Instructor CD for
Commercial Refrigeration for A/C Technicians
Chapter 2
Evaporators
Learning Objectives
• Describe the functions of an evaporator
• Explain evaporator temperature and
evaporator temperature difference (TD)
• Describe evaporator types and styles
• Explain latent heat, sensible heat, and
super heat
• Explain the relationship of coil temperature
difference (TD) and humidity
• Describe evaporator defrost methods
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Refrigeration Evaporator
• The function of the refrigeration system:
– Transfer heat form one place to another
• The evaporator’s job:
– Absorb heat from the space
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Forced Air Evaporator
Cool
air
out
Warm air in
Courtesy of
Carrier Corp.
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What is Evaporator Temperature?
•
•
The temperature of the refrigerant inside
the evaporator tubing
How do you measure evaporator
temperature?
1. Determine the suction pressure
2. Refer to the P/T chart for that refrigerant
3. Choose the temperature of the refrigerant at
that suction pressure
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Pressure / Temperature (PT) Charts
• From the system pressure
– You can find its temperature
• If you know the evaporator temperature
– You can find the suction pressure
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Pressure
PSIG
48
49
50
52
54
56
58
60
62
64
66
68
70
Temperature
134a
52
53
54
56
57
58
60
62
63
65
66
68
69
22
24
25
26
28
29
31
32
34
35
37
38
40
41
404A
14
15
16
17
19
20
22
23
25
26
27
29
30
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What is the suction
pressure if
the evaporator
temperature is:
25° for R22 Walk-in?
49 psig
25° for R404 Walk-in?
62 psig
What is the evaporator
temperature of R404,
if the suction pressure
is 50 psig? 16°
7
Using the Low Side Gauge
• Read suction pressure
• Read suction temperature
– Note: The evaporator temperature is the
same as the suction temperature when the
pressure is taken close to the evaporator
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Reading the Low Side Gauge
69 psig
40º (R22)
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Reading the Low Side Gauge
49 psig
25º (R22)
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How Evaporators Absorb Heat
• Refrigerant enters as liquid droplets
• Warm air causes droplets to boil
• Heat is absorbed into the refrigerant
This is called “latent” heat
• Refrigerant becomes “saturated” vapor
• Now it can only absorb “sensible” heat
This is called “superheat”
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Total Superheat
Evaporator section of R22 A/C
Evaporation Starts
Fully Evaporated
(Saturated)
Coil Superheat
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Understanding Evaporator TD
Type
Space
Evaporator Evaporator
Refrigeration Temperature Temperature
TD
Air Conditioning
75°
40°
35°
Reach-in Refrigerator 40°
20°
20°
0°
-20°
20°
Walk-in Refrigerator
35°
25°
10°
Walk-in Freezer
-10°
-20°
10°
Reach-in Freezer
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Slant Coil
Courtesy of
Carrier
UPFLOW OR
DOWNFLOW
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HORIZONTAL
FLOW
14
Reach-in Evaporator
20° TD
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Multiple Evaporators in a Large Walk-in
10° TD
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Meat Cutting Room
8° TD
Evaporators designed for
low air flow and to
maintain high humidity
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Evaporator TD
(temperature difference)
Difference between space temperature and
evaporator temperature
The lower the TD, the higher the humidity
Type System
A/C
Reach-in
Walk-in
Coil TD
35º
20º
10º
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Humidity
50%
65%
85%
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Refrigeration and Humidity
Evaporators Dehumidify
Remove moisture from the refrigerated space
What effects humidity?
Temperature Difference (TD)
(TD = Entering air temperature - coil temperature)
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How TD Affects Humidity
To condense moisture out of the air, the air must
be cooled below its dew point.
20° TD
55º
Water
35° TD
75º
Ambient
40º
Water
No condensation
Condensation means moisture removal
TD versus ∆T
TD (Temperature Difference) =
Air temperature entering the evaporator minus
refrigerant temperature inside the evaporator
∆T (Delta T) =
Air temperature entering the evaporator minus
air temperature leaving the evaporator
Note:
∆T used most in air conditioning
TD used most in commercial refrigeration
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Typical A/C Evaporator
∆T=20o
75º
TD=35o
40º
55º
Humidity=50%
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Commercial Refrigeration Evaporators
Walk-in Refrigerator R22
Walk-in Freezer R404A
Temp
Medium Temp
 30º
35º
-10º
25º
TD=10º
49
psig
Low
H=85%
TD=10º
 -15º
-20º
16
psig
H=N/A
Reach-in Refrigerator R22
Reach-in Freezer R404A
Medium Temp
Low Temp
 30º
40º
TD=20º
20º
43
psig
 -10º
0º
-20º
H=65%
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TD=20º
16
psig H=N/A
23
Heat Exchange Efficiency
• Liquid absorbs more heat than vapor
• Boiling of liquid droplets absorbs even more
– When water boils it absorbs 970 times more
heat than it can absorb in a liquid state at 212°
• Refrigerant in a DX (direct expansion)
evaporator vaporizes to absorb latent heat
• Saturated vapor can only absorb sensible
heat
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Flow Effect on Heat Exchange
• Too much heat load boils refrigerant quickly
– Refrigerant molecules move faster
– Results in higher pressure and temperature
• Too little heat load decreases refrigerant
boiling
– Refrigerant molecules move slower
– Pressures and temperature drop
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Distributor Tubes on Evaporators
• Location:
– Outlet of the metering device
• Application:
– Used on multicircuit evaporators
• Why needed?
– Refrigerant is a mixture of vapor & liquid
– Tends to feed liquid to the bottom circuits
– Proper distributor feeds evenly
• Sizing:
– Coil manufacturer determines proper distributor
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Distributor
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Distributor Section
Refrigerant from TEV
Distributors
Courtesy of Sporlan
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Improving Refrigerant Flow
Sometimes disks are used to make sure refrigerant
is fed evenly to each distributor tube
Courtesy of Sporlan
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Refrigerant flow through distributors to the coil
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Walk-in freezer
evaporator
Frosting of U-Bends
Most frost is
on first row
of tubing
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Measuring Superheat
Superheat =
Coil outlet temperature - Evaporator temperature
Superheat too high :
• “Starving”
Superheat too low:
• “Flooding”
The following picture shows where to take the coil
outlet temperature.
Note: Chapter 6 - Metering Devices will cover superheat in
greater detail.
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Take suction line
temperature
at the
Taking
suction
TEV sensing bulb
line temperature
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“Hot Pull Down”
When space temperature is above normal
there is a heavy load on the evaporator.
Following example: 35° box rises to 60°
• Pressure and TD relationship not valid
• Evaporator is starving
• Metering device performance is limited
Note: do not check superheat during hot pull down.
Space temperature should be within 5° of its design
conditions.
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Commercial Refrigeration System (40°)
High Heat Load
20º
Refrigerant boils off at coil inlet
30º
30º
Higher coil temperatures
Tstat won’t satisfy
Frost may start at inlet
EVAPORATOR
20º
30º
50º
RETURN AIR 60o
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Evaporator Troubleshooting
There are only 3 main problems:
1. Air flow:
A. Dirty filter, dirty or iced evaporator
B. Blower / ductwork problems
2. Refrigerant:
A. Metering device
B. Refrigerant charge (too little, or too much)
3. Load:
A. Too high
B. Too low (tstat set too low)
Note: Chapter 7, Refrigeration System Troubleshooting,
will cover this in more detail.
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Commercial Refrigerator Air Defrost
• Medium temperature refrigerators will frost:
– If space temperature is 36° to 40°
– And evaporator temperatures are 15° to 25°
– Then coil frosting is normal
• Coil will defrost during its “off-cycle”
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Commercial Refrigerator (40°)
Air Defrost
Slight frost is normal during refrigeration cycle
40º
20º
Refrigerator’s 20° coil builds frost
Tstat becomes satisfied
Compressor shuts off
Fans continue to run
EVAPORATOR
Coil temperature rises to 40 °
Frost melts
20º
40º
30º
Coil clear, ready for next cycle
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25º
40º
40º
Space Temp. 40o
38
Commercial Refrigeration Defrost
• Medium temperature refrigerators use the
thermostat “off-cycle” to melt frost
accumulation
• Sometimes a time clock is needed to extend
the length of the “off-cycle”
• The following slide is a common wiring
arrangement for a walk-in refrigerator
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Walk-in Refrigerator with Pump-down Solenoid
Refrigerating
Cycle
Off-Cycle Defrost
Thermostat
Satisfied
TS
120V
Solenoid Opens
L
N
Solenoid Closes
JUNCTION BOX
EVAPORATOR COIL
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Common Time Clock
• Used for cycling outdoor lights, heaters, etc.
• Clock in the example has one set of
normally closed (NC) contacts
• It is cycling a refrigeration compressor for
an extended off-cycle
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Trippers
“OFF” @ 2:00 AM
Dial
Rotation
“ON” @ 4:00 AM
Time of Day
115 Volt Time
Clock
(1) NC Switch
L1
Load
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N
42
Time Clock in Refrigeration
• “Planned” off-cycle defrost
• This gives the evaporator extra time to air
defrost
• The clock shuts off the compressor while
the evaporator fans continue to run
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Defrost Clock on a Walk-in Refrigerator
Refrigerating
“Planned”
off-cycleCycle
defrost
Contacts
open
DEFROST CLOCK
Circuit
opens
TIMER MOTOR
Compressor off for 1-2 hours
Evaporator will air defrost
N
2
TS
1
Change
wiring
120v
Solenoid Closes
L
N
JUNCTION BOX
EVAPORATOR COIL
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Low Temperature Evaporator Design
• Heaters
– A freezer requires some heat to help defrost
the evaporator
• Defrost termination and fan delay
– Controls are needed to return system to
freeze
• Heater Safety
– Prevents overheating of freezer
• A clock is needed to control defrost and
freeze cycles
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Electric Defrost Heaters
Heater Safety
DTFD
(defrost termination / fan delay)
Wiring Terminal Board
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Defrost Clock (Back and Front)
SOLENOID
SLIDE
CONTACTS
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Defrost Settings and Wire Terminals
• The clock on the next slide is scheduled
for 4 defrosts in 24 hours
– The “failsafe” setting is a backup to the
defrost termination switch
– If the defrost lasts too long (about 45 minutes
on the sample clock) the failsafe will put the
system back into freeze
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Time Clock
6
Paragon 8145-20
60
FAIL SAFE TIME
30
90
12
FAIL SAFE
SET POINT
DEFROST
TRIPPERS
CLOCK TIME
6
3
FREEZE
DEFROST
1
X
4
DEF. TERM.
2
N
COMMON
POWER
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Setting the Time
• Sometimes a defrost clock has to be reset
for the proper time of day
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Time
Clock
6
60
30
90
12
Time
Set
now
Time
set
for 6 pm
6
3
1
X
4
2
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N
51
Defrost Clock and Wiring
• The following slide is a pictorial diagram
of the clock, the evaporator, and the
controls
– The color coded wiring as illustrated is
typical of most walk-in freezers
– The sample system is in the freeze cycle
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Wiring Diagram for W/I Freezer with Electric Defrost & Remote C.U.
DEFROST SOLENOID
CLOCK
MOTOR
3
4
↔
SLIDE
3 4
X
X
EVAP.
FAN
DEFROST TERMINATION
FAN DELAY
Brn
DEFROST
HEATERS
R
1
N
2
N
Blk
THERMOSTAT
SOLENOID
VALVE
LINE VOLTAGE TO CLOCK
DRAIN
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LINE
HEATER
53
Defrost Clock and Heaters
• Defrost is “time initiated”
• The clock movement pushes the slide
bar on the rear of the clock
• The slide changes the switch contact
positions from freeze to defrost
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Defrost Cycle and Termination
DEFROST SOLENOID
Start Defrost Cycle
CLOCK
MOTOR
End Defrost Cycle
3
4
3 4
X
FAN
DEFROST TERMINATION
FAN DELAY
Brn
DEFROST
HEATERS
R
1-3
1-3closed
open 2-4
2-4closed
open
1
X
EVAP.
N
2
N
Blk
THERMOSTAT
SOLENOID
VALVE
LINE VOLTAGE TO CLOCK
DRAIN
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LINE
HEATER
55
Switching from Defrost Back Into Freeze
• When the defrost termination switch is
warm enough (usually 55°), the clock’s
defrost solenoid is energized
• The solenoid mechanically moves the slide,
switching the contacts out of defrost and
back into freeze
• The fan is delayed until the temperature of
the evaporator is down to approximately
25°
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Return to Freeze Cycle
DEFROST SOLENOID
Energized solenoid moves slide
CLOCK
MOTOR
Evap Cools, Fan Starts
3
4
3 4
X
FAN
DEFROST TERMINATION
FAN DELAY
Brn
DEFROST
HEATERS
R
1-3
2-4closed
open
1-3closed
open 2-4
1
X
EVAP.
N
2
N
Blk
THERMOSTAT
SOLENOID
VALVE
LINE VOLTAGE TO CLOCK
DRAIN
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LINE
HEATER
57
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