Ch 7 Troubleshooting Flashcards
- Cap tube system
- Low evap temp/pressure
- High superheat
- Low condenser temp
- Low subcool
Undercharged
- TXV system
- Low evap temp/pressure
- High superheat
- Low condenser temp
- Low subcool
- Bubbling sightglass
Undercharged
*does not always indicate a leak; if system was charged in warm weather it may be insufficient in cold
- Cap tube system
- high condenser temp/pressure
- high subcool
- slightly high evap temp
- normal superheat
Overcharged
*could have been overcharged in the cold, or mistakenly with normal sightglass bubbling
- cap tube system
- high condenser temp
- normal subcool
- high evap temp/pressure
- low to no superheat
Condenser airflow problems
- TXV system
- high condenser temp
- about normal subcool
- slightly higher evap temp
- normal superheat (unless extreme)
Condenser airflow problems
- cap tube system
- high condenser temp
- high subcool
- high evap temp
- low to no superheat
Noncondensables
*pulling a vacuum and weighing in refrigerant will prevent contamination
- TXV system
- high condenser temp
- very high subcool
- normal or slightly high evap temp
- normal or slightly low superheat
Noncondensables
- large system
- high side head pressure remains high for a few minutes after the compressor is turned off
noncondensables
- large system
- with refrigerant at ambient temps, condensing temp based on head pressure does not match chart psig at ambient
noncondensables
- cap tube system
- low condensing pressure
- normal subcool
- high evap pressure
- high superheat
inefficient compressor
- TXV system
- low condenser discharge pressure
- normal subcool
- high evap suction pressure
- high superheat
inefficient compressor
what is the primary indicator of an inefficient compressor?
higher than normal suction pressure WITH lower than normal head pressure
How can you check a compressor through vacuum testing?
It will be unable to hold a vacuum for more than a few minutes before the low side gauge rises. This does not show compressor damage from broken connecting rod or bearing wear. Use spec sheets to check amperage pull against pressures and temps
How can you identify a fully blocked cap tube?
- evaporator in vacuum
- head pressure and subcool indicate the presence of refrigerant in the condenser
- cap tube system
- low evap temp/pressure
- high superheat
- low condensing temp
- high subcool
Partially restricted cap tube
*does not increase head pressure
What are the possible causes of high head pressure on a cap tube system?
- high ambient temp
- condenser airflow problems
- overcharge
- TXV system
- low evap temp/pressure
- high superheat
- low condenser temp
- normal subcool
- clear sightglass
Partially restricted TXV
- TXV system
- low evap temp/pressure
- high superheat
- low condenser temp
- normal subcool
- bubbling sightglass
- temperature drops drastically across liquid line
Restriction in liquid line after the receiver
*most commonly a kink or a filter drier in need of replacement - more than a 3 degree F drop across the filter drier indicates replacement
-TXV system
-low evap temp/pressure
-high superheat
-high condenser temp/pressure
-high subcool
bubbling sightglass
restriction in the liquid line before the receiver
*very rare, most likely @ the receiver inlet. only other location is condenser U-bends
- cap tube system
- low evap temp/pressure
- low or no superheat
- low condenser temp/pressure
- normal subcool
Evaporator problems
*some superheat at the compressor suggests the refrigerant is vaporizing in the suction line but not in the evaporator
- TXV system
- low evap temp/pressure
- low to normal superheat
- low condensing temp
- normal subcool
Evaporator problems
What are the signs of evaporator oil logging?
- a history of not maintaining temperatures
- symptoms don’t fit with other issues
- TXV hunting, floodback, or can’t adjust superheat
- some U-bends near the evap outlet aren’t sweating or frosting like others
- history of compressor changes
- sight glass oil level varies a lot through cycle
- noisy or vibrating compressor
- evidence of flooding or slugging
- absence of a pump-down solenoid on a remote commercial refrig condensing unit
- freezer has only one defrost
- evidence of incorrect piping practices: no p-trap with compressor above evap, suction lines not sloped or too large
- diagnosis w/o pressure gauges
- low subcool
- low evap ΔT
- low condenser ΔT
undercharged
- diagnosis w/o pressure gauges
- slightly high subcool (7-10F)
- low evap ΔT
- low condenser ΔT
- suction line is only slightly cool
stopped cap tube
- diagnosis w/o pressure gauges
- little or no subcool
- low or no evap ΔT
- low or no condenser ΔT
- warm suction line @the compressor
- hot compressor dome
bad compressor discharge valves
For diagnosis w/o pressure gauges, where should you take temperature readings for condenser ΔT?
-air temp entering condenser minus air temp BETWEEN the condenser and fan on outlet side
For diagnosis w/o pressure gauges, where should you take temperature readings for evaporator ΔT?
-air temp entering evap minus air temp leaving evap
superheat rule
if refrigerant is fully vaporized, added heat will only increase its temperature, not its pressure
subcool rule
if refrigerant is fully condensed, any heat removal will decrease temperature but not pressure
What superheat indicates flooding?
under 5F
What superheat indicates starving?
over 20F
What is normal for condensing temperature in refrigeration? (TD)
30F above ambient +/- 5F
What is normal for subcool in refrigeration?
5-20F
What is normal evaporator temp for walk-ins and reach ins? (TD)
walk-in: 10F below box +/- 5F
reach-in: 20F below box +/-10F
What is normal superheat in refrigeration?
5-20F, 10F is standard
What can cause high superheat?
refrigerant vaporized too early, high load, metering device under-feeding
Normal superheat
Only accurate when temp operating w/I 5F or design conditions
Normal superheat by application
A/C :15F
Refrigerator: 10F
Freezer: 6F
(5-20F for this chapter)
What happens to condensing temp when the ambient temp drops?
Condensing temp drops
What happens to condensing temp when the load on the evap drops?
The condensing temp drops
What happens to condensing temp when the ambient increases?
Condensing temp increases
What happens to condensing temp when the load on the evap increases
The condensing temp increases
What happens in a TXV system when the ambient drops
The valve adjust flow to maintain superheat
What happens in a TXV system when the load on the evap drops
The valve adjusts the flow to maintain superheat
What happens in a TXV system when the ambient increases
The valve adjusts the rate of flow to maintain superheat
A TXV is designed to:
Provide enough refrigerant to absorb a certain amount of heat in an evap while maintaining superheat.
What happens to superheat in a cap tube system when the ambient drops?
Less refrigerant to the evaporator, higher superheat
What happens to superheat in a cap tube system when the load on the evaporator drops?
More refrigerant than needed for the load, lower superheat.
What happens to superheat in a cap tube system when the ambient increases?
More refrigerant to the evaporator, lower superheat.
What in happens in a cap tube system to superheat when the load on the evaporator increases?
Not enough refrigerant for the load, superheat increases.
What is the standard evaporator TD for medium temp (refrigeration)?
30F for both walk ins and reach ins
What is normal condenser temp for freezers (split)?
25F above ambient
For diagnosis w/o gauges, what should be the difference in temp between the liquid line leaving the condenser and the line at the outlet of the filter drier?
5F
What is standard design box temp for refrigerators, walk in and reach in?
Walk in fridge: 35F
Reach in fridge: 38F
What is standard design box temp for a freezer, reach in and walk in?
Reach in: 0F
Walk in: -10F
