Where the Water Comes From and Where It Goes

Why this matters

Water in a compressed air system is a closed account. Every pound that comes in with the intake air leaves somewhere, and there are only four places it can go. Three of them are drains you fit on purpose. The fourth is the tool, and it is the default: whatever the first three do not take, the tools receive. Most shops have never seen the account written down, which is why they argue about the dryer while the largest term in the budget leaves through a fitting nobody looks at, and the smallest term ruins the work.

Before you open any drain path

A drain leg, a filter bowl and a receiver all hold stored pressure energy after the compressor stops. To open one, close its upstream isolation valve, open the vent, confirm the section gauge reads zero with the vent still open, and lock and tag that isolation under 29 CFR 1910.147 before a wrench touches the joint.

To operate a manual drain rather than open one, stand to the side of the discharge path with it directed into a container or a floor drain, wear eye protection and hearing protection, and never look into the path or point it at a person: air driven into a break in the skin can enter the bloodstream. Air rooms commonly reach the 85 dBA eight-hour time-weighted average action level, at which point hearing protection and the rest of 29 CFR 1910.95 apply. Never clear a blocked drain line with a blow gun. 29 CFR 1910.242(b) permits compressed air for cleaning only when reduced to less than 30 psi and then only with effective chip guarding and personal protective equipment, and a plugged drain line is cleared by replacing it.

If a drain leg on an outdoor or unheated run has frozen, isolate and verify zero before doing anything about the ice, then thaw it with heat trace listed for the piping or another controlled electric method used per that listing. Never put an open flame on a pressurised line, and never on a plastic line at any pressure, because heated plastic piping releases decomposition products and a flame on a frozen pressurised line raises pressure in a section whose vent may itself be blocked.

The account has one input

Water enters exactly one way: dissolved as vapour in the air the compressor draws. Nothing else in the system makes water.

The quantity is the mass of air you move times the water each pound of that air was carrying, and the second term comes off a psychrometric chart at the intake's temperature and relative humidity. It is not a small number, and it is not stable: the amount of water saturated air holds roughly doubles for every 20 F rise, so the same machine on a humid summer afternoon brings in several times what it brought in on a cold morning.

The intake is also the one place in the system where every option is free. An intake drawing from a cool, dry, shaded outside wall brings in less water than one drawing from the ceiling of a hot production floor, and neither costs anything to run.

The four exits

 intake  --> [ airend ] --> [ aftercooler ]
                                  |
                        exit 1: wet receiver drain
                                  |
                             [ dryer ]
                                  |
                          exit 2: dryer drain or purge
                                  |
                    warm branch --+-- cold branch
                        |               |
                  no exit needed exit 3: drip leg
                                          |
                                no exit fitted: the tool

Exit 1, the aftercooler and wet receiver drain. The airend delivers hot air holding all its water as vapour. The aftercooler drops the temperature and most of the load falls out as liquid immediately. This is normal and expected, not a fault.

Exit 2, the dryer's drain or purge. Whatever survives exit 1 arrives at the dryer, which takes the air down to its rated pressure dew point and sheds the difference.

Exit 3, the low points in distribution. Anything that condenses because a length of pressurised pipe is colder than the dew point of the air in it. If every run stays warm, this exit carries nothing. If a branch leaves conditioned space, it carries whatever the temperature difference produces, and it only carries it if a drip leg with a working drain was fitted there.

The fourth is not an exit you fit. It is the tool, and it takes the remainder. Some of that remainder is harmless vapour. The part that is not harmless is liquid formed in a cold run with no drip leg under it, which arrives as slugs.

The budget for one shift

A 100 scfm package serves a plant. Over an 8 hour shift it runs loaded for 6 hours, so it moves 100 x 60 x 6 = 36,000 standard cubic feet. At the standard-air density of 0.075 lb per cubic foot referenced to 68 F and 14.7 psia, that is 2,700 lb of air.

Intake conditions are 90 F at 60 percent relative humidity. Read off a sea-level psychrometric chart, that air carries about 0.019 lb of water per lb of dry air, so the shift brings in 2,700 x 0.019 = 51.3 lb of water, which is 6.15 gallons.

To follow it through the system, one relationship does all the work: at a fixed temperature, the mass of water a given mass of air can carry falls approximately in proportion to absolute pressure. That approximation holds where the saturation vapour pressure is small next to the total pressure, which is true everywhere in this example and would not be near boiling or at low pressure. Discharge is 100 psig, so the absolute ratio against a 14.7 psia intake is 114.7 / 14.7 = 7.8.

Exit 1. The aftercooler brings the air to 100 F at line pressure. Saturated air at 100 F holds about 0.0432 lb per lb at atmospheric, so at 7.8 times the absolute pressure it holds 0.0432 / 7.8 = 0.00554 lb per lb. Carried onward: 2,700 x 0.00554 = 15.0 lb. Dropped here: 51.3 - 15.0 = 36.3 lb, which is 4.35 gallons, or 70.8 percent of the shift's water.

Exit 2. A refrigerated dryer rated at a 38 F pressure dew point. Saturated air at 38 F holds about 0.00485 lb per lb at atmospheric, so at line pressure 0.00485 / 7.8 = 0.000622 lb per lb. Carried onward: 2,700 x 0.000622 = 1.69 lb. Dropped here: 15.0 - 1.69 = 13.3 lb, which is 1.59 gallons, or 25.9 percent.

Exit 3. One branch runs through an unheated dock and the pressurised metal reaches 28 F on a winter morning. Saturated air at 28 F holds about 0.0031 lb per lb at atmospheric, so 0.0031 / 7.8 = 0.000397 lb per lb at line pressure, and 2,700 x 0.000397 = 1.07 lb can stay as vapour. Condensed in that branch: 1.69 - 1.07 = 0.62 lb, which is 0.074 gallons, or 1.2 percent.

The remainder. 1.07 lb, 0.128 gallons, 2.1 percent, leaves with the air as vapour and does no harm at all.

The four terms are 36.3, 13.3, 0.62 and 1.07 lb, summing to 51.29 lb against the 51.3 that entered, so the account closes.

What the budget actually tells you

The largest term is not the dryer. More than two thirds of the shift's water left at the aftercooler and wet receiver, before the dryer saw any of it. A plugged or ignored drain there does not slightly increase the dryer's load, it multiplies it: send 36.3 lb into a dryer that was expected to shed 13.3 and it will not hold its rated dew point, and the fault will be written up as a dryer problem.

The term that ruins the work is the smallest one. The 0.62 lb in the cold branch is 1.2 percent of the shift's water and it is the only term that reaches a tool as liquid. Everything larger left through a drain. So the size of a term tells you almost nothing about the damage it does; what matters is whether an exit was provided where the water forms.

The remainder is not a failure. The 1.07 lb leaving as vapour is the air performing exactly as specified. A plant chasing that last term is buying a dryer class it does not need.

The failure mode this exposes. A shop that finds water at a tool goes to the dryer, because the dryer is the machine whose job title says moisture. In this budget the dryer is doing 25.9 percent of the work and is not where the failure is. The two places to look first are the exit carrying 70.8 percent, which is a drain most people have never operated, and the cold branch with no drip leg, which is not in the air room at all.

What moves each line

The intake line moves with the weather and with where the intake sits. Saturated air at 50 F holds about 0.0077 lb per lb, so at 60 percent relative humidity the intake term falls to roughly 0.0046 lb per lb and the shift's input drops by about a factor of four, which is why a winter commissioning reading proves nothing about August.

Exit 1 moves with cooling. The aftercooler's outlet temperature is set by its approach to the cooling medium, so a fouled coil, a hot air room or a blocked airflow path raises it and pushes load downstream. Take the approach from the manufacturer's data rather than assuming ambient.

Exit 2 moves with the dryer's delivered dew point, which is not its nameplate and has to be measured at line pressure under load.

Exit 3 moves with the coldest pressurised metal, which is a pipe walk and not a calculation.

How to verify you got this right

Catch the condensate. Put a container under the wet receiver drain for a measured period on a warm day, using the stand-aside procedure above, and compare what you collect against the exit 1 line in your own budget. Agreement inside a factor of about two is a good result at this level of approximation, because the intake humidity is an estimate and the aftercooler outlet temperature is a spot reading. A collection that comes in at a small fraction of prediction means the drain is passing air rather than water, which is its own problem, or that the aftercooler is not cooling.

Then check the account closes. Add your four exits and compare against the input. If the exits sum to materially less than the input, the missing water is going somewhere you have not identified, and the tools are the default destination.

References

• 29 CFR 1910.147, control of hazardous energy, for isolating and verifying depressurisation of a drain leg, filter bowl or receiver before opening it
• 29 CFR 1910.242(b), compressed air used for cleaning, and 29 CFR 1910.95, occupational noise exposure, including the 85 dBA eight-hour time-weighted average action level
• A sea-level psychrometric chart for saturated humidity ratios at the intake, aftercooler outlet and dew point temperatures used above
• Aftercooler and dryer manufacturer data for approach temperature, rating conditions and correction factors
• See related: Why Moisture Is the Central Problem in Compressed Air; What Pressure Dew Point Means and Why It Is the Specification; How to Tell Whether a Dryer Is Doing Its Job