Dew point calculator

Calculate the dew point from air temperature and relative humidity and check whether a chilled water pipe, tank or valve at a given surface temperature will collect condensation.

What the dew point tells you

Air always contains some water vapour. The dew point is the temperature to which the air must cool for that vapour to start condensing. Any surface colder than the dew point of the surrounding air becomes wet. In a chilled water system that means pipes, valves, pump housings, buffer tanks and the evaporator itself. Water dripping from insulation is the visible part; the invisible part is corrosion under the insulation and wet insulation that no longer insulates.

How to use the calculator

1. Measure the air temperature and the relative humidity near the cold surface, not in the office next door. A plant room or a production hall can be much more humid.
2. Enter both values. The tool shows the dew point, the vapour pressure and the absolute humidity.
3. Enter the surface temperature you want to check, for example the chilled water supply temperature for an uninsulated pipe, or the measured temperature of the insulation surface.
4. Read the margin. If the surface is colder than the dew point, condensation will form.

The Magnus formula

The tool uses the Magnus approximation, which is accurate to a few tenths of a kelvin in the range of building and plant room conditions:

g = ln(RH / 100) + a x T / (b + T)

Td = b x g / (a – g)

T is the air temperature in °C, RH the relative humidity in per cent, a = 17.62 and b = 243.12 °C. The tool also inverts the formula to show the humidity at which a surface at a given temperature starts to condense.

Typical values

Air temperature	RH 50 %	RH 60 %	RH 70 %	RH 80 %
20 °C	9.3 °C	12.0 °C	14.4 °C	16.4 °C
22 °C	11.1 °C	13.9 °C	16.3 °C	18.4 °C
24 °C	12.9 °C	15.8 °C	18.2 °C	20.3 °C
26 °C	14.8 °C	17.6 °C	20.1 °C	22.3 °C
28 °C	16.6 °C	19.5 °C	22.0 °C	24.2 °C
30 °C	18.4 °C	21.4 °C	23.9 °C	26.2 °C
32 °C	20.3 °C	23.3 °C	25.8 °C	28.1 °C

With chilled water at 6 to 7 °C, an uninsulated pipe condenses in almost any indoor climate. Even a mildly cooled circuit at 15 °C condenses on a warm humid day: at 28 °C and 60 % humidity the dew point is above 19 °C.

Plant rooms and production halls

Humidity indoors is often higher than people expect. A production hall with washing processes, a food plant or a basement plant room in summer can easily reach 70 % relative humidity at 25 °C, which is a dew point of about 19 °C. Measure in the season with the highest humidity, typically late summer, and repeat the measurement at different times of day. A data logger left near the pipes for a week gives a far better picture than a single reading.

Insulation needs

Insulation prevents condensation only if its outer surface stays above the dew point and if no moist air reaches the cold pipe. That is why chilled water pipes need a closed-cell material, such as elastomeric foam, with sealed seams and ends, and a thickness chosen for the worst local conditions. Mineral wool without a perfect vapour barrier gets wet over time and loses its effect.

• Insulate valves, strainers, pump housings and flanges as well. They are the usual points where dripping starts.
• Seal the insulation ends at every interruption, so moist air cannot travel along the pipe under the insulation.
• Check the insulation surface temperature with an infrared thermometer on a humid day; the margin to the dew point should be at least 2 K.
• In rooms with high humidity, such as food plants or areas with open water, plan thicker insulation than the standard tables suggest.

Condensation on cold parts of the chiller itself, such as the suction line, is normal on many units, but water dripping into the electrical panel or onto the base frame is not. For pipe sizes and flows see the chilled water pipe sizing tool, and for the background on water circuits our guide to chilled water systems.

Frequently asked questions

Why does an insulated pipe still drip?

Usually because the insulation has gaps, open seams or uninsulated valves, or because moist air has travelled under the insulation from an open end. Check the joints and fittings before replacing the insulation.

What surface temperature should I enter for an insulated pipe?

The temperature of the outer insulation surface, measured with a contact or infrared thermometer. For an uninsulated pipe, the fluid temperature is close enough.

How accurate is the Magnus formula?

Within about 0.1 to 0.4 K in the range from -40 to 50 °C, which is far better than the accuracy of most humidity sensors used on site. The humidity reading is the larger source of error.

Can I raise the chilled water temperature to stop condensation?

Sometimes. If the process or the coils allow a higher supply temperature, raising it improves the chiller efficiency as well. But in humid rooms the dew point is often above any useful chilled water temperature, so insulation remains necessary.