Introduction to Steam, Humidity and Water Quality for Steam Generators
What is Steam? In engineering, steam refers to vaporized water. It is a chemically pure, invisible gas (not a mist) which, at standard atmospheric pressure (1 atmosphere), has a temperature greater than 100 degrees Celsius. It occupies about ~1,600 or more volume than the same mass of liquid water. Steam can, of course, be at a much higher temperature than the boiling point of water at any pressure. Such steam is referred to as high-temperature or superheated steam. Steam is a capacious reservoir for energy because of water’s high heat of vaporization. Steam can form at any pressure, but the boiling point (called the phase change temperature) increases with pressure. Beyond (>647.096K and >22.064 MPa), steam is neither gas nor water but is found in the supercritical state, a particular state described further below.
What is Humidity? There are several ways to understand the Humidity of the air (a term generally used only below 100°C, at 1 atmosphere). Water vapor and air have limited solubility below 100°C. Above the saturation amount that can mix in the air as water vapor, the excess water vapor will condense to water. Relative Humidity (RH): compares how much water vapor is present in the mood to how much water vapor would be in the air if the air were fully saturated. Relative Humidity is a ratio that compares the amount of water vapor in the air with the amount of water vapor present at saturation. The Relative Humidity is given as a percentage: the amount of water vapor is expressed as a percent of saturation. The saturation amount increases with temperature. At high temperatures above the boiling point, it is more appropriate to talk about Specific Humidity (also called the humidity ratio), which is the mass of water vapor present in a unit mass of dry air; that is, it is the ratio of the water vapor to the mass of dry air. For example, at 25oC and one atmospheric pressure, if the Specific Humidity is between 2 (i.e., two gms of water vapor and 100 gms of dry air), the relative Humidity (RH) is at 100%. At 30 °C (86 °F), for example, air volume can contain up to 4 percent water vapor. However, at -40 °C (-40 °F), it can hold no more than 0.2 percent. The equations for water content, partial pressure, and enthalpy of moist air are given in the adjacent column.
|Brief History of Steam Production Methods
|Nature of Device
|Who developed it first?
|Late 1700 –
(about 0.5-2.0 lbs of CO2 per 3414 BTU of heat energy from fossil fuels)
|Combustion fired steam-boilers.
|These were the earliest boilers.
Source Wiki: Stirling Boiler Company, Ohio, USA. Later merged with Babcock and Wilcox.
Saturated steam at low temperature.
(about 0.5-2.0 lbs of CO2 per 3414 BTU of heat energy from fossil fuels)
|Higher pressure. Combustion-fired and some electric boilers.
|Several manufacturers. Steam generation required high-pressure boilers and was bulky.
Advances were made post World War II. primarily for size and safety. Low-quality steam.
Zero NOx, Zero CO2. Deep Decarbonization Product.
|Electric high-temperature. Steam generators
|850°C and higher
Instant Steam. Variable flow. Variable pressure. Variable temperature. Steam above the inversion temperature.
|Compact Modern OAB® Steam Generators
|MHI Inc. USA.
Compact, Instant, and Reliable.
Now the control of steam processes and humidity is easy. The instant steam generators directly sense humidity signals to regulate the amount of water conversion.
Modern, high-quality superheated steam is employed for several critical applications. High-temperature steam is helpful for pharmaceutical, biopharmaceutical, comfort, utility, and chemical uses because of a lack of water droplets. Noncondensing superheated steam is the most energy-efficient method of steam use. Using dry superheated steam above the inversion temperature of steam leads to the best antimicrobial and most rapid drying steam applications. The wicking properties and oxygen control of this type of steam are attractive features of high-temperature steam. High-temperature steam availability leads to high-productivity applications. Steam generator products are classified into two types, namely:
What is superheated steam? Steam can be saturated or superheated. When at the boiling temperature (which depends on the pressure), the type of steam is called saturated steam. Saturated steam is often a mixture of water and gas. When above the boiling temperature, it is called superheated steam, especially when water droplets are no longer present (i.e., dry or high-quality steam). At sea level and one-atmosphere pressure (101KPa, 1 bar), steam boils at ~100°C (~212°F), the saturated steam temperature for this pressure. At 1-bar, above this ~100°C temperature. The steam is superheated above the corresponding boiling point at a lower or higher pressure than 1 bar. Steam generators make superheated steam. Modern steam generators use sophisticated electric and computer controls, making them versatile for different applications from chemical reactions, dry cleaning, and antimicrobial use to dry ores and wet materials, including food drying, efficiently. Modern steam generators offer significant benefits in downstream efficiencies and dryness.
What is Supercritical Steam: There’s a unique mix of temperature and pressure – called the critical point – where the difference between liquid and gas ceases to exist. This happens at 374 °C (705 °F) and 218 Bar for water. This type of steam forms beyond the critical point (>647.096K and >22.064 MPa). About ~30% of free monomeric H2O molecules exist at the critical point. The rest contain different types of bonds of H2. Supercritical water has very low surface tension with its gas or liquid phase; therefore, no interfaces can delineate the liquid/gas interface. Above 647.096K, supercritical steam cannot be liquefied by increasing the pressure. Supercritical steam is used for generating power.
Another example of a supercritical material is Supercritical CO2 (above 304.13 K and 7.38 MPa). CO2 in this state is used to decaffeinate coffee beans. Its viscosity and diffusivity are like a gas, penetrating the beans quickly. However, its density is like that of a liquid. It binds to caffeine (this property is much more important than its supercritical fluid properties). Supercritical CO2 is also used in dry cleaning.
A pound of water x 0.016 = cubic feet at 62.2 °F.
A pound of water x 0.12 = gallons of water
1 gallon of H2O liquid = 8.33 lb. @ 62.2 degrees FahThe mineral content of calcium and magnesium in the water determines the hardness of water(°C)
The freezing temperature of water at one Bar is 32 Fahrenheit (°F) = 0 Celsius (°C) (Very mild pressure dependence)
Soft and Hard Water:
A boiler feedwater treatment system usually incorporates the following:
- Reverse osmosis (RO).
- Primary ion exchange.
- Deaeration or Degasification.
Feedwater is piped to a steam generator to form continuous high-temperature dry steam. The condensate may be combined with treated makeup water and re-fed.
Softening. Water softening removes hardness due to calcium and magnesium in the water. A softening resin accomplishes it, typically a strong acidic resin, that allows it to capture and remove hardness ions from the stream effectively.
Makeup Water Intake. Replacement or makeup is drawn from treated city supplies or raw water treatment systems. Steam generators can use the condensate return. Water to a steam generator must be to the specifications of RO and DI water.
Filtration Typically, water is filtered to remove sediment, turbidity, and organic material. Membrane filtration units may be the most cost-effective when used for pretreatment.
Deaeration or Degasification Following all other treatment steps, the makeup water and condensate from the boiler system are combined and removed gasified for corrosion prevention.
- Water Hardness Ratings:
The mineral content of calcium and magnesium in the water determines the hardness of the water Water softness description is reported as Grains/Gallon or Parts/Million (ppm).
- Less than 1 GPG or less than 17.1 ppm is soft
1.0 to 3.5 GPG or 17.2 to 60 ppm is slightly hard
3.6 to 7.0 GPG or 61 to 120 ppm is moderately hard
7.1 to 10.5GPG or 121 to 180 ppm is hard
10.6 GPG & over 181 ppm & over is very hard
|Resistivity at 20°C (ohm-m)
|Conductivity at 20°C (S/m)
|2×101 to 2×103
|5×10−4 to 5×10−2
|109 to 1015
|10−15 to 10−9
More soap is required when washing with hard water compared to hard water.
DI water is very soft. DI-grade water is purified with almost all its mineral ions removed, such as cations like sodium, calcium, iron, and copper, and anions like chloride and sulfate. The DI process leverages specially manufactured ion-exchange resins that exchange hydrogen (H+) and hydroxyl (OH-) ions for dissolved minerals and then recombine to form water (H2O). Primary Ion Exchange – Deionizers may be used instead of membrane filtration for large volumes of water or high-pressure boilers. Ion exchange typically produces water of comparatively higher quality and resistivity and provides better yields.
Reverse Osmosis (RO) cleans tap water to make it roughly 90% to 99% pure. Deionization (DI) filters exchange positive hydrogen and negative hydroxyl molecules for positive and negative contaminant molecules in water. DI filtering and other processes are sometimes called “water polishing.” RO can remove bacteria, salts, organics, silica, and hardness. RO and nanofiltration both employ membrane filtration to capture contaminants. RO systems for industrial purposes typically provide a 65 to 75 percent recovery rate; a very efficient water use RO results in exceptionally pure water.
Typical water feed requirements for ultraclean steam generation.
- RO and DI water, or highly purified water
- Free of Amines, Chlorine, and Chlorides
- Silica less than one ppm
- Total suspended solids – close to none
- Total hardness less than 1ppm
- For conductivity, see the table above
Steam generators require soft water and very low or negligible particles. Low TDS.
Does high pressure enable the outcome of steam or chemical reaction for H2O? The short answer is seldom, especially above 100°C. For the long answer, click here.
More important is having an open-continuous, dry, high-quality steam system made by the OAB and HGA generators, where molecules of H2O can enable more of the desired reaction because of continuous production. This is a steam generator.
High-quality superheated steam has applications in drying, cooking, proper and complete bacterial inactivation, fracking, chemical processes engineering, comfort heating, chemical processes, fuel production, tablet making, mixing, and materials processing High-temperature steam in one atmosphere packs the proper punch required for these applications while minimizing the dangers of using high-pressure boilers This type of superheated steam is used in applications requiring a critical need to reduce the processing time Superheated steam often offers a higher heat transfer coefficient and high enthalpy content, enabling many unique applications When at a high temperature, significantly above the inversion temperature, such steam is often considered a non-toxic antimicrobial agent Superheated steam at high temperatures also offers superior reactions, for example, in energy reactions such as bio-fuels, reforming, hydrogen production, ammonia production, and denaturing, all with rapid heat transfer kinetics In the US, more than 90% of electric power is produced using steam as a working fluid, mainly by steam turbines Condensation of steam to water occurs downstream, but such wet-steam conditions must be carefully controlled to avoid excessive blade erosion and preserve energy efficiency Oxidation and erosion tests can be done with Steam Generators There is no moisture from the start-up in many modern steam generators The HGAs and OAB produce high-quality pure steam The HGA-M is for applications requiring steam-gas(air) mixtures The Mightysteam® and SaniZap® models are helpful for steam cleaning at several levels of cleaning The OAB® and GHGA models are used for industrial and R&D purposes The high-temperature steam also tends to be useful for pharmaceutical, biopharmaceutical, comfort industry, utility, and chemical uses because of a lack of droplets and rapid antimicrobial action for several orders of log reductions in a short duration.
The standard steam diagrams for process design are given below.
What is Humidity? Below the saturation temperature, one uses the term Relative Humidity (RH) – a term used when steam is mixed with air. Relative Humidity is a ratio that compares the amount of water vapor in the air with the amount of water vapor present at saturation. Relative Humidity is a percentage: water vapor is expressed as a percent of saturation.
The gases in the atmosphere exert a certain amount of pressure (about 1013 millibars at sea level). Vapor pressure measures the air’s water vapor content using the partial pressure of the water vapor in the air (Pressure may be expressed using a variety of units: pascals, millibars, pounds per square inch (psi), among others). Since water vapor is one of the gases in the air, it contributes to the total air pressure. The contribution by water vapor is relatively small since it only makes up a few percent of the total mass of a parcel of air. The vapor pressure of the water in the air at sea level, at a temperature of 20oC, is ~24 mbar at saturation (about 3% by volume).
Does the saturation pressure of H2O in the air depend on the total pressure? Yes, as the total pressure of a system decreases, the Relative Humidity will decrease. Likewise, as the total pressure of a system increases, the Relative Humidity will increase until saturation is reached.
Suppose 10 grams of water vapor were present in each kilogram o. dry air, and should the air at that temperature is saturated at 30 grams of water vapor per kilogram of dry air, the Relative Humidity is 10/30=33.3%. A parcel of air at sea level, at a temperature of 25oC, would be completely saturated if there were ~20 grams of water vapor in every kilogram of dry air. We use a mixing ratio: grams of water vapor per kilogram of fully saturated air. If this air contained 20 grams of water vapor per kilogram of dry air, we would say that the Relative humidity is 100%. If a parcel of air (at sea level and 25oC) had 10 grams of water vapor per kilogram of dry air, what is its relative humidity? Answer: The Relative Humidity would be 50%. Ten grams of water vapor/kg dry air compared to the maximum possible 20 grams of water vapor/kg dry air is 10/20=50%. If the parcel of air (at sea level at 25oC) had 18 grams of water vapor per kilogram of dry air, what is its Relative Humidity? The Relative Humidity would be 18/20=90%. In the examples above, the temperature is taken to be about 25°C. Another more technical term is the ratio of the actual vapor pressure to the saturation vapor pressure. You will note below that the HGA-M produces a lot of steam with very high specific Humidity because above 100C in one atmosphere, air and steam mix well as any other “ideal gasses could” Below 100C, the RH is an important limitation on how much water vapor can mix with air For one-atmosphere condition, above 100C, one should use the term specific humidity, which is the mass of water vapor (i.e., steam) ratio when mixed with a unit mass of dry air.
Water saturation pressure at a temperature
(total pressure 1 atmosphere)
(This chart is helpful for comfort heating)
The enthalpy of vaporization and the molar volume change fall with increasing pressure from 1 Bar to 10 Bar.
A must for cleaning (Mobile Platform)
Calculating your parameters.
Endotoxins, microbes, and bacteria are known to be inactivated by heat and H2O. All MHI superheated steam productions produce high-temperature steam that, during protection, encounters temperatures over 500C. Several models’ steam output temperature and humidity conditions are controllable, as discussed below. Refer to the HGA-M manual to see how your HGA-M is configured/rated. The mass fraction of steam in the final flow is about 18% for a valve setting, which gives, for example, 200ml/15.5 minutes (i.e., speciHumiditydity is about 21%). We are assuming that the water vapor is ideal and that the enthalpy of the water vapor in the air can be taken to be the enthalpy of saturated vapor at the same temperature (~ 2501.3+ 1.82 T (kJ/kg)) Temperature, T, is in units of degrees centigrade. Of course, after a specific temperature and pressure ~374C and 22.06 MPa called the critical temperature Tc and critical pressure Pc, respectively, no amount of pressure can cause condensation. The superheated steam generator can produce a steam-air temperature over this temperature, but the output is not at critical conditions because the pressure is lower!
The HGA and OAB operate at close to 100% power efficiency from which the steam temperature can be calculated n the equation below for the efficiency of HGA-M, h is enthalpy and T, the temperature is given in Kelvin (Kelvin=273+Centigrade):
h (enthalpy per kg) of steam is obtained from the figures above at 1-atmosphere pressure, steam tables, or the Mollier diagram Or; you can measure the temperature from the exit thermocouple.
GENERAL INSTRUCTIONS FOR HGA-M
Instructions on how to use and protect the HGA-M are enclosed with the product. It’s a unique, designed device with ease of use. Turn on the air and monitor with a flow meter so the SCFM does not fall below 1.4. Higher airflows give lower temperatures, or you may control power with a separately obtained controller. Then turn on the heater and finally open the water metering valve. Steam-air will be the product. Remember to read the manual for the shutdown procedure.
Q: When do you dry with steam instead of hot air? Answer- steam has many benefits. The main one is the availability of a gas with a lot of stored enthalpy at a lower temperature than corresponding dry air with the same enthalpy, So if you were interested in drying paper with an ignition temperature of, say, 450F, then the use of superheated steam at a much lower temperature may produce the same drying efficiency, SOt air at a high temperature which could be more than the paper ignition temperature.
Follow all safety procedures. NOTE STEAM IS A ODORLESS GAS AT VERY HIGH TEMPERATURES STEAM, LIKE OTHER HOT GASSES, WILL BURN YOU. STEAM PACKS A LARGE AMOUNT OF ENTHALPY SO THAT THE BURN COULD BE SEVERE; DO NOT ALLOW THE STEAM TO FALL ON THE SKIN. WEAR GOGGLES AND GLOVES And Protection for your clothes, ALWAYS.
A selection of steam tables is given below. It is best to use a standard text or International Association for the Properties of Water and Steam (IAPWS)
If you want superheated, high-quality steam, you should now search for the HGA or OAB models that address your needs. The information below refers to the HGA-M models only where an air or gas-steam mixture is required. You may enco. Humiditydity is a design term or property when using air or gas mixtures with steam.
IT IS EASY TO BE CONFUSED BETWEEN RELATIVE AND SPECIFIC HUMIDITY. RELATIVE HUMIDITY DEPENDS ON TEMPERATURE; SPECIFIC HUMIDITY ONLY RELATED TO THE MASS FRACTION.
See below from the public site http://www.pals.iastate.edu/mteor/mt206/lectures/feb7/tsld020.htm.
Potential uses: For layering, epoxy drying, and other film use, superheated steam is required at one atmospheric pressure, Ideal for steam drying or steam oxidation. Attempt use also for precipitating crystals of several small sizes, including nanocrystals from solutions. Precipitation droplet sizes may be controlled by controlling the cooling rate, impingement conditions, and surface type.
The steam temperature depends on the water valve setting and air inflow setting.
HGA-M (typical settings at Full Power):
- Air 1.45 CFM (inlet at ~30C) and water 330ml in 45 mins (inlet at ~30C) yield a steam-air temperature of about 350°C.
- Air 1.4CFM (inlet at ~30C) and water 200ml in 20 mins (inlet at ~30C) yield a steam-air temperature of about 250°C.
- Air 1.8CFM (inlet at ~30C) and water 200ml in 20 mins (inlet at ~30C) yield a steam-air temperature of about 150°C.
The graph below gives a fair idea of adjusting the HGA750-1 for different specific humidity levels. Note as the specific humidity increase, there is a corresponding decrease in overall temperature as total energy is conserved. The gas thermocouple is correct as the exit for the graph below the steam. If you try and reduce the steam gas temperature too much, you may not be able to get superheated steam, and instead, a heated mist may be the output product. The red line graph required that a power controller is in use.
Your results may vary The values above should be considered approximate Because of the placement of the thermocouple, restrictions on flows, and other random errors usually present in multivariate measurements. The user must optimize all valve settings for the best result for specific applications.
SUPERHEATED STEAM IS AN ODORLESS GAS (not to be confused with mist). Output: constant steam air (superheated steam). Safety precautions must be taken when n dealing with hot gases.
DO NOT USE UNITS WITH COMBUSTIBLE LIQUIDS; THE DANGER OF SUPERHEATED STEAM SHOULD BE WELL UNDERSTOOD. PLEASE WEAR GLOVES, GLASSES, AND A HARD HAT. PROTECTIVE CLOTHING IS REQUIRED; STEAM CAN PENETRATE CLOTHES.
The product uses cleaning technologies, drying technologies, curing technologies, and nanotechnologies.
Patents issued applied and are pending for the HGA.
A control thermocouple for the hot air generator part is included. Steam output temperature thermocouples and brackets are sold separately, or users may provide their own. An electrical 110-120V 50/60Hz supply is required for this unit. 1KW system requires compressed air input.
RELATIVE HUMIDITY DEPENDS ON TEMPERATURE; SPECIFIC HUMIDITY IS RELATED TO THE MASS ONLY. IT IS EASY TO BE CONFUSED BETWEEN RELATIVE AND SPECIFIC HUMIDITY. See below from http://www.pals.iastate.edu/mteor/mt206/lectures/feb7/tsld020.htm.
Water vapor pressure
In a closed container partly filled with water, water vapor will be in the space above the water. The concentration of water vapor depends only on the temperature. It is not dependent on the amount of water and is only very slightly influenced by the air pressure in the container. The water vapor exerts pressure on the walls of the container. The empirical equations given below give a good approximation to the saturation water vapor pressure at temperatures within the limits of the earth’s climate.
Saturation vapor pressure, ps, in pascals:
ps = 610.94 *exp(( T*17.6294) / ( T + 243 ) )
where T is the temperature in degrees Celsius
The svp below freezing can be corrected after using the equation above, thus:
ps ice = -4.86 + 0.855*ps + 0.000244*ps2
The next formula gives a direct result for the saturation vapor pressure over ice:
ps ice = 611.2 exp( 22.587*T / ( T+273)
The Pascal (Pa) is the SI unit of pressure = newtons / m2. Atmospheric pressure is about 100,000 Pa (standard atmospheric pressure is 101,300 Pa).
Water vapor concentration
The relationship between vapor pressure and concentration is defined for any gas by the equation:
p = nRT/V
p is the pressure in Pa, V is the volume in cubic meters, T is the temperature (degrees Celsius + 273.16), n is the quantity of gas expressed in molar mass ( 0.018 kg in the case of water ), R is the gas constant: 8.31 Joules/mol.K
To convert the water vapor pressure to concentration in kg/m3: ( Kg / 0.018 ) / V = p / RT
kg/m3 = 0.002166 *p / ( T+ 273.16 ) where p is the actual vapor pressure
Relative Humidity (RH) is the ratio of the actual water vapor pressure to the saturation water vapor pressure at the prevailing temperature.
RH = p/ps
RH is usually expressed as a percentage rather than as a fraction.
The RH is a ratio. It does not define the water content of the air unless the temperature is given. The reason RH is so much used in conservation is that most organic materials have an equilibrium water content that is mainly determined by the RH and is only slightly influenced by temperature.
Notice that air is not involved in the definition of RH. Airless space can have an RH. Air is the transporter of water vapor in the atmosphere and air conditioning systems, so the phrase “RH of the air” is commonly used and only occasionally misleading. The independence of RH from atmospheric pressure is not important on the ground, but it does have some relevance to calculations concerning air transport of works of art and conservation by freeze-drying.
The Dew Point
The water vapor content of air is often quoted as a dew point. This is the temperature to which the air must be cooled before dew condenses from it. At this temperature, the actual water vapor content of the air is equal to the saturation water vapor pressure. The dew point is usually calculated from the RH. The first one calculates ps, the saturation vapor pressure at the ambient temperature. The actual water vapor pressure, pa, is:
pa= ps * RH% / 100
The next step is to calculate the temperature at which pa would be the saturation vapor pressure. This means running backward the equation given above for deriving saturation vapor pressure from temperature:
Let w = ln ( pa/ 610.78 )
Dew point = w *238.3 / ( 17.294 – w )
This calculation is often used to judge the probability of condensation on windows and within walls and roofs of humidified buildings.
The dew point can also be measured directly by cooling a mirror until it fogs. The RH is then given by the ratio
RH = 100 * ps (dewpoint)/ps (ambient)
The concentration of water vapor in the air
It is sometimes convenient to quote water vapor concentration as kg/kg of dry air. This is used in air conditioning calculations and is quoted on psychrometric charts. The following calculations for water vapor concentration in air apply at ground level.
Dry air has a molar mass of 0.029 kg. It is denser than water vapor, which has a molar mass of 0.018 kg. Therefore, humid air is lighter than dry air. If the total atmospheric pressure is P and the water vapor pressure is p, the partial pressure of the dry air component is P – p. The weight ratio of the two components, water vapor and dry air, is:
kg water vapor / kg dry air = 0.018 *p / ( 0.029 *(P – p ) )
= 0.62 *p / (P – p )
At room temperature P – p is nearly equal to P, which at ground level is close to 100,000 Pa, so, approximately:
kg water vapor / kg dry air = 0.62 *10-5 *p
Thermal properties of damp air
The heat content, usually called the enthalpy of air, rises with increasing water content. This hidden heat, called latent heat by air conditioning engineers, has to be supplied or removed to change the relative humidity of the air, even at a constant temperature. This is relevant to conservators. The transfer of heat from an air stream to a wet surface, which releases water vapor to the airstream at the same time as it cools it, is the basis for psychrometry and many other microclimatic phenomena. Control of heat transfer can be used to control the drying and wetting of materials during conservation treatment.
Air at zero degrees Celsius is defined to have zero enthalpy. The enthalpy, h, in kJ/kg, at any temperature, T (K), between 0 and 60°C is approximately:
h = 1.007T – 0.026, below zero: h = 1.005T
The enthalpy of liquid water is also sometimes defined to be zero at zero degrees Celsius. To turn liquid water to vapor at the same temperature requires a very considerable amount of heat energy: ~2501 kJ/kg at 0C.
At a temperature T the heat content of water vapor is:
hw = 2501 + 1.84T
Notice that water vapor, once generated, also requires more heat than dry air to raise its temperature further: 1.84 kJ/kg.K against about 1 kJ/kg.K for dry air.
The enthalpy of moist air, in kJ/kg, is, therefore:
h = (1.007*T – 0.026) + g*(2501 + 1.84*T)
g is the water content (specific humidity) in kg/kg of dry air
The final formula in this collection is the psychrometric equation. The psychrometer is the nearest to an absolute method of measuring RH that the conservator ever needs. It is more reliable than electronic devices because it depends on the calibration of thermometers or temperature sensors, which are much more reliable than electrical RH sensors. The only limitation to the psychrometer is that it is difficult to use in confined spaces (not because it needs to be whirled around but because it releases water vapor).
The psychrometer, or wet and dry bulb thermometer, responds to the RH of the air in this way:
Unsaturated air evaporates water from the wet wick. The heat required to evaporate the water into the air stream is taken from the air stream, which cools in contact with the wet surface, thus cooling the thermometer beneath it. An equilibrium wet surface temperature is reached which is very roughly halfway between ambient temperature and dew point temperature.
The air’s potential to absorb water is proportional to the difference between the mole fraction, ma, of water vapor in the ambient air and the mole fraction, mw, of water vapor in the saturated air at the wet surface. It is this capacity to carry away water vapor that drives the temperature down to, Tw, the wet thermometer temperature, from the ambient temperature ta :
(mw – ma) = B(Ta- Tw)
B is a constant whose numerical value can be derived theoretically by some rather complicated physics.
The water vapor concentration is expressed here as a mole fraction in air rather than vapor pressure. Air is involved in the psychrometric equation because it brings the heat required to evaporate water from the wet surface. Therefore, The constant B depends on total air pressure, P. However, the mole fraction, m, is simply the ratio of vapor pressure p to total pressure P: p/P. The air pressure is the same for both ambient air and air in contact with the wet surface, so the constant B can be modified to a new value, A, which incorporates the pressure, allowing the molar fractions to be replaced by the corresponding vapor pressures:
pw – pa= A* ( Ta- Tw)
The relative humidity (as already defined) is the ratio of pa, the actual water vapor pressure of the air, tops, and the saturation water vapor pressure at ambient temperature.
RH% = 100 *pa/ ps = 100 *( pw – ( Ta- Tw) * 63) / ps
When the wet thermometer is frozen, the constant changes to 56
The psychrometric constant is taken from R.G.Wylie & T. Lalas, “Accurate psychrometer coefficients for wet and ice-covered cylinders in laminar transverse air streams,” in Moisture and Humidity 1985, published by the Instrument Society of America, pp 37 – 56. These values are slightly lower than those in general use. There are tables and slide rules for calculating RH from the psychrometer, but a programmable calculator is very handy for this job. Alternatively, click on the calculator.
Relative Humidity Chart