What Intake Conditions Do to Compressor Performance
Why this matters
A compressor's rating is a statement about the air going into it, not a property of the machine. Move the same machine to a hotter room, put a loaded filter in front of it, or truck it up a mountain and its nameplate does not change while the air it delivers does. Shops chase this backwards constantly: a plant that has slowly lost capacity gets a compressor overhaul or a capacity quote when the actual finding is a 25 degree intake temperature rise and a filter nobody has changed. The intake survey below takes about twenty minutes, needs a thermometer and a differential gauge, and is the cheapest finding available on any compressed air call.
Reading intake conditions means working around a running machine. A machine sitting quiet in auto is not off: its pressure switch will start it with nobody standing at it, which is the hazard here rather than the running machine. Take intake readings either with the machine deliberately held loaded and a second person at the panel, or with the disconnect locked open under 29 CFR 1910.147. Do not put a probe into the intake of a machine that is merely idle. Keep clear of couplings, belts and cooling fans, take readings at the manufacturer's provided ports from outside the enclosure wherever they exist, and never reach into an open package while it is energised. Wear hearing protection under a program meeting 29 CFR 1910.95 for the time you are in the room. When a temperature probe goes into the intake stream, secure the probe and its lead mechanically so neither can be drawn in; a probe pulled into an airend destroys it.
The one relationship: capacity is set at the intake
A positive displacement compressor sweeps a fixed volume per revolution. What varies is how much mass is in that volume, and mass is what the plant uses. So delivered capacity tracks intake density, and intake density is proportional to the absolute intake pressure divided by the absolute intake temperature.
Two conversions to fix before anything else, because every figure below rests on them:
- Absolute temperature in degrees Rankine is degrees Fahrenheit plus 460. That rounded offset is fine here; the more precise 459.67 matters where a calculation raises temperature to a fourth power, which nothing in this article does.
- Absolute pressure. At sea level the standard atmosphere is about 14.7 psia. At about 5,000 feet it is about 12.2 psia. Intake restriction subtracts from whatever the local barometric pressure is, and 1 inch of water gauge is 0.03613 psi.
There is a second effect stacked on top of the density one. Compression ratio is absolute discharge pressure divided by absolute intake pressure, and the work per unit of air rises with that ratio. So anything that lowers intake pressure both reduces the mass delivered and raises the work per unit delivered. The two effects push the same way, which is why intake restriction costs more than the density arithmetic alone suggests.
The three intake variables, and the direction each pushes
Temperature. Hotter intake air is less dense, so delivered mass falls. It also carries more water vapour into the machine, which lands as more condensate downstream and more load on the dryer; the moisture articles in this library own that half. Hotter intake also raises discharge temperature, which pushes an oil-injected machine toward its high-temperature protection.
Pressure. Lower absolute intake pressure, whether from altitude or from restriction across the filter and inlet piping, reduces delivered mass and raises compression ratio. Restriction is the part you control; altitude is the part you design around.
What is in the air. Dust that gets past a compromised filter goes into the airend and shortens its life. Water drawn in from a poorly sited intake does the same. Neither of these shows up as a capacity number, which is exactly why they get missed by anyone who only measures flow. A filter that is bypassing because it is torn or badly seated will show a low differential and a healthy capacity right up until the machine fails.
The intake survey record
Ten fields. Fill every one, because the value of this record is comparison against the next visit, and a blank field breaks the comparison.
| Field | What it is and how you get it |
|---|---|
| Date and hour-meter reading | Anchors the record to the machine's life, not the calendar |
| Intake air source | Room air, ducted from outdoors, or ducted from another space. Name it |
| Outdoor dry bulb at the time | For comparison against the intake reading |
| Intake temperature | Measured in the stream at the filter inlet, probe secured |
| Filter differential at full load | Across the element, machine loaded, not idling |
| Filter differential when new | From the package data, so you know how much of the reading is the element |
| Site elevation and local barometric pressure | Sets the absolute intake pressure before restriction |
| Discharge pressure setting | Needed for the compression ratio |
| Observed discharge temperature | The stacked consequence of a hot, restricted intake |
| Filter element condition, by eye | Seated, intact, no bypass path. This is the field with no number |
Filled in: one plant at sea level
A single-shift machine shop, one oil-injected rotary screw in a small room off the shop floor, complaint of gradually falling air pressure over about two years with no new equipment installed.
The record reads: intake source is room air, drawn from the compressor room itself. Outdoor dry bulb 70 F. Intake temperature at the filter inlet 95 F. Filter differential at full load 10 inches water gauge, against 3 inches when new. Site elevation is near sea level, local barometric about 14.7 psia. Discharge setting 100 psig. Element by eye: loaded, seated correctly, no bypass path.
The temperature term. Outdoor air at 70 F is 530 R. Room air at 95 F is 555 R. The density ratio is 530 divided by 555, which is 0.955. Drawing 95 F room air instead of 70 F outdoor air costs 4.5 percent of delivered mass, and the machine's nameplate was written at a reference condition nearer the cooler figure.
The pressure term. Ten inches of water gauge is 10 times 0.03613, or 0.361 psi. Absolute intake pressure is 14.7 minus 0.361, which is 14.34 psia. The ratio to unrestricted is 14.34 divided by 14.7, which is 0.9754, so the total restriction costs 2.5 percent of delivered mass. Now use the field you collected for exactly this: three of those ten inches were there when the element was new, and the machine was rated with them in place. The recoverable share is the seven inches the element has added, 0.253 psi, against a clean intake of 14.592 psia, which is 0.9827, or about 1.7 percent. That is what a new element buys back; 2.5 percent is what the intake path as a whole costs.
Both together. 0.955 times 0.9754 is 0.9315. This machine is delivering about 93 percent of what it would deliver on clean 70 F air, so 6.9 percent has gone missing without anything mechanical failing.
The work term on top of it. Absolute discharge at a 100 psig setting is 114.7 psia. Against a clean 14.7 psia intake, compression ratio is 7.80. Against the restricted 14.34 psia intake, it is 8.00, a 2.5 percent increase, of which about 1.8 points is the element and the rest is the intake path the machine was rated with. So the machine is making 6.9 percent less air while each unit of that air costs slightly more shaft work than it did when the filter was new.
What the shop does with that. Two actions. Change the element first, not because it is the larger recovery, because it is not, but because it is a stock item, it takes an hour, and it removes the element's share of the compression-ratio penalty along with its 1.7 percent of mass. Ducting the intake from outdoors is the bigger number, recovering the 4.5 percent on a 70 F day and more on a cold one, and it also stops the machine feeding on its own rejected heat. The article on compressor room ventilation in this library owns why the room is at 95 F in the first place, and it is worth reading before ducting, because a room whose ventilation is marginal gets worse when you take the compressor's intake flow out of it.
The failure mode of skipping this. The plant reads the same 6.9 percent as a worn machine and buys capacity. The new machine draws the same 95 F air through a filter that will load at the same rate, gives back its own 6.9 percent within two years, and the plant now runs a larger machine at part load in a hotter room.
The same machine at 5,000 feet
Run the identical numbers at elevation, because this is where the intake relationship stops being a maintenance item and becomes a sizing decision.
Local barometric pressure near 12.2 psia against 14.7 at sea level is a density ratio of 0.830 at the same intake temperature. That is 17 percent less delivered mass from altitude alone, before any restriction or temperature rise.
The compression ratio moves further. Holding the same 100 psig gauge setting the plant actually needs, absolute discharge is 112.2 psia and the ratio is 112.2 divided by 12.2, which is 9.20, against 7.80 at sea level. That is 17.9 percent more compression ratio and correspondingly more work per unit of air.
Both terms point the same direction, and that is the part that surprises people: at altitude you get less air and each unit of it costs more. A machine specified off a sea-level catalogue figure and installed at 5,000 feet is short by a margin large enough to be the entire complaint. Manufacturers publish derating data for their own machines and that data, not this arithmetic, is what belongs on a quote; the arithmetic is here so you can tell before you call whether altitude is worth asking about.
When an intake problem is not a capacity problem
The survey's last field has no number in it, and it is the one that catches the failures the arithmetic cannot see.
A filter element that is torn, that is not seated on its gasket, or whose housing has been closed on a fold of the element will read a low differential and give an apparently healthy capacity while passing unfiltered air into the airend. That is a wear path, not a performance path, and it presents months later as high oil consumption, rising discharge temperature, or an airend that ends its life early. Look at the element and the sealing face every time, and treat a differential that has dropped since the last visit without an element change as a bypass finding, not good news.
An intake sited where it can draw water, whether from a roof duct without a hood, from a wall louvre that gets wind-driven rain, or from a floor-level opening in a wash-down area, ingests liquid into a machine designed for vapour. And an intake sited in the package's own discharge airstream feeds the compressor its own rejected heat, which is the most common single cause of an intake temperature well above room temperature and is a layout error, not a maintenance one.
When you replace an element, shut the machine down, isolate and lock and tag the electrical disconnect and the air outlet under 29 CFR 1910.147, relieve the sump pressure by the manufacturer's stated procedure before opening any intake path on an oil-injected machine, and confirm zero at a gauge open to the sump. Bag the old element rather than blowing it out; blowing accumulated dust off an element makes a respirable aerosol from whatever the plant has been breathing past, and where that accumulation is significant the control is respiratory protection selected under a program meeting 29 CFR 1910.134, not gloves and glasses.
References
- 29 CFR 1910.147, control of hazardous energy, general industry: the basis for the isolate, lock, tag, relieve and verified-zero sequence before opening an intake path or sump on a compressor package.
- 29 CFR 1910.95, occupational noise exposure, general industry: the basis for hearing protection while taking readings on a running machine in a compressor room.
- 29 CFR 1910.134, respiratory protection, general industry: the basis for the respiratory control required rather than blowing out a loaded intake element.
- Compressor package manufacturer's derating and technical data, for the machine's reference intake condition, its as-new filter differential, and its published altitude and temperature derates.
- See related: Why a Compressor Room Needs More Ventilation Than It Has; Where the Water Comes From and Where It Goes; What Heat Recovery From a Compressor Can Realistically Deliver.