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68 Cards in this Set
- Front
- Back
- 3rd side (hint)
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True or false - ac and high temp systems require defrost |
False |
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Coil temps that run above what temperature need defrost |
30F |
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How long are run cycles? |
20-22hrs |
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off cycle defrost will be used when evap Is below? And space temp is between? |
When evap is below 30F Space is between 35-40F |
Temps |
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Cap tubes use what component for defrost |
Constant cut in Tstat |
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Evap fans must stay on during what type of defrost |
Force off cycle |
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Forced off cycle supplies how many hours defrost to run time |
1 hour defrost to 2 hours run time |
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Colder applications will utilize what device with forced off cycle defrost |
Time clocks |
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Three methods of shutting down compressors |
Turn it off Liquid line solenoid Suction stop EPR |
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EPR operate by? |
Inlet pressure regulator to a minimum setting |
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Suction stop EPR's have two points of operation, they are? |
A point where the valve remains closed (cut out) and opens once pressures reach a certain point |
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supplementary defrost is used when the coil temp and space temp are below ____ and ______ |
When coil temp is below 32F Space temp is 35F or lower |
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Types of supp heat (5) |
Reverse Electric Kool-gas Hot-gas Water |
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Compressor is pumped down and shut off with the evap fans during which type of defrost |
Electric |
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Explain electric sequence |
- LLS de-energized - system pumps down -Fans shut off -Heaters are energized -Drip time countdown begins System resumes operation |
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Why are the fans shut off during electric defrost |
To maximize heating of the coil To not release moisture to the box |
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Electric heaters should have what type of device associated with them? |
High limit |
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What happens upon termination in electric defrost? |
Fans are delayed to allow the coil to re-freeze |
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What initiates defrost? |
A time clock |
Tick tock |
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4 factors that terminate a defrost are? |
Time Temp Pressure Optical |
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Hot gas defrost are used on how many evaps? |
Single and multiple evap systems |
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Hot gas from the discharge manifold defrosts in what type of supplementary defrost |
Hot gas defrost |
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Two types of hot gas defrost are |
One on one Staggered |
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Hot gas lines for defrost flow rates are |
1000-2000 ft/min |
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A problem with one on one hot gas defrosts is? How can this problem be fixed? |
Runs out of hot gas Drop out the condenser to raise head pressure |
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Where are one on one hot gas defrosts found |
Ice machines |
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Another name for an ice machine defrost is? |
Harvest cycle |
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One on one systems use how many evaps, how many comps? |
1 of each |
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What does a vaporizer do? |
Vaporizes left over liquid from the accumulator after the compressor |
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This device controls the flow of hot gas during defrosr |
Hot gas regulator |
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Initial setting of the hot gas regulator? |
15-25 psi differential |
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Most efficient method of defrost is |
Hot gas defrost |
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Why are fans cycled off during hot gas defrost? |
To prevent adding excess heat and moisture |
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A main problem faced with hot gas defrost is |
Thermal stress |
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How can you avoid shocking a cold evap |
Using a 2 stage defrost with a smaller solenoid - soft gas |
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Two types of defrost staggered systems |
2 and 3 pipe systems |
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One evap is defrosted at 33% of capacity at any one time during |
Hot gas staggered systems |
33 - gg |
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Hot gas line, suction line, and liquid line are all included on an evaporator within |
3 pipe systems |
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2 pipe systems have what kind of lines on their evaporators |
Suction line Liquid line |
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Hot gas line to ASC must equal the discharge line size on what type of defrosts? |
One on one |
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How big are hot gas lines to be sized in a three pipe system |
66% of the discharge line |
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Saturated vapor defrost is also known as |
Kool gas defrost |
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Kool gas defrost is commonly found on |
Rack systems |
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Kool gas defrost uses what from what device to defrost coils |
Saturated vapour The reciever |
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Less thermal shock is occured with this defrost method |
Kool gas defrost |
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Refrigerant condensed where? During defrost is redirected where? To feed what? During kool defrost |
Evap Liquid line header Other coils not in defrost |
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Must be a 2 pipe defrost system |
Kool gas defrost |
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Kool gas defrost typically last |
10 minutes |
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Defrosting how many systems at a time is preferred? |
1 at a time |
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What must be kept up to achieve quick defrosts in kool gas |
Reciever pressure |
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_____ approaches _____ in kool gas |
Efficiency Hot gas |
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Min water temp in water defrost is |
50F |
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Water defrost has min gpm/sqft ? For how long? |
3gpm 5-15 min |
Gpm x what = total.min |
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Water defrost systems are commonly used where? |
Ammonia systems |
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How long is a normal reverse cycle defrost |
3-7 minutes |
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Reverse cycling defrost terminates by |
Pressure control |
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A rapid decrease in liquid flow due to sudden closing if a valve is |
Sudden liquid deceleration |
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What device will prevent split L's caused by liquid hammers |
12" stand pipe |
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High pressure refrigeration injected into a cold coil is called |
Condensation induced shock |
Something shock |
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Volume causes a coil to collapse in |
Condensation induced shock |
Type of shock |
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These three forces can produce pressures of ____ to _____ psi |
Atmospheric Vapour Liquid 2000-4000 |
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Condensate is pitch is |
4"/ft |
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Traps are located |
Outside of the space |
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Continuous heater protection is how many watts at how many degrees |
20 watts /ft 0F 30 watts /ft -20F |
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What type of defrost is this? |
Kool gas |
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Name this defrost |
Two pipe |
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Name this defrost |
3 pipe |
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What type if defrost is this |
One on one |
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