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HVAC > Test 5 > Flashcards

Test 5 Flashcards

1
Q

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

A

refrigeration equipment
•

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2
Q

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?

A

self-defrosting

Yes

“off-cycle” coil
defrosting
•air temperature, coil temperature

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3
Q

To Adjust a Commercial Thermostat

you should set the cut-in…

And set the …

What is cut-out?

A

temperature (high event)
differential
• Cut-out = Cut-in minus Differential

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4
Q

reach-ins

A

have a 20°F TD.

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5
Q

Walk-ins have

A

about a 10°F TD

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6
Q

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

A

medium temp refrigeration
– Space temperatures 35° to 40°

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7
Q

• When compressor is running is coil temp below freezing?

Does frost begin to build then…

A

Yes
Yes

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8
Q

• 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

A

yes
– Coil rises above 32°
Yes

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9
Q

Coil Temperature Sensing Thermostats

Where is Tstat bulb is mounted in?
What does it respond to?

Does the coil temp drop as box cools?

A

evaporator fins
– evaporator temperature, not air
– yea
•

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10
Q

When Frost forms on tubing what does the Tstat do?

A

– shuts off compressor

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11
Q

Then the Compressor stays off until what?

A

coil is defrosted

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12
Q

coil sensing tstats allow coil to completely what?

A

defrost before
compressor restarts

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13
Q

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

A

– to match system TD
•

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14
Q

Maintains box temperature

A

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

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15
Q

• Sensor in evaporator ensures defrosting

So the compressor stays off until?

A

– Compressor stays off until cut-in reached

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16
Q

Electronic Thermostats are more accurate than?

They also have wider… and temperature…

A

electro-mechanical
• application, ranges

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17
Q

Electronic Thermostats also have a

A

Wider application and temperature ranges

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18
Q

What do Pressure Controls do?
🛑🏁⚡️

And they energize and d’énergies refrigeration components?

A

• Stop and start electrical current flow

– Yes

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19
Q

• Switching arrangement of pressure controls:

A
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20
Q

What opens Opens on fall:

Low pressure…

Low ambient…

Oil safety…

A

• control
• fan control
• switch

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21
Q

Does high pressure safety control Opens on rise?

A

Yes

22
Q

Low Pressure Control

When pressure falls the

A

switch opens

23
Q

Cut-out =

A

Cut-in minus Differential

24
Q

To Adjusting the control you need to

Set the cut-in …

And set the…

A
  1. Set the cut-in pressure
  2. Set the differential
25
Q

When Setting Low Pressure Controls you must always

A

• Follow manufacturer recommendations

26
Q

If the cut-out too low it may

A

overheat compressor

27
Q

If Differential too close, it could cause

A

short-cycling

28
Q

Are outdoor Outdoor units are affected by low ambient?

Cut-in must be above what?

A

Yes

– lowest ambient

29
Q

Fixed Low Pressure Controls
Means that there is adjustment

A

on the control
–

30
Q

Low Pressure Control as a Thermostat
Is used in some indoor…

A

reach-in refrigerators

31
Q

• “Rule of thumb” settings:
– Control Cut-in: at a pressure equal to…

A

• the desired box temperature

32
Q

– Control Cut-out:
At pressure equal to

A

• At a pressure equal to 28⁰ below desired box
temperature

33
Q

Benefits of using LP controls

Maintain box

Ensures coil

And there’s low pressure

A
  1. temperature
  2. defrosting
  3. safety
34
Q

Note: Low pressure controls are used as temperature controls

A

only on indoor units

35
Q

High Pressure Control opens circuit

A

on pressure rise

36
Q

• Setting control cut-out you should and could?

A

– should Check with manufacturer, or
– could remember 155° condensing temperature:

37
Q

• R134a

A

= 283 psig cut-out

38
Q

• R22 =

A

382 psig cut-out

39
Q

• R404A =

A

427 psig cut-out

40
Q

• Low ambient:
–

A

When condensing temperature drops below 90°

41
Q

• What is the problem with low ambient?
It would lower…
Low liquid pressure

A

– condensing pressure,
–affects metering device

42
Q

Another problem with low ambient is that is starves …. And lowers…

A

• evaporator and cooling

43
Q

• Solution to low ambient

A

– Use Head Pressure Controls

44
Q

Head Pressure Control

A

• Three common methods:
1. Fan cycling
2. Dampers
3. Flooding

45
Q

Fan Cycle Control
• A pressure switch controls the condenser fan:

When head pressure falls

When head pressure rises,

A

–, the fan shuts off
– the fan starts

46
Q

Setting a Fan Cycle Control

Is A fan cycle is a “reverse acting”HP control?

Do Contacts close on pressure rise?

A

Yes
Yes

47
Q

• “Rule of Thumb” pressure settings for control:

Cut-out at

Cut-in at

A

– 90° condensing
– 110° condensing

48
Q

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?

A

• opens

stops

close

Yes

49
Q

Oil Pressure Control Sequence

If low pressure continues for 120 seconds:

Control circuit

Compressor

Alarm contacts

A

– opens
– stops
– close

50
Q

Oil Pressure Safety Control protects compressor from what?

A

• lack of lubrication

51
Q

Oil pressure safety control Operates on a pressure differential between what two things?

A

– Oil pump pressure and suction pressure

HVAC (16 decks)

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