Write a Program That Reads the Relative Humidity Between 0 and 1and the Temperature

Heating, ventilation, and air-conditioning

Bruce Hyndman , in Clinical Engineering Handbook (Second Edition), 2020

Humidification

The psychrometric chart ( Fig. ii) is a tool for understanding the relationships between the various parameters of supply air and the relative humidity. This template allows a designer or operator to "work backwards" from a desired room relative humidity to the desired condition of the air every bit it enters the supply duct. Control of humidity is non required by nearly codes in all areas but is for some in most geographic areas. The Uniform Mechanical Code (IAPMO, 2018) calls for OR relative humidities of fifty%–60%. In a warm and humid climate, the supply air can be drawn into a organisation and and so exposed to cooling coils that driblet the air temperature to 55°F. According to the psychrometric chart, if the relative humidity of the exterior air is eighty% and the outside air temperature is 85°F, when the air is cooled to 55°F, vapor will condense out of the air and the relative humidity will be 100%. And then, if the air is heated to lxx°F without the addition of water the humidity will change to almost threescore%, according to the chart. In climates of low humidity, information technology is necessary to inject water vapor into the supply air to increase the humidity to the target level. This can be done with steam or other methods; steam can be provided by steam boilers or electrical steam generators that boil water.

Fig. 2. Psychrometric chart.

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Heating, Ventilation, and Air conditioning

Bruce Hyndman , in Clinical Engineering Handbook, 2004

Humidification

The psychrometric chart ( Figure 108-2) is a tool for understanding the relationships between the diverse parameters of supply air and the relative humidity. This template allows a designer or operator to "work backwards" from a desired room relative humidity to the desired condition of the air every bit it enters the supply duct. Control of humidity is not required by most codes in all areas, merely is for some in almost geographic areas. The Uniform Mechanical Code (Uniform Mechanical Code, 2003) calls for OR relative humidities of l%-60%. In a warm and humid climate, supply air tin be drawn into a system and so exposed to cooling coils that drop the air temperature to 55°F. According to the psychrometric chart, if the relative humidity of the outside air is 80% and the outside air temperature is 85°F, when the air is cooled to 55°F, vapor will condense out of the air, and the relative humidity will exist 100%. And then, if the air is heated to 70F without the improver of water, the humidity will alter to about sixty%, according to the nautical chart. In climates of depression humidity, information technology is necessary to inject water vapor into the supply air to increase the humidity to the target level. This can exist washed with steam or other methods. Steam tin can be provided by steam boilers or electrical steam generators that boil h2o directly.

Effigy 108-two. Psychrometric chart.

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Properties of Humid Air

Roger Legg , in Air Conditioning System Pattern, 2017

The Psychrometric Nautical chart

The psychrometric nautical chart is a most useful design tool for air conditioning engineers. A typical nautical chart is shown in Fig. 1.xv [4]. The air backdrop on the chart are:

Fig. 1.15. Psychrometric chart.

(Reproduced from Section C1 of the CIBSE Guide, with the permission from the Chartered Institute of Building Services Engineers.)

•

dry out-bulb temperature,

•

sling wet-bulb temperature,

•

moisture content,

•

specific enthalpy,

•

specific book,

•

percentage saturation.

When working with the nautical chart, the following points should be noted:

•

Sling wet-bulb is used in preference to screen wet-seedling temperature as it is considered to be the more than consequent of the two measurements.

•

Vapour pressure level is not given. This property is rarely required by the air workout engineer.

•

Dew-point temperature can exist obtained for a given air condition as previously described.

•

The lines of constant dry out-bulb temperature are not at right angles to the lines of constant wet content. This is because the nautical chart is based on enthalpy and moisture content and dry out-seedling temperature is added subsequently, adamant from the enthalpy equation. (See Example 2.1 in the following affiliate.)

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Sensors and Auxiliary Devices

The reading text for this course was originally written by, ... Robert McDowall P. Eng. , in Fundamentals of HVAC Control Systems, 2008

4.3 Humidity and the Psychrometric Nautical chart

Humidity is the moisture content in air. Before we consider humidity sensors, it is important that you understand what is being measured and how the diverse measurements relating to moisture content and temperature interact. Since wet and temperature relate to the energy, or enthalpy, of the moist air we will also introduce that effect.

The relationships between temperature, wet content, and free energy are most easily understood using a visual aid called the " psychrometric chart ."

The psychrometric chart is an manufacture-standard tool that is used to visualize the interrelationships between dry air, moisture, and energy. If y'all are responsible for the pattern or maintenance of any attribute of air-workout in buildings, a clear and comfy understanding of the chart will make your chore easier.

Initially, the chart tin be intimidating, but as you work with it you volition discover that the relationships that it illustrates are relatively easy to sympathise. Once you are comfortable with it, yous will observe that it is a tool that can make it easier to troubleshoot air-conditioning problems in buildings. In this form, nosotros will merely introduce the psychrometric chart and provide a very cursory overview of its structure.

The psychrometric chart is congenital upon two simple concepts.

1.

Indoor air is a mixture of dry air and water vapor.

2.

At that place is a specific amount of energy in the mixture at a specific temperature and pressure level.

Indoor Air is a Mixture of Dry Air and Water Vapor

The air nosotros live in is a mixture of both dry out air and water vapor. Both are invisible gases. The water vapor in air is likewise called moisture or humidity. The quantity of water vapor in air is expressed as "pounds of water vapor per pound of air." This ratio is chosen the "humidity ratio," abbreviation W, and the units are pounds of water/pound of dry air, lb _{due west}/lb _da, often abbreviated to lb/lb.

The exact backdrop of moist air vary with pressure level. Every bit force per unit area reduces as distance increases the properties of moist air alter with altitude. Typically, psychrometric charts are printed based on standard pressure at bounding main level. For the balance of this section we will consider pressure equally constant.

To understand the relationship between water vapor, air, and temperature, we will consider two conditions:

a.

The air temperature is constant, simply the quantity of water vapor is increasing.

b.

The air temperature is dropping, but the quantity of h2o vapor is constant.

a. The temperature is constant, merely the quantity of water vapor is increasing. If the temperature remains constant, and so as the quantity of water vapor in the air increases, the humidity increases. All the same, at every temperature indicate, at that place is a maximum amount of water vapor that can co-exist with the air. The betoken at which this maximum is reached is called the saturation point. If more than water vapor is added after the saturation point is reached, and so an equal amount of water vapor condenses and takes the course of either water aerosol or ice crystals.

Outdoors, we run into water droplets in the air as fog, clouds, or rain and we see water ice crystals in the air equally snow or hail. The psychrometric nautical chart only considers the conditions up to the saturation point; therefore, information technology only considers the effects of water in the vapor stage and does non bargain with water droplets or ice crystals.

b. The temperature is dropping, merely the quantity of water vapor is constant. If the air is cooled sufficiently, it reaches the saturation line. If it is cooled even more, wet volition condense out and dew forms.

For example, if a cold canned drink is taken out of the refrigerator and left for a few minutes, the container gets damp. This is because the moist air is in contact with the chilled container. The container cools the air that it comes in contact with to a temperature that is below saturation, and dew forms. This temperature, at which the air starts to produce condensation, is called the dew-indicate temperature.

Relative Humidity

Effigy 4-9 is a plot of the maximum quantity of water vapor per pound of air against air temperature. The X-axis is temperature. The Y-axis is the proportion of h2o vapor to dry air, measured in pounds of water vapor per pound of dry out air. The curved "maximum water vapor line" is called the "saturation line." It is also known as 100% relative humidity, abbreviated to 100% rh. At any point on the saturation line, the air has 100% of the water vapor per pound of air that can coexist with dry air at that temperature.

Figure 4-9. Psychrometric Chart – Saturation Line

When the same volume of air contains but half the weight of h2o vapor that it has the capacity to hold at that temperature, we call it 50% relative humidity or 50% rh. This is shown in Figure 4-10 . Air at any bespeak on the 50% rh line has one-half the water vapor that the aforementioned book of air could have at that temperature.

Figure 4-10. Psychrometric Chart – 50% Relative Humidity Line

Every bit you can see on the nautical chart, the maximum amount of water vapor that moist air tin can contain increases speedily with increasing temperature. For example, moist air at the freezing signal, 32°F, can contain but 0.four% of its weight equally h2o vapor. All the same, indoors, at a temperature of 72°F, the moist air can comprise nearly 1.vii% of its weight as water vapor – over four times as much.

Consider Figure 4-11 , and this example:

Figure iv-xi. Psychrometric Chart – Change in Relative Humidity with Change in Temperature

On a miserable wet mean solar day it might be 36°F outside, with the air rather humid, at lxx% relative humidity. Bring that air into your building. Heat it to 70°F. This brings the relative humidity downward to about 20%. This change in relative humidity is shown in Figure 4-12 , from Point 1 → ii. A absurd damp 24-hour interval outside provides air for a dry day indoors! Note that the accented corporeality of h2o vapor in the air has remained the aforementioned, at 0.003 pounds of water vapor per pound of dry air, merely as the temperature rises, the relative humidity falls.

Effigy 4-12. Psychrometric Chart – Enthalpy

Here is an example for you to attempt, using Figure iv-11 .

Suppose information technology is a warm day with an outside temperature of 90°F and relative humidity at twoscore%. We have an air-conditioned infinite that is at 73°F. Some of the outside air leaks into our air-conditioned infinite. This leakage is called infiltration.

Plot the procedure on Figure 4-12 .

one.

Find the start condition, 90°F and 40% rh, moisture content 0.012   lb/lb.

2.

Then cool this air: movement left, at constant moisture content to 73°F.

3.

Detect that the cooled air now has a relative humidity of about lxx%.

Relative humidity of 70% is loftier enough to cause mold problems in buildings. Therefore, in hot moist climates, to prevent infiltration and mold generation, it is valuable to maintain a small positive pressure in buildings.

In that location is a specific amount of energy in the air mixture at a specific temperature and pressure. This brings us to the 2d concept that the psychrometric chart illustrates. At that place is a specific amount of energy in the air h2o-vapor mixture at a specific temperature. The energy of this mixture is dependent on two measures:

•

The temperature of the air.

•

The proportion of h2o vapor in the air.

There is more energy in air at college temperatures. The addition of heat to enhance the temperature is called adding "sensible heat." In that location is too more energy when there is more water vapor in the air. The energy that the water vapor contains is referred to equally its " latent heat."

The mensurate of the total energy of both the sensible heat in the air and the latent heat in the h2o vapor is ordinarily called "enthalpy." Enthalpy can be raised by calculation energy to the mixture of dry out air and water vapor. This tin be achieved by calculation either or both:

•

sensible heat to the air

•

more water vapor, which increases the latent heat of the mixture.

On the psychrometric chart, lines of constant enthalpy gradient down from left to right as shown in Figure iv-13 and are labeled "Enthalpy."

Figure four-thirteen. Psychrometric Chart – Heating Air From 47°F to 72°F

The zero is arbitrarily called as zippo at 0°F and zero moisture content. The unit measure for enthalpy is British Thermal Units per pound of dry air, abbreviated as Btu/lb.

Heating

The process of heating involves the addition of sensible heat energy. Figure 4-13 illustrates outside air at 47°F and almost 90% relative humidity that has been heated to 72°F. This process increases the enthalpy in the air from approximately 18–24 Btu/lb. Note that the process line is horizontal because no water vapor is being added to, or removed from the air – we are just heating the mixture. In the process, the relative humidity drops from almost xc% rh down to nearly 36% rh.

Here is an example for you lot to attempt.

Plot this process on Figure 4-fourteen .

Effigy 4-14. Psychrometric Chart – Adding Moisture with Steam

Suppose it is a absurd mean solar day with an outside temperature of xl°F and 60% rh. We have an air-conditioned space and the air is heated to 70°F. There is no change in the amount of water vapor in the air. The enthalpy rises from nearly xiii Btu/lb to 20 Btu/lb, an increase of vii Btu/lb.

As you can run into, the humidity would have dropped to xx% rh. This is quite dry out so let us presume that we are to raise the humidity to a more than comfortable 40%. As you can encounter on the chart, this raises the enthalpy by an additional 3.five Btu/lb.

Humidification

The addition of water vapor to air is a process called "humidification." Humidification occurs when h2o absorbs energy, evaporates into water vapor, and mixes with air. The energy that the water absorbs is called "latent heat."

There are ii means for humidification to occur. In both methods, energy is added to the water to create water vapor.

one.

Water can be heated. When oestrus energy is added to the water, the water is transformed to its gaseous country, steam, which mixes into the air. In Figure 4-14 , the vertical line, from Point one to Point 2, shows this procedure. The heat, energy, 3.five Btu/lb, is put into the water to generate steam (vaporize it), which is then mixed with the air.

In practical steam humidifiers, the added steam is hotter than the air and the piping loses some heat into the air. Therefore, the air is both humidified and heated due to the addition of the water vapor. This combined humidification and heating would be shown by a line which slopes up and a little to the right in Figure 4-14 .

two.

Allow the water evaporate into the air past spraying a fine mist of water aerosol into the air. The fine h2o droplets absorb heat from the air as they evaporate. Alternatively, just using the same evaporation procedure, air can be passed over a moisture material, or wet surface, enabling the water to evaporate into the air.

In an evaporative humidifier, the evaporating h2o absorbs rut from the air to provide its latent heat for evaporation. As a result, the air temperature drops, every bit information technology is humidified. The procedure occurs with no external addition or removal of heat. Information technology is called an adiabatic process. Since, at that place is no change in the heat energy (enthalpy) in the air stream, the add-on of moisture, by evaporation, occurs along a line of constant enthalpy.

Figure four-fifteen shows the procedure. From Indicate 1, the moisture evaporates into the air and the temperature falls to 56°F (Point ii). During this evaporation, the relative humidity rises to well-nigh 65%. To reach our target of 70°F and 40% rh we must now heat the moistened air at Signal 2 from 56°F to 70°F (Point 3) requiring 3.five Btu/lb of dry air.

Effigy 4-xv. Psychrometric Nautical chart – Calculation Wet, Evaporative Humidifier

To summarize, we can humidify by adding rut to h2o to produce steam and mixing the steam with the air, or nosotros can evaporate the moisture and heat the moistened air. Nosotros accomplish the same issue with the same input of heat by two unlike methods.

It has go much easier to control humidity in buildings but do be aware of the consequences. In a cold climate, maintaining higher humidity has a twenty-four hour period-to-day energy cost. If humidity is maintained too loftier for the building, serious damage from condensation on the inside can occur. Within the walls, ice tin can cause serious structural damage to the outside wall facing. In the boiling climate, dehumidification is costly but failure to continuously limit the maximum humidity tin lead to mold problems resulting in building closure. The Kalia Belfry Hilton, Hawaii, mold problem involved endmost the 453-room hotel for refurbishing at a cost of over $US50 million.

One last issue is the term wet-seedling temperature. We have discussed the fact that moisture evaporating into air cools the air. This belongings is used to obtain the wet bulb temperature. If a standard thermometer has its sensing bulb covered in a little sock of moisture cotton gauze, and air blows quickly over it, the evaporation will cool the thermometer. An equilibrium temperature is reached which depends on the dry-bulb temperature and relative humidity. If the air is very dry out, evaporation will exist rapid and the cooling effect large. In saturated air the evaporation is zero and cooling zero, so dry-bulb temperature equals moisture-bulb temperature at saturation.

Lines of constant wet-seedling temperature tin be drawn on the psychrometric nautical chart. They are almost parallel to the enthalpy lines and the error is not significant in normal HVAC except at high temperatures and low relative humidity.

If, for instance, the dry out-bulb temperature was sixty°F and wet-bulb was 50°F, we tin can plot these on the chart as shown in Figure 4-16 and notice the relative humidity to exist 50%. If the temperature were 70°F and wet seedling still 50°F the relative humidity would be downwardly at about 20%. Call up, the greater the wet-seedling temperature depression the lower the relative humidity.

Figure four-16. Plot of Dry-bulb and Wet-seedling Temperatures

This has been a very brief introduction to the concepts of the psychrometric chart. A typical published chart looks complicated as it has all the lines printed, but the unproblematic underlying ideas are:

•

Indoor air is a mixture of dry air and water vapor.

•

There is a specific corporeality of total energy, called enthalpy, in the mixture at a specific temperature, moisture content, and pressure level.

•

In that location is a maximum limit to the amount of h2o vapor in the mixture at any particular temperature.

At present that nosotros have an understanding of the relationships of dry air, moisture, and energy, at a particular force per unit area let us consider relative humidity, dew-point, and enthalpy sensors.

Figure 4-17 shows a section of a simple building with an air-conditioning unit drawing return air from the ceiling plenum and supplying to three spaces, A, B, and C. Each space has private temperature control with its own thermostat and heater. The air handling unit has a relative humidity sensor in the eye space B. Assuming like activities and the same temperature in each room the relative humidity volition also be the aforementioned in each room.

Figure 4-17. Edifice Arrangement

Now let us assume that room A occupant likes it warmer. What will happen to the relative humidity in space A? Go down, up, or stay the same? Aye, it will become downwardly. So the obvious affair to do is to average the relative humidity.

We tin reach this by moving the relative humidity sensor to the inlet of the air handling unit. If the occupants had a relative humidity sensor on their desk they could correctly complain that the relative humidity is going up and down. However, the control system records would show that system is maintaining the humidity perfectly constant. Both are correct, how can this be?

The lights produce heat that heats the render air higher up the ceiling. During the night the lights are off so the return air from the rooms is at the same temperature as the render air into the unit. In the daytime, when the lights are on, the return air is heated in the plenum by the lights and the relative humidity drops. The air handling unit of measurement compensates for this past raising the moisture content. This raises the humidity level in the spaces while keeping the relative humidity constant at the air-handler intake.

At present permit united states imagine that the roof of this edifice is non perfectly insulated. When the sun shines on the roof, the heat from the sun will also oestrus the plenum. This volition also crusade the relative humidity in the return air to become downwards and the air-handler will reply by raising wet content in the system.

In this type of situation, information technology is a bad idea to use return air relative humidity to control multiple spaces. It is better to either use one space as the master space and maintain its humidity, or to use a dew-signal sensor. The dew-point sensor tin can be mounted anywhere in the system as it is non influenced by the dry out-bulb temperature. With the dew-point sensor in the return air it sensor will average the moisture content in all the spaces.

Since the relative humidity varies with temperature information technology is amend to specify the control range for a space in terms of dry-bulb temperature and dew-signal (or humidity ratio) rather than relative humidity.

Use of Enthalpy Sensors

Enthalpy sensors tin be effective at reducing cooling costs, especially in boiling climates. Consider a organisation with an air economizer. The question to be answered is: "When should the system terminate using 100% exterior air and revert to minimum exterior air?" Consider a plant where the return air is at 75°F and 50% relative humidity. This is shown on Figure iv-xviii . The enthalpy for this return air is also shown in bold. If the system uses temperature to make the determination, a temperature of 65°F would ensure that the switch was fabricated before the outside air enthalpy rises to a higher place the return enthalpy, except for the very occasional possibility that the humidity is over ninety%. This temperature setting avoids ever bringing in air with a college enthalpy than the return air. The conditions for using return air are shown by the hatched area, all temperatures below 65°F and whatever moisture content.

Effigy iv-eighteen. Temperature Versus Enthalpy for Switching off Economizer

Switching at 65°F ensured that excessively loftier enthalpy exterior air is virtually never used simply information technology besides switches the plant well before it needs to in many situations. If, instead the switch is fabricated based on enthalpy one has 2 choices. One could utilize a single enthalpy sensor set at the design render air enthalpy (the bold line in Figure iv-xviii ). This would allow outside air to be used in the additional shaded expanse.

Better, would exist to use an enthalpy sensor in the render air and outside air and make the decision to drop to minimum exterior air when the outside air enthalpy rose to the return air enthalpy.

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All-air Systems

Roger Legg , in Air Conditioning Organization Blueprint, 2017

Winter Operation

The functioning at the wintertime design conditions is now considered, using the organization designed for summer performance. The air conditioned infinite at present has a sensible oestrus loss, $q_{\mathrm{Southward}}^{\prime }$ , and a latent heat proceeds, q ₅₀.

Since the air mass period charge per unit, determined for summer functioning, remains constant, the supply air temperature is given by:

(half-dozen.5) $t_{\mathrm{Smax}}=t_{\mathrm{R}}+\frac{q_{\mathrm{South}}^{\prime }}{{\stackrel{.}{m}}_{\mathrm{a}}{c}_{\mathrm{p}\mathrm{a}\mathrm{s}}}$

Referring to Fig. 6.iii, a procedure similar to that for the summer operation is followed.

Fig. six.three. System using 100% outdoor air, wintertime performance.

On the psychrometric chart:

•

plot the room condition and the RRL,

•

place the supply air condition S on the RRL,

•

plot the winter outdoor pattern status O.

The atmospheric condition on the chart show that it is necessary to oestrus and humidify the outdoor air from condition O to condition S. Since in that location is already an after heater in the organization, a sensible-heating process line may depict through Southward. The chart shows that the outdoor air now requires heating and humidifying from O to B. To reach this, the system can be designed in one of the two means:

•

Sensible heating followed by adiabatic humidification;

•

Sensible heating followed by steam humidification.

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Packed Towers

A. Kayode Coker , in Ludwig's Applied Procedure Design for Chemical and Petrochemical Plants (Quaternary Edition), Book 2, 2010

xiv.55 Preliminary Pattern Estimate of New Belfry

Refer to psychrometric chart, Figures 14-138A and B, for basic considerations in establishing tower conditions.

Figure 14-138A. Psychrometric chart, reference barometric pressure of 29.92 in. Hg.

Used past permission of Westinghouse Electric Co., Sturtevant Div.

Figure xiv-138B. Directions for using Effigy 14-124A.

Used by permission of Westinghouse Electric Co., Sturtevant Div.

i.

Determine the inlet water temperature to the tower. This is approximately the outlet temperature from the cooling h2o load.

2.

Determine the heat load to be performed by the belfry, based on required water inlet and outlet temperatures and flow rates.

3.

Establish the wet bulb temperature for the air at the geographical site of the belfry. Use weather condition agency records if other data are non available. Employ circumspection; do not select too high a value.

4.

Prepare a plot of the saturation curve for air-water. Establish the operating line by starting at the point set past the outlet cold water temperature and the enthalpy of air at the wet bulb temperature, and with a slope L′/Thousand_a assumed between 0.ix and 2.7. See Figure 14-123.

5.

Graphically integrate, by plotting l/h′-h vs. t, reading (h′-h) from the operating-equilibrium line plot for various values of temperature. Come across Figure 14-139.

Figure 14-139. Graphical integration to determine number of transfer units.

6.

The value of the integral is equal to the number of transfer units, so gear up it equal to Equation 14-149 and solve for the number of decks needed, Northward. Select the desired deck from Figures xiv-124A and B and the constants A′ and n from Table xiv-53.

7.

If the number of decks required is unreasonable from a height standpoint, the process must exist repeated using a new assumed L′/G_a, or a new approach, or a new wet bulb temperature, or some combination of these.

eight.

For the causeless Fifty′/1000_a and known L′, summate the required air rate G_a.

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Cardinal Relationships of Estrus and Mass Transfer in Solar Seawater Desalination Systems

Hongfei Zheng , in Solar Energy Desalination Engineering science, 2017

3.two.1 Psychrometric Chart (h-d Diagram)

For convenient adding, the coordinate graphs with state parameters for moist air are applied to determine the states and parameters of moist air and analyze its thermodynamic process. The most common one is the psychrometric chart (h-d diagram).

There are only two independent coordinates in a two-dimensional coordinate airplane normally. However, the land of moist air depends on temperature t, moisture content d, and barometric force per unit area p _b . Therefore three contained coordinates are required. If the atmospheric pressure level is provided, t and d tin be drawn in a 2-D coordinate aeroplane. Different atmospheric pressures p _b correspond to dissimilar diagrams. A suitable h-d diagram should be selected co-ordinate to local atmospheric pressure.

In Prc, the psychrometric chart in use has enthalpy every bit the vertical axis and moisture content as the horizontal axis, also chosen the h-d diagram, as shown in Fig. iii.one and Appendix Effigy ane. The plotting process of it is introduced in the following role.

Figure three.1. Psychrometric chart.

1.

Isenthalpic lines and lines of constant moisture content

In guild to make the diagram clearer, isenthalpic lines are parallel with the 135° tilt angle relative to the vertical axis. The lines of constant moisture content are parallel to the vertical centrality. The origin is the state of dry air at t  =   0 and d  =   0.

2.

Isothermal lines

Isothermal lines are plotted based on the formula h  =   ane.01t  + d(2501   +   1.86t), and the linear relationship between h and d tin be establish when the temperature is constant, as a result of which the isothermal lines in the h-d diagram is a serial of straight lines. According to the equation, 1.01t is y-intercept and (2501   +   one.86t) is the slope; when t values are unlike, the slopes of each isothermal lines are different. Hence, isothermal lines are not parallel to each other, but they can be assumed as parallel lines because 1.86t is much less than 2501, causing less obvious effects on slope past t.

3.

Lines for constant relative humidity

The lines for constant RH are determined by formula $d=0.622\frac{\phi ·p_{\mathrm{five}·s}}{p_b-\phi ·p_{v·\mathrm{due\; south}}}$ . Under a certain atmospheric force per unit area p _b , d depends on p _5,s only when RH φ is constant. p _v,southward is the single-valued function to temperature t, and the value can exist found in the tables of water vapor properties. Therefore the value of d tin can be obtained at unlike temperature t, and the line of abiding φ tin can be plotted by connecting the points on the i-d diagram according to the values of t and d. The lines of constant φ are a serial of upward-concave divergent curves. The y-axis is the line of constant φ at φ  =   0%. The line of constant φ at φ  =   100% likewise represents the country of saturated moist air; the top left of the line is the region for moist air (likewise called the "unsaturated region"), and the area at the lesser right is the region of oversaturated moist air. Due to the instability of oversaturation, condensation happens easily. The water droplets are suspended in midair and course fog, and this region is too called the "foggy region". In the region of moist air, h2o vapor is overheated and has a stable land.

4.

Lines of partial pressure of h2o vapor

The formula $d=0.622\frac{p_5}{p_b-p_v}$ can be expressed equally $p_v=\frac{p_b·d}{0.622+d}$ . When the barometric pressure B is abiding, the equation is in the form of p _v   = f(d), which ways the partial pressure of water vapor p _v only depends on moisture content d. Therefore the lines of partial pressure level of water vapor tin be obtained by drawing a horizontal line higher up the line of d and labeling the values of p ₅ corresponding to the values of d.

v.

Lines of rut humidity ratio

In the calculation of moist air, the changes of enthalpy and moisture content are important. In the thermodynamic procedure, the ratio of change in enthalpy to the change in humidity is called oestrus humidity ratio and is represented by ε, which is:

(3.19) $\epsilon =\frac{\Delta h}{\Delta d}$

The unit of Δd is thou in most cases, and then the ε in the h-d diagram should exist $\epsilon =1000\frac{\Delta h}{\Delta d}$ . The line presenting the procedure of moist air with constant ε in the h-d diagram is direct, and the unit of ε is kJ/kg or kJ/chiliad, so ε is the slope of it. Because the slope is non related to the beginning position, a series of lines with different ε tin can be plotted in the h-d diagram past applying any point every bit reference. In do, the line for the phase-alter process of air tin can be drawn by translating the referenced line with the same ε to the bespeak of initial land.

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EVAPORATIVE AIR COOLING IN ISLAMIC Architecture

AKRAM A. ZUHAIRY , A.A.1000. SAYIGH , in Energy Conservation in Buildings, 1991

Psychrometric Charts and Climatic Atmospheric condition

Fig. four . shows a hypothetical psychrometric chart showing a comparison between the cooling furnishings of the presence of a direct evaporative cooler, an A'absurd, a Salsa'beel or a fountain in a living environment. Further investigation and experimental piece of work is needed to bear witness the ranking of the cooling efficiencies of these systems.

Fig 4. A hypothetical chart shows the ranking of cooling efficiencies for different systems

The above mentioned systems works every bit follows: the air cools evaporatively forth a line of constant wet bulb temperature (WBT) moving it from weather condition at point A to these represented by the points B_ane, B₂, B_iii or B_iv, while the ambient air DBT is reduced its humidity increases. The cooled air then gets mixed with room air improving its conditions to those shown at points C_i, C₂, C₃ or C₄.

The WBT of the entering air conditions (point A in the chart) is the about limiting factor for the efficiency of these systems. The to a higher place mentioned systems are more efficient in hot dry climates. They are not going to provide the desirable comfort in hot-humid climate this is due to the fact that the humidity will increase to unpleasant level with less achievements in reducing the DBT. If the indoor conditions take a relative humidity exceeding 70–75% RH, the above mentioned systems will cease to part.

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Theoretical Grounds for Humidity

Dario Camuffo , in Microclimate for Cultural Heritage (Second Edition), 2014

2A.10 The Psychrometric Nautical chart

The following thermohygrometric parameters tin can be graphically computed with the assist of the psychrometric chart ( Fig. 2A.7).

FIGURE 2A.7. The psychrometric chart.

•

The abscissa is the bodily air temperature T (i.east. the dry bulb temperature) and the ordinate is the MR (or the SH).

•

Vertical lines are isotherms. Going upwards, the MR increases (due east.g. addition of external water, e.thou. by evaporation). Going downwards, it decreases (east.g. condensation, absorption or adsorption of vapour that is subtracted from the atmosphere; the term 'adsorption' implies the condensation of the vapour on the surface or in internal porosities of solids).

•

Horizontal lines are isohumes in terms of constant MR. A displacement to the right indicates warming of the arrangement without change in MR; a displacement to the left, cooling, and according to our previous definition, the last betoken of this "horizontal" cooling is the DP.

•

The nearly exponential curve on the left is the saturation curve and represents simultaneously the MR _sat , the RH  =   100%, the DP and the T _w .

•

The (vertical) altitude between each point of the saturation curve RH _sat   =   100% and the abscissa (RH  =   0%) is divided into 100 parts. Each of them, by definition (or nether the approximation discussed in section 2A.6), shows a given percentage of RH _saturday or MR _sat . Matched values of T and MR used as orthogonal Cartesian coordinates in a psychrometric diagram determine the actual value of the RH of the same air parcel. In the diagram, the nigh exponential curves with RH  =   10%, 20%, thirty%,… 100% are evidenced. These lines are named isohumes in terms of RH.

Tilted lines, where both the T decreases and the MR increases, are characterized by T _w   =   constant and are nearly isenthalpic. In fact, there is a small difference, beingness drawn for a nonperfect gas. The value of T _w of each thermodynamic point can be read on the saturation curve. Sometimes, the scale for enthalpy of the system is likewise added.

The enthalpy $\mathcal{H}$ of a organisation in equilibrium at pressure level p and temperature T is defined every bit

(2A.55) $\mathcal{H}\left(p,T\right)=U+pV$

where U is the internal energy of the system and V is its volume. Therefore,

(2A.56) $d\mathcal{H}=dQ+pdV$

where dQ is the amount of heat gained or lost. For a process that is isobaric and reversible

(2A.57) $d\mathcal{H}=dQ$

and for a perfect gas (i.due east. no change of state)

(2A.58) $d\mathcal{H}=c_{p}dT$

the enthalpy is a function of T only, i.e. the change in enthalpy measures the oestrus imparted to the system. In meteorology, the enthalpy is considered synonymous of sensible heat (i.due east. heat, in opposition to latent estrus) as exchanges of enthalpy coincide with exchanges of heat. Indicating past ${\mathcal{H}}_{m\upsilon }$ the enthalpy per mole of pure water in the vapour stage and by ${\mathcal{H}}_{mc}$ that in the condensed phase, nosotros get

(2A.59) ${\mathcal{H}}_{m\upsilon }-{\mathcal{H}}_{kc}={\mathrm{Fifty}}_{\upsilon }$

i.e. the molar rut of evaporation denoting the latent oestrus of vaporization for 1   mol of the associated condensed stage. Therefore, changes of enthalpy always represent transfer of estrus, either in the sensible or in the latent class.

For an isolated air packet, all the processes that are simultaneously adiabatic and isobaric are also isenthalpic. A process that is isenthalpic is also adiabatic, as well as isentropic, as the entropy $\mathsf{S}$ is thermodynamically defined every bit

(2A.60) $\mathsf{Southward}=\int \frac{dQ}{T}.$

In fact, all these processes are characterized past dQ  =   0. By definition, a process that is adiabatic simply not isobaric (east.one thousand. the upward ascent of an air bundle) is isentropic (i.e. dQ  =   0) but not isenthalpic, because of the work done against the external pressure during the variation of the volume of the air packet. The sum of enthalpies of the phases of a closed organisation is conserved in an adiabatic isobaric process.

Sometimes, the isochoric lines, i.e. the lines with equal specific volume, are also reported, expressed in m^three  kg⁻¹.

Each point in the chart shows directly, or by interpolation, the complete thermodynamic land of an air parcel defined by the following variables:

•

T, obtained by post-obit downwards the isotherm to the abscissa, and the MR, by post-obit the horizontal isohume to the ordinate;

•

DP, obtained by post-obit the same isohume to the saturation curve;

•

T _w , obtained past following the iso wet bulb line (i.e. the isenthalpic) to the saturation bend;

•

RH, obtained by considering on the aforementioned vertical the distance between the thermodynamic betoken and the abscissa and expressing information technology as a percentage of the whole distance between the abscissa and the saturation curve, or interpolating with the nearly exponential curves with RH  =   const passing close to the indicate.

A few examples may be useful to analyze the method and to become familiar with the nautical chart.

one.

Let united states of america consider the thermodynamic signal (Fig. 2A.8(a)) characterized past t  =   25   °C and RH  =   lxx%; the vertical isotherm and the curve of equal saturation 70% meet at a point where the horizontal isohume shows on the right MR  =   14   g   kg⁻¹ and on the left DP  =   nineteen   °C. Wet can be isothermally added to MR _sat   =   20   k   kg^−ane, or isenthalpically to MR _sat   =   xv.75   g   kg⁻¹, and the corresponding temperature is T _{due west}   =   21   °C.

FIGURE 2A.eight. (a) Thermodynamic label of an air bundle with t  =   25   °C and RH  =   70%. (b) Thermodynamic transformation needed to cool an air mass initially at t  =   30   °C and RH  =   60% to t  =   25   °C only with the same RH. (c) Example while drying cloths in a bathroom, testing diverse drying strategies.

2.

In Fig. 2A.eight(b), we desire to lower the air temperature in a room, initially at t  =   30   °C and RH  =   lx% (Point A), in social club to reach 25   °C merely with the aforementioned RH (Betoken B). We should remove some rut and some wet by condensation on a common cold surface. The air conditioning system will start cooling the air. Initially, the cooling will occur with unchanged MR (i.e. 16.iii   g   kg⁻ⁱ, with a horizontal deportation forth the isohume to left); at around 21   °C the DP will be reached. Further cooling will cause the thermodynamic system to follow the saturation curve RH  =   100% with condensation on the refrigerating surface and subtraction of vapour from the air. The system will condense water until the moist air will reach 17   °C and MR _sabbatum   =   12   g   kg^−ane.

Removing the fraction of liquid h2o that has been condensed and heating the air packet to 25   °C with a horizontal displacement, the desired temperature and humidity will exist reached.

3.

Suppose (Fig. 2A.8(c)) to hang out the clammy laundry in a bathroom, when the external conditions are characterized by t  =   0   °C and fog, i.e.RH  =   100%; (Point A) the MR is 3.8   g   kg^−ane. The book of the bathroom is 25   m³ and contains the total mass 25   thousand^three  ×   1.255   kg   m⁻³  =   31.4   kg of air. For simplicity in this case, nosotros will neglect the small density changes. The external air that entered the room was heated tot  =   20   °C (Bespeak B); as in that location is no subtraction or addition of moisture, the indoor RH is RH  =   25%, i.east. very dry. At this indoor temperature, the maximum allowable MR is MR _sat   =   14.viii   chiliad   kg⁻¹ (Point C) and the amount of h2o that can evaporate is given by the number of kilograms of air independent in the bath, multiplied by the moisture capacity of each air kilogram, minus the initial wet content, i.e. 31.4   kg   ×   (14.8   −   3.eight)   g   kg⁻¹  =   345   m, which is a very footling amount, corresponding, east.thousand. to 1 sock. Heating the bathroom to 25   °C (Point D), the MR _sat will increase to MR _sabbatum   =   20   g   kg^−one, which allows a further evaporation (Point Due east) of 31.4   kg   ×   (20   −   14.eight)   thousand   kg⁻¹  =   163   thou, i.e. half a sock. This shows that the disquisitional factor for indoor evaporation is the limited ambient volume and piddling advantage is obtained by room overheating. Damp laundry needs open windows and continual air exchanges to dry.

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SOLAR CROP DRYING

Gordon Yaciuk , in Solar Free energy Conversion 2, 1981

Psychrometrics

A knowledge of the drying process requires a knowledge of psychrometrics also as a knowledge of the concept of sensible heating, adiabatic saturation, and equilibrium wet content.

Normal atmospheric air is a mixture of dry out air and water vapour. A psychrometric chart is a graphical representation of the concrete and thermal backdrop of atmospheric air for a particular barometric pressure. Well-nigh charts are constructed to be used at a barometric pressure of 101.325 kPa (normal atmospheric pressure at sea level). The psychrometric chart has five sets of skeleton lines equally is shown on Fig. 1.

1.

Dry out bulb temperature–is the temperature of the air as measured by an ordinary thermometer (represented by vertical line on chart)

2.

Wet bulb temperature–is the temperature of the air as measured by an ordinary thermometer whose glass bulb is covered by a moisture cloth or gauze (represented past oblique lines at small angle)

three.

Dewpoint temperature–is the temperature at which moisture condenses on a surface (represented past horizontal line with figures read from left side)

4.

Relative humidity–is the ratio of actual partial force per unit area of the water vapour to the saturation partial pressure level at the aforementioned temperature (represented past lines sweeping upward from left side of chart)

5.

Specific volume–is the volume occupied past a unit weight of dry air (represented by steep astute angled lines)

If any ii values amidst the above are known all other values tin can be obtained. In improver to the v sets of skeleton lines we have the post-obit:

1.

Humidity ratio–is the weight of water vapour diffused through or mixed with a unit of measurement weight of dry air (read on horizontal line from correct-mitt side)

two.

Enthalpy of an air-water mixture–is equal to enthalpy of the air plus enthalpy of the water vapour (read from moisture bulb lines)

The humidity ratio and enthalpy of an air-water mixture are required to determine the size of the heat source.

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