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2 PUMP / SYSTEM CURVE RELATIONSHIP

# 2 PUMP / SYSTEM CURVE RELATIONSHIP

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CUT-OFF FOR SELECTED
IMPELLER DIAMETER
IMPELLER SIZE
MOTOR POWER (STANDARD SIZES)

MOTOR WILL OVERLOAD IF OPERATING POINT SHIFTS TO THE

FOR DESIGN CONDITIONS AND IMPELLER
DIAMETER

DESIGN

DESIGN
OPERATING
POINT

MOTOR B
MOTOR A
DESIGN FLOW
FLOW

EFFICIENCY CURVES - SELECTION AT OR TO LEFT OF MAXIMUM
MAINTAINS HIGH EFFICIENCY IF ACTUAL OPERATING POINT
OCCURS TO RIGHT OF DESIGN OPERATING POINT

FIGURE 8-6 TYPICAL DESIGN PUMP SELECTION
POINT (FROM ABBREVIATED CURVE)
The same procedure carried out for a 116 gpm (7.0 L/s)
flow rate would result in a 10.1 (3 m) pressure drop.
These points may be plotted on a foot head versus gpm
chart as shown in Figure 8−7. Connection of these three
points, along with other condition combinations, de−
scribes a system curve. The system curve is a statement
of the change in pipe friction drop with water flow
change for a fixed piping circuit. This is a most impor−
tant working tool for pump application.

Equation 8-2
H2
+
H1

ǒ Ǔ
Q2
Q1

2

Where:
H = HeadĊft wg (m wg)

Q = Fluid flowĊgpm (L/s or m3/s)

50
(15)
40
(12)
30
(9)
20
(6)
10
(3)
0

The operation of the pump in Figure 8−7 installed in the
piping circuit described by the system curve must be
at the intersection of the pump curve with the system
curve because of the First Law of Thermodynamics.

PUMP CURVE

POINT OF OPERATION

8.2.3

SYSTEM CURVE
50
(3)

100
(6)

150
(9)

200
(12)

250
(15)

CAPACITY - US GALLONS PER MINUTE
(LITERS PER SECOND)

FIGURE 8-7 SYSTEM CURVE PLOTTED
ON PUMP CURVE
8.8

Open System Curve

In plotting the system curve for an open system, the
statics of the system must be analyzed in addition to
the friction loss. The different static conditions are il−
lustrated in Figure 8−8.
A typical cooling tower application is illustrated in
Figure 8−9. In this system, the pump is drawing water
from the tower sump and discharging it through the
condenser to the tower nozzles, at a 10 foot (3 m) high−
er elevation than the sump level.

HVAC SYSTEMS Testing, Adjusting & Balancing • Third Edition

TOTAL
STATIC

TOTAL
STATIC
STATIC
SUCTION

STATIC
STATIC
DISCHARGE DISCHARGE

TOTAL
STATIC

STATIC
SUCTION

STATIC
DISCHARGE

STATIC
SUCTION
LIFT

LESS THAN

STATIC SUCTION LIFT
PLUS
STATIC DISCHARGE

GREATER THAN
STATIC DISCHARGE

FIGURE 8-8 TYPICAL OPEN SYSTEMS

This system curve cannot be applied directly to the
pump curve and the intersection taken as the accurate
pumping point for the open system. A false evaluation
using this criteria, but without evaluating the static
height of the tower, is shown in Figure 8−10.
The illustration is false because the pump must also
provide the necessary energy to raise water from the
tower sump to the spray nozzles. In this case, the pump
NOZZLES

10 FT(3 m) TOTAL
STATIC

COOLING
TOWER
SUMP

must raise each pound of water 10 feet (3 m) in height,
or it must provide 10 feet (3 m) of energy head due to
the static difference in height between the water levels.
The static difference of 10 feet (3 m) must be added to
the piping pressure drop to provide total required head
for each of the gpm points previously noted. The re−
vised fluid flow versus total required head is shown in
Table 8−3.

60
(18)

Total friction loss (suction and discharge piping, con−
denser, nozzles, etc.) is 30 foot (9 m) at a design flow
rate of 200 gpm (12 L/s), the change in piping pressure
drop for a change in water flow rates is determined and
plotted to develop a system curve.

PUMP No. 1
PERFORMANCE
CURVE

50
(15)
40
(12)

FALSE OPERATING
POINT

30
(9)
20
(6)

SYSTEM
CURVE

10
(3)

STATIC
DISCHARGE
STATIC
SUCTION

0

75
(4.5)

150
(9.0)

225
(13.5)

300
(18.0)

375
(22.5)

CAPACITY - US GALLONS PER MINUTE
(LITERS PER SECOND)
CONDENSER

FIGURE 8-9 TYPICAL COOLING
TOWER APPLICATION

FIGURE 8-10 SYSTEM CURVE FOR
OPEN CIRCUIT FALSE OPERATING
POINT

HVAC SYSTEMS Testing, Adjusting & Balancing • Third Edition

8.9

PUMP NO.2
PERFORMANCE
CURVE

POINT 4

POINT 2

50
(15)

POINT 1

TRUE OPERATING
POINT PUMP NO.2

40
(12)

DESIGN SYSTEM

PUMP NO.1
PERFORMANCE
CURVE

30
(9)

SYSTEM
CURVE

20
(6)

TRUE OPERATING POINT
PUMP NO.1
10
(3)
10’(3m)
0
75
(4.5)

150
(9.0)

225
(13.5)

300
(18.0)

375
(22.5)

CAPACITY - US GALLONS PER MINUTE
(LITERS PER SECOND)

60
(18)

OVERPRESSURE WITH
CONSTANT SPEED PUMP
POINT 3

ACTUAL SYSTEM

POINT 5

FIGURE 8-11 SYSTEM CURVE FOR
OPEN CIRCUIT TRUE OPERATING
POINT

50% DESIGN
100% DESIGN
FLOW

FLOW

SYSTEM FLOW

FIGURE 8-12 PUMP OPERATING
POINTS

The correct procedure for plotting a system curve for
the circuit shown in Figure 8−9 is illustrated in Figure
8−11.
8.2.4

Pump Operating Points

In Figure 8−12, if the system is of the free flowing type
without control valves, with an actual system head
curve as shown, the pump will operate at Point 2, not
Point 1; the pump will produce a higher flow rate than
design flow rate. If the system is of the controlled flow
type with two way valves on all heating or cooling
coils, at design flow, the pump will operate at Point 1
and will create an over−pressure on the coils and con−
trol valves equal to the head difference between Points
1 and 3. If the system flow is reduced to 50 percent of
design on such a system, the over pressure will in−
crease to the amount between Points 4 and 5. Pump op−
eration will be as follows.

sure, consumed by the control valves.
Recognizing that over−pressure can occur in con−
trolled flow systems where coils are equipped with two
way control valves, the selection of pumps for these
systems must include methods of limiting over−pres−
sure to an economic minimum. These methods in−
clude:
a.

multiple pumps operating in parallel

b.

multiple pumps operating in series

NON-CONTROLLED FLOW

c.

multi−speed pumps

On a system without control valves, the pump will al−
ways operate at the point of intersection of the pump

d.

variable speed pumps

8.2.4.1

8.2.4.2

CONTROLLED FLOW

On controlled flow systems, the pump will follow its
head capacity curve, the difference between pump

8.10

The actual method used on a specific hydronic system
depends on the economics of that system. The effects
of these methods on over−pressuring a particular sys−
tem can be determined by developing the system head
curve and plotting pump head capacity curves on the
same graph with the system head curve.

HVAC SYSTEMS Testing, Adjusting & Balancing • Third Edition

0
(0)
0 (0)

116
(7.0)
10 (3)

Design
163
185
200
(9.8)
(11.1)
(12.0)
20 (6)
25 (7.5)
30 (9)

10 (3)

10 (3)

10 (3)

10 (3)

10 (3)

10 (3)

10 (3)

10 (3)

20 (6)

30 (9)

35 (10.5)

40 (12)

45 (13.5)

50 (15)

GPM (L/s)
Piping System

215
(12.9)
35 (10.5)

230
(13.8)
40 (12)

Table 8-3 Flow vs Total Head (Cooling Tower Application)
8.2.5

Multiple Pumps

ONE-PUMP
OPERATION

SYSTEM DESIGN

Multiple pumps in parallel is the most common meth−
od of eliminating over−pressure. Figure 8−13A de−
scribes two pumps piped in parallel, while Figure 8−14
ity curves for single pump and two pump operation. It
is obvious from Figure 8−14 that one pump at 50 per−
cent system flow will reduce the over−pressure caused
by two−pump operation or one pump designed to han−
dle maximum design flow and head.

Figure 8−13B illustrates two pumps piped in series
with bypasses for single pump operation. Figure 8−15
indicates the use of series pumping on a hydronic sys−
tem with a system head curve consisting of a large
amount of system friction. For such systems, series
pumping can greatly reduce the overpressure on a con−
trolled flow system. Series pumping should not be
used on hydronic systems with flat system head curves
similar to the one shown in Figure 8−14.

For such a system, one pump operation with series
connection would result in the pump running at shutoff
head and producing no flow in the system.

CHECK
VALVE

CHECK VALVE

BYPASSES FOR
SINGLE PUMP OPERATION

A. PARALLEL PUMPING

B. SERIES PUMPING

FIGURE 8-13 MULTIPLE PUMPS

TWO-PUMP
OPERATION

TWO PUMPS
ONE PUMP
MAXIMUM POINTS
OF OPERATION

CURVE

INDEPENDENT

50% DESIGN
FLOW

100% DESIGN
FLOW

SYSTEM FLOW

FIGURE 8-14 PUMP AND SYSTEM
CURVES FOR PARALLEL PUMPING

8.3

PUMP INSTALLATION CRITERIA

8.3.1

Pressure Gage Location

To eliminate the effect of pipe friction, fittings, valves,
and other obstructions, the most desirable gage loca−
tion for accuracy would be at the pump flanges. How−
ever, this is not usually practical. Gages should be lo−
cated as close to the flanges as possible as shown in
Figure 8−16.
To eliminate an elevation static head correction, the
gages on suction and discharges should be at the same
height with respect to the pump centerline. If this pre−
caution is not taken, the difference in gage elevation,
even though usually of small numerical value, must be
accounted for in the gage differential.

HVAC SYSTEMS Testing, Adjusting & Balancing • Third Edition

8.11

versed or if one or both gages were located below the
horizontal pipe or pump centerline.
Gate
Valve

Unacceptable
Locations

Gate
Valve
Check
Valve

Strainer

Pump

Preferable
Location on
Each Side

Acceptable
Location
Each Side

FIGURE 8-16 GAGE LOCATION

In Figure 8−17 there is a physical difference in height
of 2 feet (0.6 m). If the gage pressure, when converted,
measured 50 feet (15 m) on the discharge and 30 feet
(9 m) on the suction, subtraction alone would indicate
a differential of 20 feet (6 m). However, with respect
to the discharge gage which is two feet (0.6 m) lower
in the piping, the suction gage reads two feet (0.6 m)
of head too little, and at the same elevation as the dis−
charge gage would read 32 feet (9.6 m). The correct
differential is then 50 − 32 = 18 feet (15 − 9.6 = 5.4 m).
A similar analysis would apply if the positions were re−

TWO-PUMP
OPERATION

Fluid Viscosity

It should be noted that as long as the head − fluid flow
curve is based on feet (meters) of head, no correction
need be made for temperature or density since feet
(meters) of head and gallons per minute (liters per sec−
ond) account for these factors. However, density does
increase the pump power requirements. The pump wa−
ter power curves are developed at near maximum den−
sity at approximately 85_F (29_C). Since density de−
creases as temperature rises, pump water power will
decrease, but the change usually is ignored. Viscosity
can change the pump impeller head capacity curve
provided the change in viscosity is greater than the
change of water viscosity between 40_F and 400_F
(4_C and 204_C). The effect on the curve is illustrated
in Figure 8−18.

2 ft (0.6m)

Pump
TWO PUMPS

100% DESIGN

8.3.2

is Not Pump Differential

ONE-PUMP

FIGURE 8-17 RELATIVE GAGE
ELEVATIONS

MAXIMUM POINTS

ONE-PUMP
OPERATION

OF OPERATION

8.3.3

Installation Criteria

50% DESIGN

Some of the important points for the TAB technician
to consider in installing a pump are:

INDEPENDENT

100% DESIGN
FLOW

a.

suction piping should be air tight and free of
air traps

b.

piping should provide a smooth flow into the
suction without unnecessary elbows

c.

suction pipe should be one or two sizes larger
than pump inlet (eccentric reducer or reduc−
ing elbow to connect inlet to piping)

d.

reduce or eliminate restrictions at pump suc−
tion

SYSTEM FLOW

FIGURE 8-15 PUMP AND SYSTEM
CURVES FOR SERIES PUMPING

8.12

HVAC SYSTEMS Testing, Adjusting & Balancing • Third Edition

e.

piping supported independently of pump cas−
ing

f.

use of vertical silent check valve in pump dis−
charge in multi−pump installations

g.

manual air vent in pump casing and piping

h.

pressure gages on suction and discharge at
same elevation

i.

when vibration isolation is used, isolate pip−
ing and pump as a system (preferable), or pro−
vide pump isolators and piping flexible con−
nectors

j.

recheck pump alignment after installation
even if guaranteed by manufacturer

k.

lubricate prior to start up

l.

check rotation, but do not run mechanical
seals dry

Water Impeller Curve

Increased Viscosity
Curve for Same
Impeller

Flow

Boiler heating surface is the area of fluid backed sur−
face exposed to the products of combustion, or the fire
side surface. Various codes and standards define al−
lowable heat transfer rates in terms of heating surface.
Boiler design provides for connections to a piping sys−
tem which delivers heated fluid to the place of use and
returns the cooled fluid to the boiler.
8.4.2

Heat exchangers or converters are used as heat sources
for many hot water heating systems. Heat exchangers
may be of three general types: a) steam−to−water, b)
water−to−water, or c) water−to−steam (generators).
Steam−to−water heat exchangers usually take the form
of shell and tube units. Steam is admitted to the shell,
and water is heated as it circulates through the tubes.
Steam−to−water converters are useful where an addi−
tion is to be made to an existing steam system and
where hot water heating is desired. They are also wide−
ly used in areas where district steam is available and
individual buildings are to be heated with a hot water
system. High rise buildings can be zoned vertically by
using steam distribution and installing converters at
various levels to serve several floors, thus limiting
maximum operating pressures in the zone.
Water−to−water heat exchangers (generally shell and
tube units) are used in high temperature water (HTW)
systems to produce lower temperature water for cer−
tain zones or in process water or domestic water ser−
vices.
Water−to−steam heat exchangers generally consist of
U−tube bundle installed in a tank or pressure vessel to
provide space for the release of steam. They are used
in HTW systems to provide process steam where re−
quired.
8.4.3

FIGURE 8-18 EFFECT OF VISCOSITY

Heat Exchangers

Water Chillers

8.4

HYDRONIC HEATING AND
COOLING SOURCES

The source of cooling in a chilled water or a dual tem−
perature system is a water chiller. There are three gen−
eral types of water chillers: (1) reciprocating, (2) cen−
trifugal, and (3) absorption. For further information,
see the 2000 ASHRAE Systems and Equipment Hand−
book.

8.4.1

Boilers

8.4.4

A boiler is a cast iron or steel pressure vessel heat ex−
changer, designed with and for fuel burning devices
and other equipment to burn fossil fuels (or use electric
current) and transfer the released heat to water (in wa−
ter boilers) or to water and steam (in steam boilers).

Heat Pumps

A heat pump may serve as a source for both hot water
and chilled water in a dual temperature system. Heat
pumps are described in the 2000 ASHRAE Systems
and Equipment Handbook. Water temperatures avail−
able are generally low in winter (about 90_F to 130_F

HVAC SYSTEMS Testing, Adjusting & Balancing • Third Edition

8.13

or 32_C to 54_C) and terminal heat transfer must be
designed for operation under these conditions. In some
cases, a supplementary heat source is used to raise
temperature levels.
8.5

TERMINAL HEATING AND
COOLING UNITS

8.5.1

General

Many types of terminal units are available for central
water systems. Some are suited to only one type of sys−
tem and others may be used in all types of systems.
Terminal units may be classified in several ways.
8.5.1.1

Natural Convection

Natural convection units include cast iron radiators,
cabinet convectors, baseboard and finned tube radi−
ation are used in heating systems.
8.5.1.2

Forced Convection

Forced convection units include unit heaters, unit ven−
tilators, fan coil units, induction units, air handling
units, heating and cooling coils in central station units,
and most process heat exchangers. Fan coil units, unit
ventilators, and central station units can be used for
heating, ventilating and cooling.
8.5.1.3

panels, and certain special types of cast iron radiation.
All transfer some heat by convection. Such units are
generally used for heating in low temperature water
(LTW) systems. However, special designs of overhead
radiant surfaces, both tubular and panel, are being used
in medium and high temperature water systems to take
advantage of the lowered surface requirements
achieved through the use of high surface temperatures.
Panel cooling is applied in conjunction with control of
space humidity to maintain the space dew point below
the panel surface temperature.
8.5.1.4

Mixing Different Types Of Units

In any single circuit having similar loads and a single
control point, the terminal units should be of similar
response types. Cast iron radiation should not be
installed in the same controlled circuit as baseboard or
fin tube type units. Caution should be exercised when
including fan operated units with natural convection
units on the same pumping circuit.
8.14

8.5.2

Cast iron radiation and cabinet convectors have been
widely used in LTW systems. Ceiling hung radiators
frequently were used where floor space was not avail−
able for other types of units. Convectors are used ex−
tensively in areas where high output is needed and lim−
ited space is available, and where linear heat
distribution is not desired. Typical areas heated with
radiators or convectors include corridors, entries, toi−
let rooms, storage areas, work rooms, and kitchens.
8.5.3

Baseboard and fin tube radiation permits the blanket−
ing of exposed surfaces for maximum comfort. Base−
board and fin tube elements are generally rated at vari−
ous average water temperatures and at one or more
water velocities. Velocity corrections may be applied.
Many designers feel that these units are thus limited to
systems designed to a 20_F (11_C) temperature drop.
However, careful selection can result in successful ap−
plication with temperature drops much higher than
20_F (11_C).
8.5.4

Unit Ventilators

Unit ventilators, originally developed for specific ap−
plication in school classrooms, are being used today in
a much wider range of applications. Unit ventilators
consist of a forced convection heating or cooling unit
with dampers permitting introduction of controlled
amounts of outdoor air to provide a complete cycle of
heating, ventilating, ventilation cooling, or mechani−
cal cooling as required. Condensation may be a prob−
lem during summer operation unless chilled water
flow is stopped when fans are not operating. Conden−
sate drains are necessary. Comparatively low supply
temperature and rise may be required.
8.5.5

Fan Coil And Induction Units

Fan coil units are generally used, with or without out−
door air, in dual temperature water systems. The same
coil is often used for both heating and cooling. Individ−
ual control is usually achieved by the use of valves, or
by using intermittent or multi−speed fan operation. Hot
water ratings are usually based on flow rates or tem−
perature drops at various entering water and air tem−
peratures. Temperature drops of 40_F to 60_F (22_C
to 33_C) frequently are used. Induction units are simi−
lar to fan coil units except that air circulation is pro−
vided by a central air system which handles part of the
8.5.6

Unit Heaters

Unit heaters are available in several types: a) horizon−
tal propeller fan, b) downblow or c) cabinet. They are

HVAC SYSTEMS Testing, Adjusting & Balancing • Third Edition

used where high output in a small space is required,
and where no cooling is to be added. Cabinet units are
frequently applied in corridors and at entrances to

blanket doors which are frequently opened. Normally,
unit heaters do not provide ventilation air.

HVAC SYSTEMS Testing, Adjusting & Balancing • Third Edition

8.15