Moisture content

EPA Method 4, 40 CFR Part 60, Appendix A-3

Stack gas is part water vapor, sometimes a large part. Method 4 pulls a measured sample through ice-cold impingers, catches the water, and weighs it, so every other result can be stated on a dry basis.

Answers: What fraction of the stack gas, by volume, is water vapor?
Equipment: Heated probe, impingers in an ice bath, indicating silica gel, pump, dry gas meter, balance or graduated cylinder
Produces: The moisture fraction B_{ws}, a number between 0 and 1
Feeds into: Wet molecular weight for Method 2, isokinetic setup for Method 5, and every dry-basis concentration

Why moisture matters

Emission limits are almost always written on a dry basis: grains per dry standard cubic foot, ppm by volume dry, pounds per hour calculated from dry standard flow. The sampling equipment agrees. The impingers strip the water out before the gas reaches the dry gas meter, so the meter only ever measures the dry part of the sample.

Moisture also changes how heavy the gas is. Water (18 lb/lb-mole) is lighter than dry flue gas (about 29–30 lb/lb-mole), so wet gas has a lower molecular weight, M_s. Method 2 uses M_s to turn the pitot reading into velocity, and Method 5 uses it to set the isokinetic sampling rate. Get moisture wrong and the flow, the isokinetic rate, and the dry-basis concentration all move with it.

What moisture does to the numbers

Change the moisture and watch velocity and concentration respond.

Moisture, B_{ws}

Wet molecular weight, M_s

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Stack velocity

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Velocity error if moisture is ignored

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A 100 ppm wet reading, dry basis

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Fixed conditions: dry molecular weight 30.16 lb/lb-mole, stack temperature 350 °F, stack pressure 29.5 in. Hg, average \sqrt{\Delta p} of 0.90, and C_p = 0.84.

Ignoring moisture makes the gas look heavier than it is, so the calculated velocity comes out low. A wet-basis analyzer reading has to be divided by (1 - B_{ws}) to put it on the same dry basis as the limit.

Dry and wet concentrations

C_{dry} = \frac{C_{wet}}{1 - B_{ws}} \qquad M_s = M_d\,(1 - B_{ws}) + 18.0\,B_{ws}

M_d: dry molecular weight from Method 3, lb/lb-mole
18.0: molecular weight of water, lb/lb-mole

The reference method and the quick estimates

Method 4 describes two kinds of measurement, and they serve different purposes.

The reference method condenses and collects the water from a measured volume of gas, then weighs or measures it. This is the value you report. In most particulate tests it comes straight out of the Method 5 train, whose impingers are a Method 4 train.
Approximation methods give a quick estimate before the test so you can choose a nozzle and set the isokinetic rate. Options include wet-bulb and dry-bulb temperatures, a short run with a small condensation train, a combustion calculation, or data from an earlier test at the same source.

An approximation is a setup tool. It never replaces the reference value in the results.

The sampling train

Gas leaves the stack through a heated probe, which keeps the water as vapor until it reaches the collection system. It then bubbles through a series of glass impingers sitting in an ice bath:

Impinger 1, 100 mL of water. Most of the water vapor condenses here as the gas cools.
Impinger 2, 100 mL of water. Catches what the first one missed.
Impinger 3, empty. Traps carryover droplets.
Impinger 4, about 200–300 g of indicating silica gel. Adsorbs the last of the vapor. The gel changes color as it loads, so you can see when it is close to spent.

A temperature sensor sits at the silica gel outlet. After that come the umbilical, a vacuum gauge, a leak-free pump, and the dry gas meter that measures how much dry gas was sampled.

Impinger train during a 60-minute run

Change the stack moisture, or let the ice melt to see what goes wrong.

Stack moisture

Ice bath

Run clock

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Condensate gained

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Silica gel gain

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Gel outlet temperature

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Measured moisture

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An illustration, not a lab model. When the bath stays iced, almost all the water condenses in the first two impingers and the gel only polishes the gas. When the ice melts, more vapor reaches the gel, warm gel holds less, and the excess passes uncounted into the meter, so the measured moisture comes out low.

Running the measurement

Prepare the impingers. Add the water and silica gel, then weigh each impinger (or measure the water volume) and record the starting values.
Assemble and leak check. Pack the impingers in ice, connect the train, and leak check at about 15 in. Hg of vacuum. The leak rate limit is the same as Method 5: no more than 0.020 cfm or 4% of the average sampling rate, whichever is less.
Sample. In a particulate test the moisture train simply rides along with the Method 5 run. Run on its own, Method 4 samples at a steady rate across the Method 1 traverse points.
Keep it cold. Add ice as needed so the gas leaving the silica gel stays at or below 68 °F (20 °C).
Finish with a leak check. A post-test leak check is mandatory, at a vacuum at least as high as the highest reached during the run.
Measure the water. Weigh the impingers again, or measure the condensate to the nearest milliliter, and weigh the silica gel. The gains are the water collected.

Weigh rather than pour

Many crews weigh each impinger on a field balance instead of pouring the contents into a graduated cylinder. It is faster, avoids losing drops on the glassware, and leaves the impinger contents intact for any later analysis.

Calculating the moisture fraction

The calculation turns everything into the same units, standard cubic feet at 68 °F and 29.92 in. Hg, and then asks what share of the total was water. Liquid water is converted to the volume it would occupy as vapor at standard conditions: one milliliter of water becomes about 0.047 standard cubic feet of vapor.

Water vapor condensed in the impingers, and adsorbed on the silica gel

V_{wc(std)} = 0.04706\,(V_f - V_i) \qquad V_{wsg(std)} = 0.04715\,(W_f - W_i)

V_f - V_i: condensate gained, mL (0.04706 ft³/mL); if you weigh the impingers instead, use the weight gain in grams with 0.04715 ft³/g
W_f - W_i: silica gel weight gain, g (0.04715 ft³/g)

Dry gas volume at standard conditions, and the moisture fraction

V_{m(std)} = 17.64\,Y\,V_m\,\frac{P_{bar} + \Delta H/13.6}{T_m} \qquad B_{ws} = \frac{V_{wc(std)} + V_{wsg(std)}}{V_{wc(std)} + V_{wsg(std)} + V_{m(std)}}

V_m: dry gas meter volume, ft³
Y: dry gas meter calibration factor
P_{bar}: barometric pressure, in. Hg
\Delta H: average orifice pressure differential, in. H₂O (13.6 converts it to in. Hg)
T_m: average meter temperature, °R (°F + 460)
17.64: 528 °R ÷ 29.92 in. Hg, the standard temperature over standard pressure

Moisture calculator

Enter the field weighings. The equations update with your numbers.

Condensate measured by

Impinger water, initialmL Impinger water, finalmL Silica gel, initialg Silica gel, finalg Meter volume, V_mft³ Meter factor, Y Barometric pressurein. Hg Average \Delta Hin. H₂O Average meter temp°F

Water condensed in the impingers

Water adsorbed on the silica gel

Dry gas sampled, standard conditions

Moisture fraction

Saturated and droplet-laden gas

Gas can only hold so much water vapor at a given temperature. Above that limit the extra water exists as droplets, a fog. Downstream of wet scrubbers and quench towers the gas is often saturated and full of droplets, and the impingers cannot tell the difference: they collect droplets and vapor alike. The measured value then overstates the vapor fraction, which is what the velocity and dry-basis calculations need.

Method 4 handles this with a simple comparison. Calculate the saturation moisture at the stack temperature and pressure, compare it with the measured value, and use the lower of the two.

Saturation moisture at the stack temperature

B_{ws,sat} = \frac{p_v(T_s)}{P_s}

p_v(T_s): vapor pressure of water at the stack temperature, from steam tables or the Antoine equation, in. Hg
P_s: absolute stack pressure, in. Hg

Saturation check

Move the stack temperature and the measured moisture. Points above the curve are carrying droplets.

Stack temperature

Measured moisture

Stack pressure

Saturation moisture

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Value to use

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Vapor pressure from the Antoine equation for water. At 212 °F and atmospheric pressure the saturation fraction reaches 100%: the gas could be pure steam. Hot, dry combustion gas sits far below the curve, so the check only bites at wet, cool sources.

Acceptance criteria

Method 4 at a glance

Gas leaving the silica gel at or below 68 °F (20 °C) throughout the run.
Leak rate no more than 0.020 cfm or 4% of the average sampling rate, whichever is less. The post-test leak check is mandatory.
Impinger 1 and 2 start with 100 mL of water each; impinger 3 starts empty; impinger 4 holds about 200–300 g of indicating silica gel.
Condensate measured to the nearest milliliter (or weighed), silica gel weighed.
For saturated or droplet-laden gas, report the lower of the measured and saturation moisture values.

When Method 4 is run by itself rather than inside a Method 5 train, it adds rules on the sampling rate and minimum sample volume. Check the current method text for those values.

Field mistakes that cost runs

Letting the ice run low

On a hot platform an ice bath can melt within a run. Once the outlet climbs above 68 °F, vapor slips past the silica gel and the moisture comes out low. Check the outlet temperature at every traverse point and keep a cooler of ice on the platform.

Condensation in the probe

If the probe runs cold, water condenses in the liner. In a Method 5 train it ends up in the probe rinse instead of the impingers, so moisture reads low and the particulate catch picks up dissolved material. Keep the probe heated and check it.

Spent silica gel

If the gel has changed color all the way through before the run ends, it has stopped adsorbing and water is reaching the meter. Use enough gel for the expected moisture, and swap it between runs.

Reporting a supersaturated value

A measured 25% moisture at a 120 °F scrubber outlet is not vapor; it is droplets. Always run the saturation check on wet sources before the number goes into the velocity calculation.