-
A refrigeration designed for the storage of flowers or candy is considered a _______ type
High temperature
-
If a low temp evap is forming frost, what would be the problem
no problem,frost formation is normal
-
If an evap has a pressure drop of 10 PSIG and the low side pressure is 15 PSIG, what is the pressure at the inlet of the evap
25 Psig
-
If a Low temp evap is forming excess ice, which of the following could be a possible problem
- The unit is not staying in defrost long enough
- the evap fan motor is defective
- the defrost heater is defective
-
Refrigerant entering the evap is approximently _______ vapor
25%
-
Excess evap superheat _______
will reduce the cooling capacity of the cooling system
-
if a high temp evap is forming ice, which of the following is a possible problem
evap fan not running
-
A typical supper heat for an evap operating under normal condition would be _______
8 - 12 * F
-
What type of defrost, and fins would A forced draft high temp evap have_______
- no need for defrost
- it's fins are very close together
-
The heat required for off- cycle defrost is obtained from blank
the space temp when the compressor is off
-
If the application requires a coil design to operate below 32*F, there must be provision for _______
Defrost
-
how would a forced draft medium temp evap be defrosted
by off cycle defrost, when the compressor is off.
-
A direct expansion-type evap is also called what
dry-type evaporator
-
If a R-22 refrigerant system is operating wit a 25.4 psig suction line pressure, what is the evap supper heat if the outlet temp is 35*F
9*F
-
The fin spacing on a low temp evap is_______than on a medium-temp evap
farther apart
-
During the defrost cycle of a forced draft low temp evap. the evap fan would _____
turn off
-
An evap made up of two pieces off sheet-metal that has an impression of refrigerant piping is called a __________evap
Stamped
-
The refrigerant in the evap must be cooler than the air passing over it for heat to transfer to the evap
TRUE or FALSE
True
-
A stamped plate evap usually does not use a fan to move air across it
TRUE or FALSE
TRUE
-
When the velocity of the air moving over the evap is slow the film factor acts as an insulates and slows the rate of heat exchange
TRUE or FALSE
True
-
Boiling temp. is the same as
evaporator temp.
-
fresh food is maintained at ________
35*F
-
How do you get TD?
What does TD controls?
- Evap. tamp. - box tamp. =TD.
- It controls humidity
-
What is air conditioning considered to be and what is the typical coil temp.
- High temp/ 47*F to 60*F
- Medium temp 28*F to 40*F
- low temp -20*F to 0*F
-
What are the refrig temp ranges?
- High temp/ 47*F to 60*F
- Medium temp 28*F to 40*F
- low temp -20*F to 0*F
-
Low side gauge converted to a temp =
Boiling (actual)
-
Box temp - evap temp=
Boiling (design)
-
What is the definition of TD?
It's the difference between the refrigerant temp inside the coil and the space being cooled.
-
When is the biggest heat lose in the system?
when the refrigerant changes state this is called latent heat or hidden heat
-
Common materials used in the evaporator
nickel, copper, aluminum, steal, bras, stainless steal
-
Rapid heat transfer rates between two ______
liquids
-
Slower heat transfer rates between two ________
vapor
-
Relationship between the medium giving up heat and the heat exchange surface
film factor
-
Film acts as an insulator when ?
velocity is low
-
Temperature difference between two mediums
- large temp difference = high heat transfer rate 20 * TD
- Small temp difference = ow heat transfer rate 10*F TD
-
Natural convection (draft) evaporators
- Made up of fins and tubes
- physically large, very low velocity (film factor)
- Located high, near the ceiling of the cooler
-
Mechanical draft evaps
- Use fans to move air across the coil
- It also improves heat transfer rate
- Physically smaller then natural draft evaps
-
Stamped plate evaporators are made of what, and are designed to do what
- Made of metal plate stamped with grooves that provide a path for the refrigerant
-
What are the functions of the fins in the evap
to increase surface area
-
What is considered and excessive pressure drop in the evap
2 1/2 or more
-
Multiple coils reduce _________
Reduces the temp differences drastically and lowers pressure drop.
-
What is the header
It is the outlet of each circuit that is tapped into the side of a straight pipe it will become the suction line
-
More liquid enters the coil then vaper
-
Latent and heat is more concentrated then sensible heat
-
The greatest heat transfer takes place during latent and heat transpher
-
Flooded evap is designed to operate _______
Full of liquid
-
What devices used to prevent liquid from entering the compresser
- Low side float
- Accumulator
-
Chiller barrel is a flooded evap
-
what is a dry evaporator
when 100% of the the refrigerant boils before leaving the coil.
-
evaporator outlet temp- the evaporator saturation temp=
superheat
-
what does high superheat indicate
underfed evaporator or starved evaporator
-
what does low superheat indicate
overfed evaporator or flooded evaporator
-
closing TXV will do what to the superheat
lower the superheat
-
opening the TXV will do what to the superheat
raise the superheat
-
when does hot pull down occur
- any time your box goes above C/I of the low pressure switch LPS.
- ie: excessively loaded box
-
When should you take superheat readings
when the system is @ 5*F above the C/O
-
Pressure drop in the evaporator should be _____
as low as possible below 2 1/2 #
-
What is done when the pressure drop in the evap is higher than 2 1/2#
- a multiple circuit evap is used
-
liquid type evap are commonly called chiller barres. how much superheat is good
0-1 superheat
-
Low-Temp evaporators are designed differently. fins are spaced farther apart to accommodate frost buildup without sacrificing airflow
-
low temp evap methods to defrost the coil
- hot gas defrost sends the compressor discharge gas to the evap to melt the ice
- electric heaters can also be used.
-
high temp coil uses what type of defrost
none. no need because the box temp is not designed to go below 32*F
-
Medium temp coil uses what type of defrost
random defrost. anytime the compressor shuts down, it defrosts by the space temp inside the box by blowing the warmer air over the coil.
-
low temp coil uses what type of defrost
planned defrost. relies on a timer , and some type of mechanical device to defrost the coil
-
FPI. Fins Per Inch. how many FPI does ac, med and low temp coils have
- ac has 15+ fpi
- med has 7 to 14 fpi usually 10
- low has 6 fpi
-
Where is Hot gas defrost piped in the system
It will be piped from the discharge line to the evaporator after the TXV, but before the distributors
-
How does Hot gas defrost work.
- Compressor discharge gas pumped directly to the evaporator coil during the defrost cycle
- hot gas from the compressor condenses to a liquid in the evap coil.
- Liquid from the evap vaporizes in the accumulator before returning to the compressor
-
How does Electric Defrost work.
- Electric heating element are located next to the evaporator coil
- The compressor and the fan motor are not running during defrost
- At the end of the defrost cycle, the compressor cycles on before the fan, to allow the evap coil to get cold before the fan energized
|
|
