1

HVACR317 – Core for Refrigeration

Commercial Defrost Sequence

TIMER MOTOR

2

NO

HGSNC

EFM

TS

LPS

HPS

LAC CFM

R C

S

1 2 5

SC

DIAGRAM “A”

1

COMPRESSOR

• THE FIRST DIAGRAM REPRESENTS A TYPICAL SELF CONTAINED REACH-IN BOX

• IT’S A LOW TEMP SYSTEM USING HOT GAS DEFROST

• COMPRESSOR IS CSIR• THE LAC (LOW AMBIENT CONTROL) WOULD

BE USED IF THE CONDENSING UNIT IS INSTALLED OUTDOORS

• THE RUN CAP IS OPTIONAL

H.O. SLIDE 1

TIMER MOTOR

2

NO

HGSNC

EFM

TS

LPS

HPS

LAC CFM

R C

S

1 2 5

SC

POWER SWITCH CLOSED

2

- LIKE ANY SYSTEM…FAN, FAN, AND COMPRESSOR WILL SEQUENCE IN PARALLEL

- THE START CAP WILL PROVIDE THE REQUIRED TORQUE TO THE START WINDING

H.O. SLIDE 2

TIMER MOTOR

2

NO

HGSNC

EFM

TS

LPS

HPS

LAC CFM

R C

S

1 2 5

SC

NORMAL RUNNING CYCLE

3

- THE BEMF ENERGIZES THE RELAY AND DROPS OUT THE START CAP

- THE SYSTEM WILL CYCLE NORMALLY ON THE (HONEYWELL / WHITE ROGERS / JOHNSON CONTROLS) REMOTE BULB THERMOSTAT

H.O. SLIDE 3

TIMER MOTOR

2

NO

HGSNC

EFM

TS

LPS

HPS

LAC CFM

R C

S

1 2 5

SC

BOX REACHES 0 – 10 º F

4

- WHEN THE BOX TEMP REACHES 0 -10 DEGREES (TYPICAL SETTING) THE T-STAT OPENS….EFM AND TM CONTINUE TO RUN

H.O. SLIDE 4

TIMER MOTOR

2

NO

HGSNC

EFM

TS

LPS

HPS

LAC CFM

R C

S

1 2 5

SC

BOX TEMP GOES UP…TSTAT CLOSES

TIMER MOTOR

2

NO

HGSNC

EFM

TS

LPS

HPS

LAC CFM

R C

S

1 2 5

SC

DEFROST CYCLE

5

- WHEN THE TIMER SWITCHES TO DEFROST, COMPRESSOR STAYS ON, AND HGS ENERGIZES

- THE EFM IS DE-ENERGIZED IN DEFROST

H.O. SLIDE 5

HOT GAS DEFROST (PIPING) REMINDER….NORMAL OPERATION…..

C

MD

CONDENSER

EVAPORATOR

OFF

HOT GAS DEFROST (PIPING) REMINDER….DEFROST CYCLE…..

C

MD

CONDENSER

EVAPORATOR

EFM SHUTS OFF

ON

TIMER MOTOR

2

NO

DH

NC

EFM

TS

LPS

HPS

LAC CFM

R C

S

1 2 5

SC

DIAGRAM “B”

6

COMPRESSOR

N

3

4

X DTT

LIMIT

• THIS SYSTEM IS ALSO LOW TEMP, BUT USING ELECTRIC DEFROST

• THIS TYPE SYSTEM HAS AN EVAP FAN “DELAY” SEQUENCE

• IT USES 2 “DTT” TYPE CONTROLS• IT ALSO HAS THE ABILITY TO DISENGAGE THE

TIMER OUT OF DEFROST• STEP BY STEP SEQUENCE IS COMING UP

H.O. SLIDE 6

TIMER MOTOR

2

NO

DH

NC

EFM

TS

LPS

HPS

LAC CFM

R C

S

1 2 5

SC

POWER ON

7

COMPRESSOR

N

3

4

X DTT

LIMIT

• JUST LIKE THE LAST SYSTEM…FAN, FAN, COMPRESSOR, ARE RUNNING…

• THE DTC TIMER IS LINE TO LINE (LIKE THE LAST SYSTEM)

• A DIFFERENT TYPE (OR MODEL) DTC IS USED … NOTICE THE EXTRA TERMINALS ON THE TIMER

• THE DEVICE BETWEEN “3” AND “X” IS A SOLENOID (MORE ON THAT LATER)

H.O. SLIDE 7

TIMER MOTOR

2

NO

DH

NC

EFM

TS

LPS

HPS

LAC CFM

R C

S

1 2 5

SC

DEFROST CYCLE

8

COMPRESSOR

N

3

4

X DTT

LIMIT

• THE ONLY PATH IS THROUGH THE HEATER AND “LIMIT”…ALL ELSE IS OFF

• THE LIMIT “ACTS AS” A DTT (LIKE THE DOMESTIC UNITS)…BUT IT HAS A HIGHER TEMP (CUT-OUT) RATING THAN THE DOMESTIC TYPES….IT’S BASICALLY A HIGH TEMP SAFETY…THE “ACTUAL” DTT IS THE SPDT ONE JUST BELOW THE “LIMIT”

H.O. SLIDE 8

TIMER MOTOR

2

NO

DH

NC

EFM

TS

LPS

HPS

LAC CFM

R C

S

1 2 5

SC

9

COMPRESSOR

N

3

4

X DTT

LIMIT

DEFROST TERMINATION

• WHEN THE SPDT DTT SENSES THAT THE EVAP COIL IS WARM ENOUGH … IT WILL SWITCH UP TO THE TOP CIRCUIT COMPLETING A PATH TO THE “CLUTCH SOLENOID” ON THE DTC

• THE SOLENOID WILL SWAP THE CONTACTS BACK INTO REFRIGERATION MODE (NO WAITING)

H.O. SLIDE 9

TIMER MOTOR

2

NO

DH

NC

EFM

TS

LPS

HPS

LAC CFM

R C

S

1 2 5

SC

10

COMPRESSOR

N

3

4

X DTT

LIMIT

DEFROST IS TERMINATED..BUT NO EFM

• THE EVAP FAN HAS NOT COME YET DUE TO THE WARM EVAP COIL….

• WHEN THE DTT COOLS BACK DOWN THE SWITCH WILL DROP BACK IN AND THE EVAP FAN WILL RE-START

• THE PURPOSE OF THIS CONTROL (AND SEQUENCE) IS TO PREVENT A “HOT PULL DOWN” SITUATION AFTER EACH DEFROST CYCLE

H.O. SLIDE 10

HOT PULL DOWN EXAMPLE: PRESENTLY THE SYSTEM IS RUNNING NORMAL (R134a). THE SYSTEM PRESSURES WILL GENERALLY RUN 5 TO 20 PSIG………

C

MD

CONDENSER

EVAPORATOR

5 150

WHEN THE SYSTEM GOES INTO DEFROST THE EVAP COIL HEATS UP AND THE PRESSURES ARE BASICALLY EQUALIZED

OFF

MD

CONDENSER

EVAPORATOR

100100

HEATERS ON

WHEN DEFROST IS TERMINATED, THE EXTREME HEAT LOAD ON THE COIL COULD CAUSE AN OVERLOAD ON THE ELECTRICAL CIRCUIT AND TRIP A BREAKER….

C

MD

CONDENSER

EVAPORATOR

50 170

…DELAYING THE EFM IS ONE WAY TO KEEP THE PRESSURES DOWN AT INITIAL START UP

TOO HIGH

TIMER MOTOR

2

NO

DH

NC

EFM

TS

LPS

HPS

LAC CFM

R C

S

1 2 5

SC

EVAP GETS COLD…EFM GOES ON

11

COMPRESSOR

N

3

4

X DTT

LIMIT

TIMER MOTOR

2

NO

DH

NC

EFM

TS

LPS

HPS

LAC CFM

R C

S

1 2 5

SC

DTT SWITCHED “DOWN”

11

COMPRESSOR

N

3

4

X DTT

LIMIT

• LIKE ALL (DOMESTIC OR COMMERCIAL) AUTOMATIC DEFROST SYSTEMS, THIS DEFROST CIRCUIT CAN BE TERMINATED BY TIME AND / OR TEMPERATURE

• THE MAIN ADVANTAGE (USING THIS TYPE TIMER) IS THE REFRIGERATION MODE CAN BE INSTANTLY BROUGHT BACK ON IF THE DTT DURATION TIME LASTED JUST A FEW MINUTES

H.O. SLIDE 11

TIMER MOTOR

2

NO

DH

NC

EFM

TS

LPS

HPS

LAC CFM

R C

S

1 2 5

SC

12

COMPRESSOR

N

3

4

X DTT

LIMIT

PURPOSE OF THE “LIMIT”

• IF THE SPDT DTT SHOULD FAIL … THE LIMIT WILL PROVIDE A LOCK OUT CONDITION IF AN OVERHEATING SITUATION SHOULD OCCUR

H.O. SLIDE 12

ELECTRIC DEFROST REMINDERS…THE ELEMENT IS EITHER ON OR BELOW THE EVAP COIL…DTT AND LIMIT ARE BOTH LOCATED ON EVAP COIL

C

MD

CONDENSER

EVAPORATOR

LPS

CFM

R C

S

1 2 5

SC

DIAGRAM “C”

13

COMPRESSOR

EFM

TM X

4

N

• THIS SYSTEM IS A MEDIUM TEMP APPLICATION (EFM IS LINE TO LINE)

• THE DEFROST CYCLE SIMPLY SHUTS DOWN THE COMPRESSOR CIRCUIT (SOMETIMES DONE IN THE LATE EVENING HOURS). A SIMPLE TIMER IS USED.

• THE LP SWITCH IS USED AS A TEMPERATURE CONTROL (NOT A GREAT WAY TO GO…BUT IT’S COMMON)

H.O. SLIDE 13

LPS

LAC CFM

R C

S

1 2 5

SC

DIAGRAM “C”

14

COMPRESSOR

EFM

TM X

4

N

USING AN LP SWITCH AS A THERMOSTAT….-- SET THE LP SW. TO ITS RECOMMENDED

SETTINGS (C.I. & C.O.)…ATTACH GAUGES-- PLACE A THERMOMETER IN THE BOX AND

CLOSE THE DOOR. -- WHEN THE BOX REACHES THE DESIRED TEMP

(EX: 35º) SET THE C.O. OF THE LP SW. TO THE LOW SIDE PRESSURE SHOWN ON GAUGE

H.O. SLIDE 14

LPS

CFM

R C

S

1 2 5

SC

DIAGRAM “D”

15

COMPRESSOR

EFM

R134A: 30 # DIFF.

CI:50 psig CO: 20 psig

-- HIGH OR MEDIUM TEMP SYSTEM-- USES OFF CYCLE DEFROST-- “OFF CYCLE” DEFROST CAN BE SET UP BY ADJUSTING THE LP SW. DIFFERENTIAL HIGHER THAN NORMAL-- THE 30 PSIG DIFFERENTIAL ALLOWS SUFFICIENT TIME FOR DEFROST

H.O. SLIDE 15