Unit 3 — Refrigeration System Fundamentals & Maintenance
Section 5 — Pressure–Enthalpy Diagram
5.3 — System Capacity and Performance
Every change in operating conditions moves the state points on the P–h diagram.
Understanding which direction they move — and why — is how a technician
predicts what a service action will do to system capacity, efficiency, and component
temperatures before touching anything.
Jump to
5.3.1 — Effect of Condensing Temperature
Condensing temperature (and pressure) is the single most controllable variable
in system performance. It is set primarily by the outdoor ambient temperature and
the condition of the condenser heat exchanger. On the P–h diagram, raising the
condensing temperature shifts the entire high-side horizontal line upward
to a higher pressure.
Higher Condensing Temperature
High-side pressure increases → Point 2 and Point 3 move up
Compression ratio increases → more compressor work (h2−h1 grows)
h3 (sat liquid enthalpy) increases → flash gas quality rises
Refrigeration effect (h1−h4) decreases
COP falls; discharge temperature rises
Caused by: dirty condenser, blocked airflow, high ambient, overcharge, non-condensables
Lower Condensing Temperature
High-side pressure decreases → Points 2 and 3 move down
Compression ratio decreases → less compressor work
h3 decreases → less flash gas, more refrigeration effect
A single 10°F rise in condensing temperature — caused by a moderately dirty
condenser coil — costs 19% in efficiency while barely affecting capacity. The compressor
is working much harder for nearly the same output. This is why condenser
coil cleaning is the highest-value preventive maintenance action on any A/C system.
5.3.2 — Effect of Evaporating Temperature
Evaporating temperature (and pressure) is set by the load on the evaporator and
the flow of refrigerant through the metering device. On the P–h diagram, lowering
the evaporating temperature shifts the low-side horizontal line downward
to a lower pressure.
Lower Evaporating Temperature
Low-side pressure decreases → Points 1 and 4 move down
Compression ratio increases → more compressor work
Refrigerant vapour density at suction decreases → less mass flow per revolution
When the evaporating temperature drops (e.g., from a dirty evaporator coil or restricted
airflow), two bad things happen simultaneously on the P–h diagram: the compression
ratio rises (increasing compressor work), AND the suction vapour becomes less dense
(reducing mass flow rate). Both effects reduce total system capacity at the same time.
This is why diagnosing “low suction pressure” must always start with
verifying airflow before suspecting a refrigerant charge problem.
5.3.3 — Effect of Subcooling and Superheat
Subcooling and superheat shift individual state points without changing the operating
pressures. Their effects are visible on the P–h diagram as horizontal movements
of Points 1 and 3 at their respective pressure levels.
🔻
More Subcooling (Point 3 moves left)
h3 decreases → h4 decreases →
refrigeration effect (h1−h4) increases.
Flash gas quality decreases → more liquid enters the evaporator.
Small improvement in capacity and COP. Maximum benefit is limited
by the available condenser surface area and ambient temperature.
🔺
More Superheat (Point 1 moves right)
h1 increases → refrigeration effect appears to increase,
but the additional enthalpy is sensible heat gained in the suction line
(not in the evaporator). Compressor work also increases because the suction
vapour is hotter and less dense. Net effect: no useful capacity gain;
discharge temperature rises; compressor is stressed.
✅
Optimal Superheat & Subcooling
Target evaporator superheat of 8–12°F (TXV systems) ensures the
compressor receives dry vapour. Target subcooling of 10–20°F ensures
liquid delivery to the metering device. Both values at target = system is
operating at its rated design point on the P–h diagram.
📋
Worked Example — Improving Subcooling from 5°F to 15°F (R-410A)
A technician finds a system with only 5°F subcooling (bubbles
visible in sight glass, suspected low charge). After verifying no leaks and adding
refrigerant, subcooling rises to 15°F. Condensing at 130°F,
evaporating at 40°F:
Before (5°F SC):
Liquid line temp = 125°F → h3 ≈ 54.2 BTU/lb
qevap = 124.5 − 54.2 = 70.3 BTU/lb
Flash gas quality x4 ≈ (54.2−42.3)÷80.2 = 0.15 (15%)
After (15°F SC):
Liquid line temp = 115°F → h3 ≈ 50.5 BTU/lb
qevap = 124.5 − 50.5 = 74.0 BTU/lb
Flash gas quality x4 ≈ (50.5−42.3)÷80.2 = 0.10 (10%)
Refrigeration effect increased by 5.3% and flash gas dropped by 5
percentage points — purely from restoring proper subcooling.
5.3.4 — Effect of Airflow and Fluid Flow Rates
The P–h diagram shows what happens inside the refrigerant circuit,
but the refrigerant can only absorb or reject heat at the rate the air or fluid
on the outside of the heat exchangers allows. Inadequate flow is the most common
field cause of performance problems.
Low suction pressure, high superheat, low subcooling, bubbles in sight glass
Excessive indoor airflow
Evaporator
Evaporating pressure rises slightly; reduced latent removal relative to sensible
High supply air temperature (insufficient cooling), reduced dehumidification
💡
Always verify airflow before diagnosing a refrigerant charge problem
Low suction pressure with high superheat can look identical on a gauge set whether
caused by a refrigerant undercharge or a dirty evaporator coil. On the P–h diagram,
both push Point 1 downward and to the right. Checking filter condition, measuring
temperature rise across the evaporator, and confirming blower amperage takes two minutes
and can avoid an unnecessary and incorrect refrigerant addition.
5.3.5 — Heat of Compression and Discharge Temperature
The heat of compression (h2−h1) appears on the P–h
diagram as the horizontal length of the compression line. Any factor that makes
this line longer wastes energy and raises discharge temperature.
⬆️
High Compression Ratio
A large pressure difference between suction and discharge means the compressor
must work harder per kg of refrigerant. On the P–h diagram: Point 2 moves
further right. Caused by: high condensing pressure (dirty condenser) combined
with low suction pressure (restricted airflow or low charge).
🌡️
High Suction Superheat
Hot suction gas entering the compressor has a higher specific volume, reducing
mass flow, and starts at a higher enthalpy, making the isentropic compression
line start further right. Both increase compressor work per unit of cooling
produced and raise discharge temperature.
🔧
Poor Isentropic Efficiency
A worn or damaged compressor with valve leakage, piston ring wear, or bearing
friction produces more heat per kg compressed. Point 2 moves further right from
the ideal isentrope. Diagnosed by comparing measured discharge enthalpy to the
theoretical isentropic value.
🌡️
Discharge Temperature as a Diagnostic Tool
Discharge line temperature (measured at the compressor discharge port, not the
condenser inlet) is one of the most useful single-point indicators of system health.
A useful rule of thumb:
Normal: 150–220°F (66–104°C) — 50–100°F above condensing saturation temperature
Above 225°F (107°C): Investigate high compression ratio or excessive superheat
Above 250°F (121°C): Risk of oil carbonisation and valve damage — shut down and diagnose
Below 150°F (66°C): May indicate liquid flooding back to the compressor (low discharge SH)
On the P–h diagram, discharge temperature is read from the isotherm passing
through Point 2. A quick field check: measure the discharge line surface temperature
and compare to the condensing saturation temperature. The difference is the discharge
superheat — it should be 50–100°F.
5.3.6 — System Load and Design
The building or process load determines how much heat the evaporator must absorb per hour.
As load varies with weather, occupancy, and internal gains, the system must adapt while
maintaining acceptable temperatures and pressures.
High Load Conditions
Evaporating pressure tends to rise (more heat to absorb)
Compressor runs longer or at higher capacity (variable speed)
Condenser must reject more total heat → discharge pressure rises
TXV opens wider to feed more refrigerant → superheat maintained
System may struggle to maintain setpoint at design limits
Low Load Conditions
Evaporating pressure may rise above design (less heat to absorb)
Single-speed systems short-cycle → poor humidity control
Variable-speed systems reduce compressor speed to match load
Risk of coil freeze-up if evaporating temp drops due to low airflow
Subcooling may increase on high side (less heat to reject)
💡
Variable-speed compressors keep the cycle closer to the ideal
A variable-speed (inverter-driven) compressor can modulate between 30% and 120%
of rated capacity. At part load, it slows down rather than cycling on and off.
On the P–h diagram, this keeps the cycle operating close to its rated
state points rather than experiencing the large pressure swings that occur during
on/off cycling. The result is more consistent superheat and subcooling, higher
average COP, and better humidity control — which is why inverter-driven
equipment dominates modern high-efficiency ratings (SEER2 ≥ 18).
5.3.7 — Diagnostic Summary — P–h Diagram Shifts at a Glance
This table connects common field problems to their effect on the cycle plotted on the
P–h diagram. Use it to predict what gauge readings and temperatures to expect
before connecting tools — and to confirm a diagnosis when readings match the pattern.
Condition
Points Affected
Direction on P–h
Effect on COP
Effect on Capacity
Dirty condenser coil
2, 3 (high side)
Points 2 & 3 shift UP
Decreases
Decreases (more flash gas)
Low refrigerant charge
1, 4 (low side) + low SC
Points 1 & 4 shift DOWN; Point 3 shifts RIGHT (less SC)
Decreases
Decreases significantly
Overcharge of refrigerant
2, 3 (high side)
Points 2 & 3 shift UP; Point 3 shifts further LEFT (more SC)
Decreases
Marginal increase then decrease
Restricted evaporator airflow
1, 4 (low side)
Points 1 & 4 shift DOWN
Decreases
Decreases significantly
Increased subcooling (proper charge)
3, 4
Points 3 & 4 shift LEFT (lower enthalpy)
Increases
Increases
Excessive suction superheat
1
Point 1 shifts RIGHT (higher enthalpy at same pressure)
Point 2 shifts RIGHT from isentrope at same pressure (higher h2)
Decreases
Decreases (less mass flow)
🛠️
Using This Table in the Field
When you connect a manifold gauge set and record suction pressure, discharge pressure,
suction line temperature, and liquid line temperature, you have enough data to plot
an approximate cycle on the P–h diagram — or at minimum, to locate
where the state points are relative to their ideal positions.
Plot both pressures as horizontal lines on the diagram
Use the P–T chart to find saturation temperatures for each pressure
Add superheat (suction line temp minus sat temp) → locate Point 1
Add subcooling (sat temp minus liquid line temp) → locate Point 3
Read h1, h3 from the diagram; set h4 = h3
Calculate qevap and compare to nameplate capacity at those conditions
Any deviation from expected values points directly to the table above