Transcript Superheat & Sub-cooling

Tennessee
Technology
Center at Pulaski
Superheat & Sub-cooling
A Technician’s Guide to
HVACR Diagnostics
Introduction

The ability to properly and accurately
measure superheat and sub-cooling and
interpret the results is very likely the single
most important skill that you, as an HVACR
service technician, can acquire

As you study this program, keep one very
important thing in mind ……….
No Test Is Valid
If The
Air Flow Is Not
Correct!!!
Superheat

A vapor is said to be “superheated” when its
temperature is higher than its saturation
temperature at the same pressure

Sub-cooling occurs to a liquid when its
temperature is below saturation for the
same pressure
Checking Superheat

Allow the system to operate for 15 –20
minutes to stabilize

Attach an accurate thermometer to the
suction line near the sensing bulb on TEV
systems or near the suction service valve on
fixed restrictor systems

Record the suction line temperature

Connect a manifold gauge set to the suction
service valve and record the suction pressure

Use a temperature/pressure chart to obtain the
saturation temperature for the suction gas at the
pressure recorded

Subtract the saturated temperature from the actual
suction line temperature – the result is the
operating superheat
Example 1

Suction line temperature =
55 F

Suction pressure =
68.5 psig

68.5 psig =
40 F
55
F – 40 F = 15 F superheat
Let’s examine some
common system
problems and see how
they affect superheat
and sub-cooling
High Superheat
Possible causes and
remedies for HIGH
SUPERHEAT
Excessive or HIGH superheat
is an indication of insufficient
refrigerant in the indoor coil for
the heat load present
This could be from insufficient
refrigerant entering the coil or from
an excessive heat load crossing the
coil
LOW REFRIGERANT
CHARGE

If there is insufficient refrigerant in the
indoor coil, all of the refrigerant will
evaporate in the first few passes of the coil.
 The excess sensible heat picked up by the
refrigerant vapor causes a higher than
normal suction gas temperature

Discharge pressure will be lower than
normal
 Suction pressure will be lower than normal
 Superheat will be higher than normal
 Sub-cooling will be lower than normal
 Current draw will be lower than normal
Liquid Line Restriction

A restriction in the liquid line will not allow
sufficient refrigerant to reach the
evaporator coil
 This causes many symptoms similar to a
low refrigerant charge
 Often there is a noticeable temperature
change at the point of the restriction

Suction pressure will be lower than normal
 Discharge pressure will be normal to lower
than normal
 Superheat will be high
 Sub-cooling will be high
 Current draw will be low
Evaporator Air Flow Too High

Excessive air flow reduces the latent
capacity of the coil thus increasing the
sensible heat load.
 This additional sensible heat results in
higher than normal suction gas temperatures
and pressures

Discharge pressure will be high
 Suction pressure will be high
 Superheat will be high
 Sub-cooling will be lower than normal
 Current draw will be higher than normal
Excessive Load Conditions

Excessive indoor coil loads will cause a
higher than normal heat content in the air
crossing the coil
 This excess heat will cause the refrigerant
liquid to boil away sooner allowing the
vapor to pick up additional superheat
 Most commonly caused by internal gains
such as an increase in occupancy load

Discharge pressure will be higher than
normal
 Suction pressure will be higher than normal
 Superheat will be high
 Sub-cooling will be lower than normal
 Current draw will be high
Metering Device Not Feeding
Properly

A restriction in a capillary tube, orifice or
TEV will reduce the amount of liquid
refrigerant entering the evaporator.
 Symptoms are the same as for a liquid line
restriction

Discharge pressure will be lower than
normal
 Suction pressure will be lower than normal
 Superheat will be higher than normal
 Sub-cooling will be higher than normal
 Current draw will be lower than normal
Low Superheat
Possible causes and
remedies for Low
SUPERHEAT
LOW SUPERHEAT

Low superheat indicates an excess of liquid
refrigerant in the evaporator coil
 Liquid refrigerant is very likely entering the
compressor
 This results in reduced compressor life and
possible imminent compressor failure
Refrigerant Overcharge

An overcharge forces excessive refrigerant
into the evaporator due to increased
pressure differential
 There is not enough heat present to
completely vaporize the excess refrigerant
 Compressor failure is likely

Discharge pressure will be higher than
normal
 Suction pressure will be higher than normal
 Superheat will be lower than normal
 Sub-cooling will be higher than normal
 Current draw will be higher than normal
TEV Overfeeding

Many symptoms similar to an overcharge
 Sensing bulb not insulated or not secured
properly
 Improperly sized valve
 Wrong valve for the application

Discharge pressure will be higher than
normal
 Suction pressure will be higher than normal
 Superheat will be lower than normal
 Sub-cooling will be lower than normal
 Current draw will be higher than normal
Low Evaporator Heat Load

Most common cause of low superheat
 Low air volume (dirty coils, filters,
restricted duct, etc.)
 Reduces the heat available to vaporize the
refrigerant
 Liquid refrigerant may enter the compressor

Discharge pressure will be lower than
normal
 Suction pressure will be lower than normal
 Superheat will be lower than normal
 Sub-cooling will be higher than normal
 Current draw will be lower than normal
Improper Metering Device

The wrong orifice
 A capillary tube the wrong size (or that has
been shortened)
 An improperly sized TEV
 Symptoms identical to device overfeeding

Discharge pressure will be higher than
normal
 Suction pressure will be higher than normal
 Superheat will be lower than normal
 Sub-cooling will be lower than normal
 Current draw will be higher than normal
Equipment Oversized

When a system is greatly oversized there is
not enough heat to vaporize the refrigerant
present in the evaporator
 Symptoms are similar to a low charge,
except that a low charge will have a high
superheat and run excessively
 Oversized unit will likely short cycle and
have a low superheat

Discharge pressure will be lower than
normal
 Suction pressure will be lower than normal
 Superheat will be lower than normal
 Sub-cooling will be higher than normal
 Current draw will be lower than normal
Condenser Air Flow

Low condenser air flow or recycled
condenser air will increase condensing
temperature thus increasing condenser
pressure
 Increased pressure drop across the metering
device results in a flooded evaporator
Caused by:

Dirty coil
 Bad motor or blade
 Shrubs, bushes or other obstructions
 Low overhangs
 Other equipment too close

Discharge pressure will be higher than
normal
 Suction pressure will be higher than normal
 Superheat will be lower than normal
 Sub-cooling will be lower than normal
 Current draw will be higher than normal
SUB-COOLING

A liquid is sub-cooled when its temperature
is below saturation at the same pressure
 Measuring sub-cooling is a good method of
confirming your diagnosis based on other
tests
 TEV systems MUST be charged by subcooling in the absence of a known charge
quantity
Measuring Sub-cooling


Allow the system to operate for 15 –20 minutes to
stabilize
Attach an accurate thermometer to the liquid line
near the inlet of the metering device whenever
possible
 The condenser outlet may be used, but will be in
error by the amount of liquid line
pressure/temperature losses

Record the liquid line temperature
 Using a gauge manifold, obtain the liquid
line pressure
 Discharge pressure may be used, but
allowances must be made for condenser
pressure drop

Using a temperature/pressure chart, convert the
pressure reading to saturation temperature
 Subtract the line temperature from the saturation
temperature
 The difference is operating sub-cooling
 In the absence of manufacturer’s data, a subcooling reading of 10 degrees or more is usually
acceptable
Sub-cooling Losses

Long liquid lines
 Liquid lines exposed to high ambient
temperatures (un-insulated)
 Low condenser air flow
 Inadequate condenser size
 Long vertical lifts
Long Liquid Lines

Long liquid lines cause increased pressure
drop due to friction losses
 Use the shortest lines possible
 Relocate equipment if necessary
Liquid lines exposed to high
ambient

High ambient increases liquid line
temperature
 Exposed liquid lines should be insulated
 Heat exchangers or auxiliary sub-coolers
may be considered.
Low condenser air flow

Low condenser air flow reduces the
condenser’s ability to reject heat
 Causes increased condensing temperature
 Clean condenser coils and clear any
obstructions
Inadequate condenser size

A condenser coil that is too small uses all of
the available space for condensing leaving
no room for sub-cooling
 May be caused by the presence of NCG’s
(non-condensable gasses)
Long vertical lifts

The weight of the refrigerant in long
vertical lifts causes a pressure drop
 HCFC-22 looses about ½ PSIG for every
foot of vertical rise
 Reduce the lift, use a heat exchanger or
artificial liquid line amplification
Practice Exercises
Exercise # 1

2 ½ ton package gas
unit
 5 years old
 No previous service
 Not cooling enough








odb = 95F
Idb = 80F
Iwb = 68F
SP = 55 PSIG
ST = 36F
DP = 210 PSIG
LT = 96F
Current = low
LOW LOAD

Superheat is low
 Sub-cooling is low to normal
 Both pressures are low
 Current draw is low
Exercise # 2

2 ½ ton package
heatpump
 15 years old
 No previous service
 Not cooling enough








odb = 95F
Idb = 80F
Iwb = 68F
SP = 55 PSIG
ST = 76F
DP = 180 PSIG
LT = 96F
Current = low
LOW CHARGE

Superheat is high
 Sub-cooling is low
 Both pressures are low
 Current draw is low