Introduction to Energy Management

Week/Lesson 9 part b

Centrifugal Pumps and Hydronic Systems

After completing this chapter, you will be able to: Describe the operation and primary parts of a

centrifugal pump Describe how a centrifugal pump adds pressure to

a fluid Identify several types of centrifugal pumps Explain pump performance characteristics such as

head, flow rate, horsepower and efficiency

Centrifugal Pumps and Hydronic Systems

Use pump performance curves to evaluate a pump’s capability

Evaluate the flow characteristics of a hydronic system

Calculate the flow requirement for a hydronic system

Centrifugal Pumps and Hydronic Systems

How a centrifugal pump works Uses centrifugal force Force is created by an impeller Force pushes outward The faster the impeller, the greater the force Volute – pump casing

Centrifugal Pumps and Hydronic Systems

Types of centrifugal pumps In-line pumps

• Low flow, low pressure• Motors are less than one horsepower• Booster or circulator pump

Centrifugal Pumps and Hydronic Systems

Close-coupled pumps• Impeller connected to motor shaft• Used in smaller applications

Flexible-coupled pumps• Impeller shaft and motor are isolated• Reduced vibration/noise transmission

Centrifugal Pumps and Hydronic Systems

Single suction Double suction End suction Casing types

• Horizontal split case• Vertical split case

Centrifugal Pumps and Hydronic Systems

Characteristics of centrifugal pumps Head

• Measure of pump pressure• Expressed in feet or psi

Flow rate (capacity)• Amount of water pumped• Expressed in gallons per

minute (gpm)

Centrifugal Pumps and Hydronic Systems

Horsepower• Brake horsepower• More pumping requires more horsepower• Higher horsepower higher energy usage

Speed• Speed of motor = speed of pump• Expressed in rotations per

minute (rpm)

Centrifugal Pumps and Hydronic Systems

Efficiency• Ratio of output to input• Input is always higher than output

Impeller diameter• Larger impellers require higher horsepower• Larger impellers move more water

Centrifugal Pumps and Hydronic Systems

Performance curves of centrifugal pumps Head-capacity curve

• Plots head against flow rate• Largest head occurs at zero flow• Shutoff head = head at zero flow• Flat or steep curves

Centrifugal Pumps and Hydronic Systems

Horsepower-capacity curve• Plots horsepower against flow rate• Increases in flow rate require more horsepower

Centrifugal Pumps and Hydronic Systems

Efficiency-capacity curve• Efficiency = water efficiency/horsepower• Water efficiency – energy content of water• Efficiency plotted against flow rate• Efficiency = zero at zero flow• Efficiency reaches a peak and then declines

Centrifugal Pumps and Hydronic Systems

Example 12-1 Flow = 90 gpm, head = 25 feet Using performance curves

• Intersection of 25 feet and 90 gpm• Pulley size = 5 ½ inches• Efficiency = 65%• Motor size = 1 horsepower

Centrifugal Pumps and Hydronic Systems

Piping characteristics of hydronic systems Open piping systems Closed piping systems Required flow rate System pressure loss

Centrifugal Pumps and Hydronic Systems

Calculating the flow rate GPM = Q/(500 x ΔT) Friction head losses System characteristic curve Elevation, static head loss

• Only a factor in open systems

Centrifugal Pumps and Hydronic Systems

Operating characteristics of hydronic systems Flow rate and pressure

• Pump head-capacity curve• System characteristic curve

Output determined by head System operating point

Centrifugal Pumps and Hydronic Systems

Example 12-2 Impeller = 5½ inches Head = 30 feet (from 25 feet) Using performance curves

• New flow rate = 60 gpm• Pump capacity has dropped

Centrifugal Pumps and Hydronic Systems

Controlling hydronic system pressure High pressure

• Valve seating problems• Weakened pipe joints

Low pressure• Instant steam formation• Pump damage

Centrifugal Pumps and Hydronic Systems

Expansion tank Water expands when heated Used to control system pressure

Air venting valves Located at highest system points Open and close automatically Help remove air from the system

Centrifugal Pumps and Hydronic Systems

Pressure bypass Monitors pressure difference

• Outlet pump pressure• Inlet pump pressure

Valve opens when differential is large Allows supply water into the return

Centrifugal Pumps and Hydronic Systems

Pump Basics

Centrifugal Pumps

From the Center of a Circle

RADIAL DIRECTIONTo the Outside of a Circle

A machine for moving fluid by accelerating the fluid RADIALLY outward.

• This machine consists of an IMPELLER rotating within a case (diffuser)

• Liquid directed into the center of the rotating impeller is picked up by the impeller’s vanes and accelerated to a higher velocity by the rotation of the impeller and discharged by centrifugal force into the case (diffuser).

Centrifugal Pumps

Centrifugal Pumps

• A collection chamber in the casing converts much of the Kinetic Energy (energy due to velocity) into Head or Pressure.

Pump Terminology

• Head is a term for expressing feet of water column• Head can also be converted to pressure

"Head"

100 feet

43.3 PSI

Reservoir of Fluid

Pressure Gauge

Conversion Factors Between Head and Pressure

• Head (feet of liquid) =Pressure in PSI x 2.31 / Sp. Gr.• Pressure in PSI = Head (in feet) x Sp. Gr. / 2.31• PSI is Pounds per Square Inch• Sp. Gr. is Specific Gravity which for water is equal to 1

– For a fluid more dense than water, Sp. Gr. is greater than 1– For a fluid less dense than water, Sp. Gr. is less than 1

Head

• Head and pressure are interchangeable terms provided that they are expressed in their correct units.

• The conversion of all pressure terms into units of equivalent head simplifies most pump calculations.

Diameter of the Impeller

Thickness of the impeller

Centrifugal Impellers

• Thicker the Impeller- More Water• Larger the DIAMETER - More Pressure• Increase the Speed - More Water and

Pressure

Impeller Vanes

“Eye of the Impeller”Water Entrance

Two Impellers in Series

Direction of Flow

• Twice the pressure• Same amount of water

Multiple Impellers in Series

• Placing impellers in series increases the amount of head produced

• The head produced = # of impellers x head of one impeller

Direction of Flow Direction of Flow

Energy Loss in Valves Function of valve type and valve position The complex flow path through valves can

result in high head loss (of course, one of the purposes of a valve is to create head loss when it is not fully open)

Ev are the loss in terms of velocity heads