Outdoor humidity measurements for HVAC applicationsWebinar

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Meet the presenter

Lars Stormbom. Product Manager at Vaisala, with over 30 years of experience of industrial and HVAC measurements.

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Covered topics

Ways to save energy by using evaporative cooling and free cooling Selecting a suitable outdoor humidity sensor Error sources that can reduce or eliminate energy savings

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Outdoor air economizers

Conditioned airAir handling unit

Return airExhaust air

Outdoor air

Dampers

Return air sensor

Outdoor air sensor

Is it more economical to use outdoor air or return air?

Δ

H2OCO2

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1: Return air is 76°F/24.4°C, 50%RH, enthalpy is 28.7 BTU/lb

2: Cold but wet outdoor air: 72°F/ 22.2°C, 90%RH, enthalpy is 33.9 BTU/lb should not be used for cooling

3: Hot but dry outdoor air: 85°F/ 29.4°C, 15.4%RH, enthalpy is 24.7 BTU/lb. Should be used instead of return air

The Economizer logic

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Outdoor air economizers

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Conditioned airAir handling unit

Return airExhaust air

Outdoor air

Dampers

Return air sensor

Outdoor air sensor

Δ

Benign conditions ex. 50%RH 76°F Humidity sensors work well

Difficult conditionsfor Humidity sensorsHigh humidity may cause drift

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What do the standards say? ASHRAE 90.1:

The system shall have an air economizer. Fixed enthalpy or differential enthalpy control is suitable for all climate zones

Required measurement accuracies for humidity sensors:– Dry-bulb and wet-bulb temperatures shall be accurate to ±2°F (1.1°C) over the range of 40°F

to 80°F (4.4 … 26.7°C).– Enthalpy and the value of a differential enthalpy sensor shall be accurate to ±3 Btu/lb over the

range of 20 to 36 Btu/lb.– Relative humidity shall be accurate to ±5% over the range of 20% to 80% rh.

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How much is too much?

Starting condition 70°F/21°C and 50%RH (25 Btu/lb / 40.8 kj/kg) Enthalpy error caused by:– 2°F/1.1°C temperature measurement error: 1.1 Btu/lb / 2.5 kj/kg– 3.6°F/2°C temperature error due to solar heating: 0.9 Btu/lb / 2.0 kj/kg– 5%RH measurement error due to drift or calibration: 0.9 Btu/lb / 2.0 kj/kg

Starting condition 86°F/30°C and 50%RH (36 Btu/lb / 64.3 kj/kg) Enthalpy error caused by:– 5%RH measurement error due to drift or calibration: 1.5 Btu/lb /3.5 kj/kg

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Cooling towersCooling towers can in some climates provide significant free cooling by evaporative cooling.

In some cases Wet-Bulb control is needed to determine when it is economical to run the cooling towers

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Some points to consider For large sites with individually controlled

zones or several cooling towers one good outdoor sensor beats several low-quality sensors

The common outdoor humidity sensor becomes one of your most important sensors that influences airflows in the whole site

Install and forget is not a good maintenance strategy for outdoor sensors. At least periodic checking is needed!

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Better: A common outdoor humidity sensor

Return air sensorΔ

Return air sensorΔ

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What to look for

A solar radiation shield with blackundersides reduces solar heating

Sturdy constructioncan take high winds and icing conditions

High quality humidity sensor

Pole mountingor wall mounting with enough offset

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Error sources: Solar radiation heating

Solar radiation can easily heat up sensors by 4-5°F (2°C).

Mitigation:• Use sensor with proper radiation shield• Mount in a location with unhindered air flow as wind will

reduce heating• A dirty shield with heat up more => cleaning may be

needed especially in urban locations• North facing walls likely better

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Error sources: Rain and condensation

Liquid water on or close to the sensor due to rain or condensation. Can cause drift of sensor and too high humidity readings.

Mitigation:• Use a sensor designed to work in humid outdoor

conditions• Good airflow (wind) will reduce effect• For industrial level applications use heated probe

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Error sources: Heavy weather

Heavy weather may cause damage to the sensor.

Mitigation:• Use a sensor designed to work in outdoor

conditions• Mount in area with unhindered air flow

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Error sources: Heat from building wall

Heat radiating from wallor black roofing:

Mitigation:• Use pole mounting when possible• Avoid dark surfaces in direct sunlight• Do not mount under eaves

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Error sources: Building exhaust air

Exhaust air

Indoor air gets into the outdoor sensor

Mitigation:• Mount away from exhaust duct• Do not mount over doors or windows that can

be opened

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Error sources: Temperature differences

Air handling unit

Outdoor air

Large temperature difference inside and outside the duct. Heat (or cold) can flow to the humidity and temperature sensor distorting the measurement or even cause condensation.

Mitigation:• Enough air flow in duct• Insulate transmitter body• Mount transmitter outside controlled area (= same conditions as outside)

86°F/30°C 90%RH

70°F/21°C

condensation!

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Error sources: Leaks

Air handling unit

Outdoor air

86°F/30°C 90%RH

70°F/21°C

Leaks

Leaks in the transmitter mounting cause conditioned air to leak into the duct and distort measurement. This effect is more important than with normal duct transmitters as the duct conditions differ so much from those outside

Mitigation:• Proper leak proofing

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Error sources: Duct air temperature change

Air handling unit

Outdoor air

86°F/30°C 90%RH

70°F/21°C

condensation!

Outside air heats up or cools down significantly before the duct transmitter installation point. Especially pronounced with low flow rates

Mitigation:• Enough air flow in duct• Mount duct transmitter as close to inlet as possible

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The main issue: Humidity sensor stability• 3 Humicap sensors in outdoors use for 12 years • Calibration checks at 0%RH and 75%RH• All results within ±1%RH!

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The weather is not always fair!

• High wind speeds (40 m/s)• Icing rain

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The ultimate solution : Heated probe

Ambient temperature sensor

Heated humidity probe avoids condensation issues

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Conclusion

Outdoor humidity measurement is:

1. USEFULL: Outdoor humidity data is usefull for optimal energy savings2. DEMANDING: The outdoor humidity measurement is more demanding

than a normal indoor humidity measurement.

Use quality sensors and keep them in good working order!

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Make calculations easy ...

The free Vaisala Humidity Calculator does Enthalpy and many other humidity parameters!http://www.vaisala.com/en/services/technicalsupport/downloads/HumidityCalculator

Works in any browser on mobiles and PC

Thank you