1. Chapter 16 A: PUMPS AND SYSTEM EFFECTS
T. Agami Reddy (rev- May 2017)
Equation of motion- Bernouill’s equation with friction
Pressure drop: D’arcy-Weisbach equation, hydraulic diameter of pipe, friction factor, Moody diagram,
Pressure drop in pipe fittings (minor losses): equiv length method and pressure loss coefficient for fittings
Description of centrifugal pumps
Ideal and actual pump power, efficiency, suction head, NPSH
Pump characteristics
System curves: Water piping characteristics, series and parallel
Pump-piping system interaction and control
Pump affinity laws
HCB 3-Chap 16A: Pumps and System Effects

2. Review- Total, Static and Velocity Pressures
Manometer arrangement
HCB 3-Chap 16A: Pumps and System Effects

3. HCB 3-Chap 16A: Pumps and System Effects
Equation of Motion
HCB 3-Chap 16A: Pumps and System Effects

4. HCB 3-Chap 16A: Pumps and System Effects
Equation of Motion -
HCB 3-Chap 16A: Pumps and System Effects

5. Equation of Motion- Examples
Application to open systems (i.e., open to atmosphere)
HCB 3-Chap 16A: Pumps and System Effects

6. Equation of Motion- Examples
Application to closed systems (i.e., a system
with no opening to the atmosphere and
the fluid is recirculated continuously)
Example:
Pressure loss due to friction is 24 ft WG.
What is the pump head required?
There is not net change in elevation,
in pressure or in velocity.
So H pump = H friction = 24 ‘ WG
HCB 3-Chap 16A: Pumps and System Effects

7. HCB 3-Chap 16A: Pumps and System Effects
Pressure Drop
For a pipe under pressure, pipe is full and D = Dh
For partially filled pipes or for non-circular pipes such as
use above equation
HCB 3-Chap 16A: Pumps and System Effects

8. HCB 3-Chap 16A: Pumps and System Effects
Pressure Drop
(16.8)
HCB 3-Chap 16A: Pumps and System Effects

9. HCB 3-Chap 16A: Pumps and System Effects
Fig The Moody diagram with friction factor as a function of Reynolds number and relative roughness as a parameter. The hydraulic diameter is Dh.
HCB 3-Chap 16A: Pumps and System Effects

10. Relative Roughness values for pipes
HCB 3-Chap 16A: Pumps and System Effects

11. Pressure Drop in Straight Pipes
Fig (b) Pipe friction chart for seasoned steel pipe in SI units at 20 C.
HCB 3-Chap 16A: Pumps and System Effects

12. HCB 3-Chap 16A: Pumps and System Effects
Example: Using the friction chart
Find the flow given the pressure drop and pipe size. Water at 80° C flows through 400 mm cast-iron pipe with a pressure drop of 225 Pa/m. What is the volumetric flow rate?
We can solve this problem by rearranging Eq (16.6) as shown in the textbook (Example 16.2).
A simpler approach is to use Figure (16.3) directly:
flow rate is about 400 L/s
However, note that this is approximate since the water temperature and the pipe material does not correspond to the standard conditions assumed in Fig. 16.3
HCB 3-Chap 16A: Pumps and System Effects

13. Temperature Correction
Fig. 16.3(d) Pressure drop correction factor for different water temperatures
HCB 3-Chap 16A: Pumps and System Effects

14. Pressure Drop in Pipe Fittings
Straight lengths of pipe are connected by elbows and tees, with valves and ancillary equipment such as filters. These have pressure drops (referred to as minor losses) that are quite significant.
Two approaches used for water flow: Kf method and equivalent length method
(16.15)
(16.16)
(16.17)
HCB 3-Chap 16A: Pumps and System Effects

15. HCB 3-Chap 16A: Pumps and System Effects
Fig Typical pipe fittings used in HVAC systems.
Fig Nomograph for pipe fitting friction factors.
HCB 3-Chap 16A: Pumps and System Effects

16. HCB 3-Chap 16A: Pumps and System Effects
(16.19)
(16.20)
Table 16.4
HCB 3-Chap 16A: Pumps and System Effects

17. HCB 3-Chap 16A: Pumps and System Effects
Piping Material
Considerations for pipe material selection: (often dictated by code)
Type of fluid, Temperature, Pressure, Oxidation / corrosion, Cost
Materials used normally
- low carbon steel pipe (black steel pipe), wall thickness specified by a schedule number: example: 20, 30,…
- for open systems use galvanized pipe (corrosion prevention)
- copper pipe wall thickness specified by a letter:
K – thickest, high pressure, refrigerant
L – intermediate, hydronic system piping
M- low pressure plumbing
Copper tubing has lower friction and no corrosion, but is less strong and more expensive
Fittings for steel pipe (screwed, welded, flanged)
elbows, tees, couplings, unions, bushings
Fittings for copper tubing
- soldering or brazing ( temp > F, stronger joint)
- flaring
- strainers (usually at suction of pumps)
HCB 3-Chap 16A: Pumps and System Effects

18. Centrifugal Pumps- Description
HCB 3-Chap 16A: Pumps and System Effects

19. HCB 3-Chap 16A: Pumps and System Effects
Power and Efficiency
Hydraulic power (water horsepower, whp)
Theoretical power to move fluid
Common to express shaft power in HP, flow in gal/min, pressure in ft. H2O. For 68ºF water,
HCB 3-Chap 16A: Pumps and System Effects

20. HCB 3-Chap 16A: Pumps and System Effects
Power and Efficiency
Brake power (brake horsepower, bhp)
Power input to pump shaft required to produce a given hydraulic power
Pump efficiency defined by relationship of hydraulic power to brake power
HCB 3-Chap 16A: Pumps and System Effects

21. HCB 3-Chap 16A: Pumps and System Effects
Fig. 16.6
HCB 3-Chap 16A: Pumps and System Effects

22. HCB 3-Chap 16A: Pumps and System Effects
Solution:
From Fig. 16.6
HCB 3-Chap 16A: Pumps and System Effects

23. HCB 3-Chap 16A: Pumps and System Effects
Contd.
HCB 3-Chap 16A: Pumps and System Effects

24. HCB 3-Chap 16A: Pumps and System Effects
System Curve
Fig Characteristic curve of a basic piping system
Head vs. flow characteristic of a piping system
Variable head (due to friction, function of flow rate)
Fixed head (head at zero flow, due to control, static lift)
HCB 3-Chap 16A: Pumps and System Effects

25. HCB 3-Chap 16A: Pumps and System Effects
System and Fan Interaction
Fig Typical pump or fan performance curve (solid curve). System curve is approximately parabolic (dashed). Intersection point is the system operating point.
HCB 3-Chap 16A: Pumps and System Effects

26. HCB 3-Chap 16A: Pumps and System Effects
Suction Head
At inlet to centrifugal impeller
Flow accelerates
Static pressure decreases
If static pressure falls below vapor pressure, cavitation will occur
Noise
Impeller damage
Loss of efficiency
Minimum suction head must be provided to prevent flashing
Net positive suction head required (NPSHR)
Determined by manufacture, a pump characteristic
Net positive suction head available (NPSHA)
A system characteristic
Under all operating conditions, we should have
NPSHA > NPSHR
HCB 3-Chap 16A: Pumps and System Effects

27. HCB 3-Chap 16A: Pumps and System Effects
Pump Characteristic
Graphical representation of pump performance
Head vs. flow
Pump efficiency
Motor power
Suction head required
Constant speed,
multiple impeller sizes
Variable speed,
fixed impeller size
Fig Example pump curve showing constant-efficiency lines (labeled with %), NPSHR, and input power (labeled in hp units) for five impeller sizes (labeled in inches).
HCB 3-Chap 16A: Pumps and System Effects

28. HCB 3-Chap 16A: Pumps and System Effects
Example:
Calculate the power required
by the pump when the flow
rate is 6 L/s
At 6 L/s, i.e., m3/s,
we find from the figure
HCB 3-Chap 16A: Pumps and System Effects

29. HCB 3-Chap 16A: Pumps and System Effects
Piping networks in series and parallel
Fig Equivalent characteristic curves for piping and ducting networks (a) series flow arrangement (b) parallel flow arrangement
HCB 3-Chap 16A: Pumps and System Effects

30. HCB 3-Chap 16A: Pumps and System Effects
Pumps in Parallel
Fig Two identical pumps in parallel
HCB 3-Chap 16A: Pumps and System Effects

31. Pump and System Interaction Using Valves
HCB 3-Chap 16A: Pumps and System Effects

32. Flow Control Using Valves
Not very
energy
efficient
control
or balancing
valves
Generally, head increases as flow decreases
Control valve adds resistance- all hydronic systems have several control valves!
Efficiency decreases rapidly
HCB 3-Chap 16A: Pumps and System Effects

33. Flow Control with Variable Speed Pump
More
energy
efficient
control
Lower head than constant speed- so less pumping power
Generally, higher efficiency
Fixed head influences amount of speed reduction
HCB 3-Chap 16A: Pumps and System Effects

34. HCB 3-Chap 16A: Pumps and System Effects
Fig Flow control can be achieved either by partial closure of control valves or by variable speed drives
HCB 3-Chap 16A: Pumps and System Effects

35. HCB 3-Chap 16A: Pumps and System Effects
Pump Selection Example
150 gpm at total head = 36 ft
Pump output depends on system characteristic
Perform system calculations first, then select pump
Operating point requires 7” impeller
However, this would give 160 gpm
which is more than required
A valve is specified to reduce the
flow to 150 gpm
Shaft power required = 2.3 hp
So one would specify say a 2.5 hp
electric motor
HCB 3-Chap 16A: Pumps and System Effects

36. HCB 3-Chap 16A: Pumps and System Effects
Pump Affinity Laws
Similarity relationships for homologous (geometrically similar) pumps
For diameters or speeds 1 and 2
Flow
Head
Power
HCB 3-Chap 16A: Pumps and System Effects

37. HCB 3-Chap 16A: Pumps and System Effects
Example
Consider the pump and system characteristic as shown. The 7” impeller at 1750 rpm pump is originally operated at point 1 (43 ft of total head at 130 gpm). We need to reduce the pump speed until the flow rate is 100 gpm. Find the new pump head, shaft power and efficiency
We note that the pump must produce 25 ft of head at a flow rate of 100 gpm.
Shaft power
Implied assumption while using the affinity laws is that the efficiency is the same (geometrically similar pumps)
HCB 3-Chap 16A: Pumps and System Effects

38. HCB 3-Chap 16A: Pumps and System Effects
Outcomes
Working knowledge of the equation of motion
Understanding the friction factor and the Moody diagram
Familiarity with pressure drop determination in open and closed systems
Be able to solve problems of pressure drop through straight pipe, both by using the D’arcy-Weisbach relation and by using specialized charts
Be able to solve problems of pressure drop through fittings both by the equivalent length method and by the loss coefficient method
Familiarity with different parts of a centrifugal pump
Understanding notions related to pump characteristics (head, power and effy vs flow)
Be able to compute ideal and actual pump power and efficiency
Be able to analyze system-pump interactions and select pumps
Understanding how pump characteristic curves vary under series/parallel
Understanding of water piping characteristics under series/ parallel configuration
Understanding flow control using valves and by varying speed
Understanding of the pump affinity laws
HCB 3-Chap 16A: Pumps and System Effects