Stephen L. Herman, Bennie Sparkman. Electricity and Controls for HVAC-R (6th edition)

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300 SECTION 5 Control Components

CONSTRUCTION

Most of the pressure switches used for com-

mercial or industrial systems are adjustable,

Figure 30–5. This feature allows the service tech-

nician to use the switch on different systems. For

example, the pressure settings are different for dif-

ferent types of refrigerant. Figure 30–6 shows a

table of common pressure settings for high- and

low-pressure switches used with different types of

refrigerant.

A dual-pressure switch is shown in

Figure 30–7. This switch incorporates both high-

and

low-pressure switches in the same housing.

Some manufacturers use a nonadjustable type

of pressure switch. This switch is commonly found

on central units designed for residential use. This

Figure 30–4

Low-pressure switch. (Source: Delmar/Cengage Learning)

Figure 30–5

Adjustable pressure switch. (Courtesy of Johnson Controls Inc.)

PRESSURE SWITCH SETTINGS

Type of

Refrigerant

High Pressure

Cut out Cut in

Low Pressure

Cut out Cut in

12 275 145 15 35

22 380 300 38 68

500 280 200 22 46

Figure 30–6

Pressure switch settings. (Source: Delmar/Cengage Learning)

type of switch is shown in Figure 30–8. The advan-

tage of this type of switch is that it is inexpensive.

When replacing this type of pressure switch, how-

ever, it must be matched to the refrigerant system.

High- and low-pressure switches are shown in the

schematic in Figure 30–9.

UNIT 30 Pressure Switches 301

Figure 30–7

Dual pressure switch. (Courtesy of Johnson Controls Inc.)

Figure 30–8

Nonadjustable pressure switch. (Source: Delmar/Cengage Learning)

LPC

56 55

75 53

COIL

HPC

52 50

68 BRN BRN

RED

BLU (208V TAP) SEE NOTE 7

30, 90

TRANSFORMER 208-230-24V

24V

LINE VOLTAGE

BLK

EFC

30A

92930

CCH

WHITE

RED DOT

OR ARROW

BRN

TOP

BLK 26

CFM-1

CAP-1

WHITE

BRN

TOP

BLK 36

CFM-2

354241

111,9,4

CMT

CAP-2

13 3,10,6

L3OL

COMP

12 2.5

16 6,RED

EFC

4,BLK 14

TOP

88 86

EFC

LEGEND

CMT CONDENSER MOTOR THERMOSTAT

CAPACITOR

COMPRESSOR CONTACTOR

CRANKCASE HEATER

CONDENSER FAN MOTOR

COMPRESSOR

EVAPORATOR FAN CONTACTOR

FUSE

RESET RELAY

TRANSFORMER

TERMINAL BLOCK

THERMAL OVERLOAD PROTECTOR

TERMINAL STRIP

EVAPORATOR FAN MOTOR

LOW-PRESSURE CONTROL

HIGH-PRESSURE CONTROL

OVERLOAD PROTECTOR

CAP

CCH

CFM

COMP

EFC

TOP

EFM

LPC

HPC

WARNING

DISCONNECT ELECTRICAL POWER

SOURCE TO PREVENT INJURY OR

DEATH FROM ELECTRICAL SHOCK

CAUTION

USE COPPER CONDUCTORS ONLY

TO PREVENT EQUIPMENT DAMAGE

Figure 30–9

High- and low-pressure switches are used to disconnect the compressor contactor. (Courtesy of Trane Corp.)

302 SECTION 5 Control Components

SUMMARY

High- and low-pressure switches are used to sense the pressure in an air conditioning and

refrigeration system.

High- and low-pressure switches generally use a bellows, which expands and contracts

with a change of pressure. The bellows operates a set of contacts.

High-pressure switches are normally closed switches that open when the pressure

becomes too high. The switch is connected in series with the coil of the compressor relay.

Low-pressure switches are normally open held-closed switches that open when the pres-

sure drops below a certain level. Low-pressure switches are connected in series with the

coil of the compressor relay also.

Some pressure switches can be adjusted and some cannot.

KEY TERMS

adjustable

bellows

dual-pressure switch

high-pressure switch

low-pressure switch

nonadjustable

REVIEW QUESTIONS

1. What device is used to construct most of the pressure switches used in the air

conditioning  eld?

2. What type of contact is used with a high-pressure switch?

3. What type of contact is used with a low-pressure switch?

4. Where in the refrigerant system is the high-pressure switch connected?

5. Where in the refrigerant system is the low-pressure switch connected?

UNIT 30 Pressure Switches 303

TROUBLESHOOTING QUESTIONS

Refer to the schematic shown in Figure 30–9 to answer the following questions.

1. Is the compressor in this unit operated by a single-phase or a three-phase motor?

A. Single-phase

B. Three-phase

2. In order for the compressor contactor to energize, the thermostat (not shown) must make

contact between which terminals?

A. R and G

B. R and T

C. R and E1

D. R and Y2

3. Are the condenser fan motors single-phase or three-phase?

A. Single-phase

B. Three-phase

4. In order to energize the EFC contactor coil, connection should be made between which

terminals?

A. E1 and G

B. T and G

C. Y1 and G

D. R and G

5. Is the evaporator fan motor single-phase or three-phase?

A. Single-phase

B. Three-phase

304

The type of  ow switch used in air conditioning

systems senses the  ow of air instead of the  ow of

liquid. The  ow switch is often referred to as a

sail

switch because it operates on the principle of a sail.

The 

ow switch is constructed from a snap-action

microswitch. A metal arm is attached to the micro-

switch. A piece of thin metal or plastic is connected

to the metal arm. The thin piece of metal or plastic

has a large surface area and offers resistance to the

 ow of air. When a large amount of air ow passes

across the sail, enough force is produced to cause

the metal arm to operate the contacts of the switch.

A  ow switch is shown in Figure 31–1.

The  ow switch is used to give a positive indi-

cation that the evaporator or condenser fan is

operating before the compressor is permitted to

operate. The air ow switch is the only positive

method of indicating that the fan is actually in

OBJECTIVES

After studying this unit the student should

be able to:

Describe the operation of a ﬂ ow switch

Draw the standard NEMA symbol for

a ﬂ ow switch

Connect a ﬂ ow switch in a circuit

The Flow

Switch

UNIT 31

UNIT 31 The Flow Switch 305

THERMOSTAT

CONTROL RELAY

CFM

CONDENSER FAN MOTOR RELAY

EFM

EVAPORATOR FAN MOTOR RELAY

COMP

COMPRESSOR RELAY

CFM EFM

operation. For example, in the circuit shown in

Figure 31–2, the thermostat controls the operation

of a control relay (CR). When the thermostat

closes its contacts, CR coil energizes. This causes

all CR contacts to close. When the  rst CR contact

closes, CFM (condenser fan motor) relay

energizes and starts the condenser fan motor. When

the second CR contact closes, EFM (evaporator

fan motor) relay coil energizes and starts the

evaporator fan motor. The third CR contact cannot

energize the compressor relay coil, however,

because it is interlocked with CFM and EFM relays.

The compressor relay coil can be energized only

after the condenser fan and evaporator fan relay

coils have energized.

The idea behind this type of control is to ensure

that the compressor cannot be started until both

the condenser and evaporator fans are operating.

This control circuit, however, does not ful ll that

requirement. This circuit does not sense if the fans

are actually operating. It does sense if the relay coils,

which control those fan motors, are energized. This

circuit cannot detect if a fan motor is not operating,

or if a belt is broken between the motor and the fan.

The circuit shown in Figure 31–3 has been

modi ed from the circuit in Figure 31–2. Notice in

this circuit that the normally open CFM and EFM

contacts connected in series with the compressor

relay have been replaced with air ow switches CFS

(condenser  ow

switch) and EFS (evapora-

tor  ow switch). These switches are operated by

the force of air created by the condenser fan or the

evaporator fan. In this circuit, the compressor can

be started only after the condenser and evaporator

fans are actually operating. If the circuit is in opera-

tion and one of the fans should stop, the compressor

Figure 31–1

Airﬂ ow switch. (Courtesy of Honeywell Inc.).

Figure 31–2

Compressor is interlocked with

condenser and evaporator

fan relays.

(Source: Delmar/Cengage Learning)

306 SECTION 5 Control Components

THERMOSTAT

CONTROL RELAY

CFM

CONDENSER FAN MOTOR RELAY

EFM

EVAPORATOR FAN MOTOR RELAY

COMP

COMPRESSOR RELAY

CFS EFS

relay will be disconnected from the circuit. This will

disconnect the compressor motor from the circuit.

Another type of  ow switch is shown in

Figure 31–4. This switch is actually a pressure switch

that detects the increase in air pressure inside the

duct when the blower is in operation. The switch

can be adjusted for more or less sensitivity. The

switch contacts connect in the same manner as

shown in Figure 31–3.

Figure 31–3

Compressor is interlocked with

airﬂ ow switches. (Source: Delmar/

Cengage Learning)

Figure 31–4

Flow switch senses the increase of air pressure inside

the duct when the blower is in operation. (Source: Delmar/

Cengage Learning)

UNIT 31 The Flow Switch 307

SUMMARY

Some  ow switches sense the  ow of liquid and some are made to sense the  ow of air.

Air ow switches are sometimes referred to as sail switches.

Air ow switches are generally used to ensure that air is moving across the evaporator coil

before the compressor is permitted to start.

Flow switch contacts are generally normally open and are connected in series with the coil

of the compressor relay.

KEY TERMS

CFM (condenser fan

motor) relay

CFS (condenser  ow

switch)

compressor relay coil

CR (control relay)

EFM (evaporator fan

motor) relay

EFS (evaporator  ow

switch)

 ow switch

sail switch

REVIEW QUESTIONS

1. What is a common name for the air ow switch?

2. What function does the air ow switch perform in a circuit?

3. What is interlocking in a control circuit?

308

The control of humidity can be very important in

some heating and air conditioning systems. Some

industries, such as mills that knit polyester and

nylon  bers, must maintain a constant humidity

because these materials contract and expand with

a change of humidity. The control of humidity is

also important in heating systems. The amount of

humidity in the air has a great effect on the comfort

of the living area. If the humidity is to be maintained

at a constant level, some device must be used to

detect the amount of humidity and then operate

some type of control.

The

humidistat is a device that can sense the

amount of humidity in the air and activate a set of

contacts if the humidity should become too high or

too low. The two most common materials used to

sense humidity are

hair and nylon. The materials

contract and expand with a change in the amount

OBJECTIVES

After studying this unit the student should

be able to:

Discuss the operation of a humidistat

Discuss how the humidistat is

connected into the control circuit

Connect a humidistat in a circuit

The Humidistat

UNIT 32

UNIT 32 The Humidistat 309

of humidity in the air. A humidistat using hair as

the sense element is shown in Figure 32–1.

If a humidi er is used in a central-heating or

air conditioning system, it is generally operated only

when the blower is in operation. For this reason,

some means is used to interlock the humidi er with

the blower. The circuit shown in Figure 32–2 uses

a humidistat to control the operation of a solenoid

coil. The solenoid coil operates a valve that supplies

water to the humidi er. Notice that the solenoid coil

is interlocked with an air ow switch. The coil can be

energized only when the sail switch indicates there

is air ow in the system. Some controls use a com-

bination of a humidistat and a sail switch. Nylon

strips are used to sense the amount of humidity in

the air and a plastic sail is used to sense the  ow of

air in the duct.

HUMIDISTAT

24 VOLTS

SOL.

SAIL SWITCH

Figure 32–1

Humidistat. (Courtesy of Honeywell Inc.).

Figure 32–2

Humidiﬁ er is interlocked with

an airﬂ ow switch.

(Source: Delmar/Cengage Learning)

SUMMARY

A humidistat is used to control the amount of humidity in the air.

The amount of humidity in the air greatly affects the comfort of the living area.

The humidistat is a device that senses the amount of humidity in the surrounding air and

controls a set of contacts.

The two most common materials used to sense humidity are hair and nylon.

A humidi er used in a central air conditioning system is generally permitted to operate

only when the blower is in operation.

KEY TERMS

hair

humidistat

nylon