Test 5 Flashcards
What are Line Voltage Thermostats (Tstats) primarily used in?
•
refrigeration equipment
•
What do Refrigeration units need?
Do the “Tstats” have adjustable differentials?
A wide differential for line voltage thermostats gives it time to do what?
What do most T-stats sense? And what do some sense?
self-defrosting
Yes
“off-cycle” coil
defrosting
•air temperature, coil temperature
To Adjust a Commercial Thermostat
you should set the cut-in…
And set the …
What is cut-out?
temperature (high event)
differential
• Cut-out = Cut-in minus Differential
reach-ins
have a 20°F TD.
Walk-ins have
about a 10°F TD
What is an Off-Cycle” Defrost required for?
And for what space range temps?
medium temp refrigeration
– Space temperatures 35° to 40°
• When compressor is running is coil temp below freezing?
Does frost begin to build then…
Yes
Yes
• When compressor cycles off do the fans keep running and circulant air?
At this point what temp does could rise above?
Would the frost melt
yes
– Coil rises above 32°
Yes
Coil Temperature Sensing Thermostats
Where is Tstat bulb is mounted in?
What does it respond to?
Does the coil temp drop as box cools?
evaporator fins
– evaporator temperature, not air
– yea
•
When Frost forms on tubing what does the Tstat do?
– shuts off compressor
Then the Compressor stays off until what?
coil is defrosted
coil sensing tstats allow coil to completely what?
defrost before
compressor restarts
There is a Factory set cut-in temperature
But adjustable cut-out
– to match system TD
•
Maintains box temperature
– Unauthorized adjustments (by customer) will not
cause coil freeze up
• Sensor in evaporator ensures defrosting
So the compressor stays off until?
– Compressor stays off until cut-in reached
Electronic Thermostats are more accurate than?
They also have wider… and temperature…
electro-mechanical
• application, ranges
Electronic Thermostats also have a
Wider application and temperature ranges
What do Pressure Controls do?
🛑🏁⚡️
And they energize and d’énergies refrigeration components?
• Stop and start electrical current flow
– Yes
• Switching arrangement of pressure controls:
What opens Opens on fall:
Low pressure…
Low ambient…
Oil safety…
• control
• fan control
• switch
Does high pressure safety control Opens on rise?
Yes
Low Pressure Control
When pressure falls the
switch opens
Cut-out =
Cut-in minus Differential
To Adjusting the control you need to
Set the cut-in …
And set the…
- Set the cut-in pressure
- Set the differential
When Setting Low Pressure Controls you must always
• Follow manufacturer recommendations
If the cut-out too low it may
overheat compressor
If Differential too close, it could cause
short-cycling
Are outdoor Outdoor units are affected by low ambient?
Cut-in must be above what?
Yes
– lowest ambient
Fixed Low Pressure Controls
Means that there is adjustment
on the control
–
Low Pressure Control as a Thermostat
Is used in some indoor…
reach-in refrigerators
• “Rule of thumb” settings:
– Control Cut-in: at a pressure equal to…
• the desired box temperature
– Control Cut-out:
At pressure equal to
• At a pressure equal to 28⁰ below desired box
temperature
Benefits of using LP controls
Maintain box
Ensures coil
And there’s low pressure
- temperature
- defrosting
- safety
Note: Low pressure controls are used as temperature controls
only on indoor units
High Pressure Control opens circuit
on pressure rise
• Setting control cut-out you should and could?
– should Check with manufacturer, or
– could remember 155° condensing temperature:
• R134a
= 283 psig cut-out
• R22 =
382 psig cut-out
• R404A =
427 psig cut-out
• Low ambient:
–
When condensing temperature drops below 90°
• What is the problem with low ambient?
It would lower…
Low liquid pressure
– condensing pressure,
–affects metering device
Another problem with low ambient is that is starves …. And lowers…
• evaporator and cooling
• Solution to low ambient
– Use Head Pressure Controls
Head Pressure Control
• Three common methods:
1. Fan cycling
2. Dampers
3. Flooding
Fan Cycle Control
• A pressure switch controls the condenser fan:
When head pressure falls
When head pressure rises,
–, the fan shuts off
– the fan starts
Setting a Fan Cycle Control
Is A fan cycle is a “reverse acting”HP control?
Do Contacts close on pressure rise?
Yes
Yes
• “Rule of Thumb” pressure settings for control:
Cut-out at
Cut-in at
– 90° condensing
– 110° condensing
Oil Pressure Control Sequence
• If low pressure continues for 120 seconds:
– Control circuit
– Compressor
– Alarm contacts
If Net Oil Pressure drops below 10 psig will Contacts close to start timer?
• opens
stops
close
Yes
Oil Pressure Control Sequence
If low pressure continues for 120 seconds:
Control circuit
Compressor
Alarm contacts
– opens
– stops
– close
Oil Pressure Safety Control protects compressor from what?
• lack of lubrication
Oil pressure safety control Operates on a pressure differential between what two things?
– Oil pump pressure and suction pressure
