must be less than the available motor horsepower ratings.
The available horsepower motor ratings are shown in the table below.
Table 1: This table shows the available NEMA motor sizes that are used to size the motors for pumps
and fans. Although the actual energy usage will be equal to the BHP, electrical engineers must size
their equipment and wiring based on the HP value provided by the mechanical engineer.
Pump manufacturers have online software that automatically shows the available pumps for a
given flow rate and pressure drop. The software will show the speed (RPM), efficiency, brake-
horsepower (BHP) and horsepower (HP) for the various pump types that can meet the required
flow rate and pressure drop.
Although the pump selection tool makes it very easy to select pumps, you should also know
how to select pumps via the pump curves. A pump curve shows the pressure and flow rate
operating points for a pump operating at various speeds or impeller diameters. If a pump speed
is selected (1,200, 1,800 or 3,600) for a pump, then the pump curve graph will show multiple
pump curves at various impeller diameters. If an impeller diameter is selected, then the pump
curve graph will show multiple pump curves at various speeds. The following figure shows a
specific pump speed, with multiple pump curves at various impeller diameters.
The graph also shows curves of pump horsepower in blue. These curves are created by
calculating the horsepower based on the pressure, flow rate and pump/motor efficiency at a
point. The efficiency curves shown on the figure are found based on a series of tests of the
actual pump.
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Figure 14: This sample pump curve shows the operating conditions for a pump at a specific pump
speed. The red curves show the operating points for this pump at various impeller diameters. The red
curves show the pressure that can be provided at various flow rates. The intersection of the red curve
and the blue horsepower curve shows the horsepower required at a certain operating point. The
intersection of the red curve and the green efficiency curves show the efficiency at a certain operating
point.
Non-Overloading Pump: A non-overloading pump is a pump with a sufficiently sized motor that
can provide enough power at all operating points along a pump curve. For example, in the
previous figure a non-overloading pump with 6” impeller diameter will have a 1-1/2 HP motor.
At every point on the red curve, the horsepower required is less than the 1-1/2 blue HP curve.
Premium Efficiency Motor: A premium efficiency motor describes a motor that has a minimum
efficiency specific to each motor horsepower in compliance with NEMA Premium Efficiency
Motor Standards. The motor standards can be found in the below links. As an example, an
energy efficient 5 HP/1,800 RPM motor will have an efficiency of 87.5% and a premium efficient
motor will have an efficiency of 89.5%.
https://www.nema.org/Policy/Energy/Efficiency/Pages/NEMA-Premium-Motors.aspx
https://www.energy.gov/sites/prod/files/2014/04/f15/amo_motors_handbook_web.pdf
Variable Frequency Drive: A variable frequency drive is used to increase the speed of the pump
rotation, which causes the pump curve to shift up and down. This causes the pump to provide
more flow/pressure as the speed is increased and less flow/pressure as the speed is decreased.
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