Where the water comes from
Nowhere, is the short answer. It was already in the air. A hundred cubic feet a minute of intake air at 85 degrees and 75 percent humidity is 424 pounds of dry air an hour carrying 0.0195 pounds of water on every pound, which is 8.25 pounds of water an hour walking into the machine. Over a ten hour day that is 82.5 pounds, or 9.9 gallons, and none of it was made by the compressor.
What the compressor does is take away the room. Squeeze the air to 124.7 psia and it occupies an eighth of the volume it did, but the water does not compress into a liquid on its own — it simply exceeds what the air at that density and temperature can hold, and the surplus condenses.
The split between the aftercooler and the dryer
At the defaults, the aftercooler cools the air to 100 degrees at line pressure and drops it from 136 grains of water per pound of dry air to 33. That is 6.23 pounds an hour of condensate, 76 percent of everything that came in, and it happens inside a few feet of the discharge.
Then the dryer takes it from a 100 degree saturation down to a 38 degree pressure dew point, which is 33 grains to 3.9, and 1.78 pounds an hour of condensate. Between them, 8.02 pounds an hour, 9.6 gallons a day and 2,403 gallons a year at 250 working days.
The aftercooler doing three quarters of the work is normal and it explains why an undersized or fouled aftercooler is such an expensive fault. The water it fails to condense arrives at the dryer, which was not sized for it, and the dryer passes what it cannot handle down the pipe.
Pressure dew point is not dew point
A dryer data sheet quotes a pressure dew point, and it is measured with the air still at line pressure. Expand the same air to atmosphere and the same water is spread through eight and a half times the volume, so the dew point falls hard. The 38 degree pressure dew point in the defaults is a minus 6.2 degree atmospheric dew point at 110 psi.
That 44 degree gap is why compressed air dried to a dew point above freezing does not fog when it leaves a blow gun, and it is also why a specification written in atmospheric terms and a data sheet written in pressure terms can look wildly inconsistent while describing the same air. Check which one a number is before comparing it with anything.
The intake is the lever
Run the sensitivity block and the pattern is obvious. Ten degrees cooler at the intake takes about a quarter off the condensate; ten degrees warmer adds about a third. Twenty points of humidity either way moves it by a similar amount. The reason is that the water air can hold climbs steeply and non-linearly with temperature — near enough doubling every twenty degrees over the range a compressor room sits in.
Which makes the intake location the only change on this page that reduces the water rather than moving where it is caught. A machine breathing 100 degree compressor room air on a July afternoon is pumping substantially more water than the same machine drawing from shade outside, and it is doing it at exactly the time of year the dryer is already working hardest.
What this does not settle
Condensate from an oil-lubricated machine is not clean water. It carries lubricant, and where it is allowed to go is a question for your local authority and whatever discharge permit covers the building. There is no guidance here on that, deliberately. Nor does the page say anything about whether a given dew point is adequate for your work — a plasma cutter, a spray booth and a nail gun have completely different answers, and all three come from the equipment rather than from arithmetic.
Questions people ask
How much water does an air compressor produce in a day?
For 100 cfm of intake air at 85 degrees and 75 percent humidity, running ten hours, about 9.6 gallons. Change the weather and it moves a lot: the same machine on a cool dry morning makes a fraction of that, and on a humid August afternoon in a hot compressor room it makes more. The arithmetic is entirely about what the intake air was carrying, which is why the page asks for the intake conditions rather than the machine model.
Why does most of the water come out at the aftercooler?
Because that is where both things happen at once — the air is at full line pressure and it is being cooled hard. At the defaults it goes from 136 grains of water per pound of dry air to 33, which is three quarters of the total, within a few feet of the discharge. The dryer only has the remaining quarter to deal with, which is why it can be much smaller than the numbers first suggest, and why a fouled aftercooler overwhelms it.
What is the difference between pressure dew point and atmospheric dew point?
The pressure the measurement is taken at. Air held at a 38 degree pressure dew point at 110 psi has an atmospheric dew point near minus 6 degrees, because expanding it spreads the same water through eight and a half times the volume. Dryer data sheets quote pressure dew point; general humidity specifications often mean atmospheric. Comparing one with the other without converting is the most common mistake in this corner of the subject, and the page prints both so it cannot happen.
Will a bigger dryer stop water reaching my tools?
Only if the dryer is what is failing. Water at a tool comes from air that was warmer or wetter than the dew point somewhere downstream of wherever it was last dried, so the usual causes are an aftercooler that is not getting the temperature down, a drain that has stopped working, a drop leg taken off the bottom of a main instead of the top, or a run of cold pipe after the dryer. Working out the condensate load first tells you whether the dryer was ever the problem. Nothing here is a verdict on a specific machine.
Does altitude change how much condensate I get?
Yes, in two directions at once, which is why the barometric field is there. Thinner air at the intake means fewer pounds of dry air per cubic foot, so less water walks in for the same cfm. But the compression ratio to reach the same gauge pressure is higher, which squeezes harder. Put the local barometric pressure in and the page handles both. At 5,000 feet, near 12.2 psia, the two effects do not cancel and the answer moves noticeably.