Test 5 Flashcards by K R

What are Line Voltage Thermostats (Tstats) primarily used in?
•

refrigeration equipment
•

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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

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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

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reach-ins

have a 20°F TD.

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Walk-ins have

about a 10°F TD

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What is an Off-Cycle” Defrost required for?
And for what space range temps?

medium temp refrigeration
– Space temperatures 35° to 40°

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• When compressor is running is coil temp below freezing?

Does frost begin to build then…

Yes
Yes

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• 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

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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
•

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When Frost forms on tubing what does the Tstat do?

– shuts off compressor

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Then the Compressor stays off until what?

coil is defrosted

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coil sensing tstats allow coil to completely what?

defrost before
compressor restarts

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There is a Factory set cut-in temperature
But adjustable cut-out

– to match system TD
•

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Maintains box temperature

– Unauthorized adjustments (by customer) will not
cause coil freeze up

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• Sensor in evaporator ensures defrosting

So the compressor stays off until?

– Compressor stays off until cut-in reached

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Electronic Thermostats are more accurate than?

They also have wider… and temperature…

electro-mechanical
• application, ranges

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Electronic Thermostats also have a

Wider application and temperature ranges

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What do Pressure Controls do?
🛑🏁⚡️

And they energize and d’énergies refrigeration components?

• Stop and start electrical current flow

– Yes

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• Switching arrangement of pressure controls:

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What opens Opens on fall:

Low pressure…

Low ambient…

Oil safety…

• control
• fan control
• switch

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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

1. temperature 2. defrosting 3. 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