Follow the measurement path before changing the cooler: the gas reaches the suction pressure and temperature elements, their signals pass through input scaling, and the flow calculation converts those readings to mass flow. If any stage uses the wrong pressure basis, temperature units, or compensation method, the displayed change will not represent compressor capacity.
What readings define the current operating point?
Layer one first. Record simultaneous, steady readings at the compressor suction flange, discharge flange, flow element, speed indication, and recycle valve. The stated point is 170 TPH at 95% speed, with suction pressure 10.7 kg/cm2(g), discharge pressure 24.5 kg/cm2(g), suction temperature 23 deg C, molecular weight near 16.5, and the antisurge valve fully closed.
| Reading | What to check | Decision |
|---|---|---|
| Suction temperature | Sensor location, calibration, heat conduction, and conversion to absolute temperature | If the element does not measure compressor-flange temperature, correct the measurement path before calculating density. |
| Suction and discharge pressure | Impulse path, zero, range, and gauge-versus-absolute handling | Use absolute pressure for density and pressure-ratio calculations. |
170 TPH flow |
Determine whether it is directly compensated mass flow or volume converted with configured gas data | If it is calculated, audit molecular weight, compressibility, pressure, and temperature inputs. |
95% speed |
Compare the actual shaft-speed signal, not only a controller demand | A speed change invalidates a temperature-only comparison. |
| Recycle position | Confirm physical valve closure and absence of reverse leakage | A closed command alone does not prove zero recycle flow. |
What does the temperature-only density calculation predict?
For a gas, density follows rho = Pabs × MW / (Z × R × Tabs). Between two suction states with unchanged gas composition, the density ratio is:
rho2/rho1 = (P2abs/P1abs) × (Z1/Z2) × (T1abs/T2abs)
The temperatures are T1 = 23 + 273.15 = 296.15 K and T2 = 15 + 273.15 = 288.15 K. If suction absolute pressure and compressibility factor remain equal, pressure and compressibility cancel:
rho2/rho1 = 296.15/288.15 = 1.02776
The cooler gas is therefore about 2.78% denser under that stated assumption. If the compressor passes the same actual inlet volume per unit time, predicted mass flow becomes:
m2 = 170 × 1.02776 = 174.72 TPH
That screening result is an increase of about 4.72 TPH. It is not a guaranteed operating point. Natural-gas compressibility can change with temperature and pressure, so use the selected property method or operating compressibility values for the final calculation.
If the process instead holds mass flow at 170 TPH, inlet actual volume falls according to Q2/Q1 = rho1/rho2 = 0.97299, or about 2.70%. Mass flow and actual volumetric flow cannot both remain fixed while density changes.
Does constant speed mean constant actual inlet volume?
No. A centrifugal compressor at fixed speed does not automatically deliver one invariant actual volume. Flow is the intersection of the compressor characteristic and the connected system characteristic. Cooling changes gas density, the relative position on the compressor map, required head, pressure losses, and possibly the downstream operating point.
A useful similarity check is the relative corrected-speed change. With shaft speed fixed, a common temperature component of corrected speed varies as N/sqrt(T). Cooling from 296.15 K to 288.15 K raises that coordinate by sqrt(296.15/288.15) = 1.01379, about 1.38%. The exact corrected-flow and corrected-speed definitions must come from the compressor map because map conventions may include reference pressure, compressibility, molecular weight, and reference temperature.
Plot both conditions on the applicable map. If the new corrected point remains inside the operating envelope, away from surge and choke limits, the machine may accept the higher mass flow. A fully closed antisurge valve describes valve position only; it does not quantify surge margin.
Which pressure condition is actually being held constant?
The phrase “remaining all conditions the same” creates mutually different branches. Fixed suction pressure, fixed discharge pressure, fixed downstream resistance, and fixed mass demand do not lead to the same result.
| Process constraint | Likely response to cooler suction gas | Next check |
|---|---|---|
| Suction and discharge pressures controlled | The operating system may pass more mass at a changed corrected-flow coordinate. | Find the new point on the map and check driver load. |
| Downstream resistance unchanged | Flow and discharge pressure can both move until compressor and system curves intersect. | Trend actual discharge pressure; do not prescribe an increase without the system curve. |
Mass flow controlled at 170 TPH
|
The controller may throttle, recycle, or change another manipulated variable. | Trend controller output and recycle movement. |
| Actual inlet volume approximately unchanged | The ideal screening estimate is about 174.72 TPH. |
Correct for Z, map position, and measured pressure changes. |
Calculate pressure ratio only from absolute pressures. The supplied values are gauge pressures, and the local atmospheric pressure needed for conversion is not stated. The density ratio above avoids that missing value only because equal suction pressure was assumed at both temperatures.
Where can the added flow be stopped?
Trace the operating point through each constraint. First inspect the suction path: a cooler or added exchanger can introduce pressure loss, partially offsetting the density gain. Measure suction pressure at the compressor flange before and after the change rather than treating upstream header pressure as compressor pressure.
Next check the compressor map using measured suction state, gas properties, speed, and flow. A movement toward choke, a maximum-flow boundary, or a discharge-pressure limit can prevent the simple 2.78% mass-flow increase. Then check gas-turbine load and compressor power. More mass flow can require more driver power even when shaft speed remains 95%.
Finally inspect the control system. Suction-pressure control, discharge-pressure control, capacity control, or antisurge control may move a valve or load command as the gas cools. The resulting controller action determines which variable remains constant. Trend commands and feedback separately so a commanded closed recycle valve is not mistaken for confirmed mechanical closure.
How should the cooling change be tested and verified?
- Capture a stable baseline at
23 deg C: suction and discharge pressures, suction temperature, molecular weight, compressibility used by the calculation, mass flow, actual shaft speed, recycle command and feedback, and driver load. - Convert pressure and temperature inputs to absolute units inside the calculation. Confirm that the flow system uses the same gas composition and compensation basis for both tests.
- Obtain the compressor-map definitions for corrected flow and corrected speed. Plot the baseline point and calculate the proposed
15 deg Cpoint with the manufacturer’s stated corrections. - Reduce suction temperature in controlled increments while holding the intended process constraint—not merely the controller setpoint—constant. At each stable point, record compressor-flange pressure and temperature.
- Compare measured mass flow with
170 × (P2abs/P1abs) × (Z1/Z2) × (296.15/288.15)only when actual inlet volume is approximately unchanged. If measured flow diverges, use the map position, suction pressure loss, control action, and recycle leakage to locate the stopping branch. - Accept the resolving branch only after the final stable
15 deg Cpoint remains inside the compressor operating envelope and within the driver-load limit while the independent flow indication agrees with the compensated calculation.
FAQ
Can I calculate the new compressor flow from temperature alone?
Only as a screening case. With equal suction pressure, equal compressibility, unchanged composition, and unchanged actual inlet volume, cooling from 23 deg C to 15 deg C predicts 174.72 TPH from a 170 TPH baseline.
Does discharge pressure always rise when suction gas is cooled?
No. Discharge pressure rises only if the compressor and downstream system settle at a higher-pressure intersection; a pressure controller or fixed discharge-pressure process can convert the density change mainly into flow or control movement.
Can I use a fully closed antisurge valve as proof of added capacity?
No. Confirm physical closure, plot the corrected operating point, and verify surge margin, maximum-flow boundary, driver load, and an independent compensated flow reading at the final stable 15 deg C condition.