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Application of the Psychrometric Chart and
Comfort Conditions
Learning Outcome
When you complete this chapter you will be able to… Solve problems using a psychrometric chart.
Learning Objectives
Here is what you will be able to do when you complete each objective.
- Interpret the psychrometric chart to find values of specific properties.
- Apply the psychrometric chart to the heating and cooling of air, and calculate heat added or removed.
- Apply the psychrometric chart to the humidification and dehumidification of air, and calculate moisture added or removed.
- Apply the psychrometric chart to combined heating/cooling and humidification problems.
- Discuss what is meant by "comfort conditions", with respect to the psychrometric chart.
Introduction
This chapter will first make the student familiar with the various properties of air-water vapour mixtures as depicted on the psychrometric chart, then look at how this chart is put to practical use. In order to find the reference point of a specific air-water vapour mixture on the chart, two values of the properties of this mixture must be known. Once the reference point has been plotted, all the remaining unknown properties can be easily determined. The following should give the student a good idea how the chart is read.
Objective One
When you complete this objective you will be able to… Interpret the psychrometric chart to find values of specific properties.
Learning Material
Application of the Psychrometric Chart
A sling psychrometer has been used to find that the dry-bulb temperature of the air in a certain area is 25°C (77°F) and the wet-bulb temperature is 18°C (64.4°F). The other five properties of the air can now be found on the psychrometric chart by using the method described below. To make it easier to follow this method, the values found are plotted on the skeleton chart in Fig. 1. However, the student should also attempt to use this procedure on the actual psychrometric chart. Figure 1. Skeleton Chart
Sample Calculation
Step 1 - Find the Reference Point
Locate 25°C on the dry-bulb scale at the bottom of the chart. Follow the vertical line starting at this point upwards until it intersects with the wet-bulb line which runs diagonally downwards from 18°C on the wet-bulb scale. The point at which these two lines intersect is the reference point.
Step 2 - Find the Relative Humidity
The curved line slanting down to the left and passing through the reference point is the relative humidity line. Read the percentage. This is 50%; thus, at 25°C dry-bulb and 18°C wet-bulb temperatures, the relative humidity is 50%. The same result is obtained on the Imperial chart using 77°F dry-bulb and 64.4°F wet-bulb temperatures.
Step 3 - Find the Dewpoint Temperature
Follow the horizontal line passing through the reference point and read the temperature at the point where this line crosses either the wet-bulb scale on the left, or the dewpoint
the conditions of the air change, while others remain constant. This is shown in the first three columns of Table 1. Figure 2. Heating and Cooling of Air Table 1 Sensible Heating and Cooling Notice that during sensible heating the dry-bulb temperature as well as the wet-bulb temperature is raised, but the dewpoint remains unchanged since the specific humidity
remains constant. The relative humidity, however, drops drastically, indicating the need for humidification with sensible heating. The enthalpy is increased since the air is heated; and, due to expansion during heating, the air volume is also increased. When air is cooled without change in moisture content, only sensible heat is removed. The effect is shown again on the psychrometric chart as a straight line, but this time the line starts at B and extends to A, as indicated by the arrow "sensible cooling". If the air in the previous example, leaving the heating coil at 40°C dry-bulb and 19°C wet-bulb temperatures, is cooled down to 10°C dry-bulb by sending it through a cooling coil, all conditions will revert to their original state. The process on the psychrometric chart will be exactly the same as that for sensible heating, as depicted in Fig. 2, except that it now works in the opposite direction. During sensible cooling, dry-bulb temperature, wet-bulb temperature, enthalpy, and specific volume decrease, but the relative humidity increases. Dewpoint and specific humidity remain constant. These changes are indicated in the last column of Table 1. Example 1: A field test on a heating coil reveals that a volume of 4.72 m^3 /s of air enters the coil at 10°C d-b, 7°C w-b. The temperature of the air leaving coil is 40°C d-b. Find: a. the mass of air flowing. b. the volume of air leaving the coil. c. the heat added per kg of air; that is, the change in enthalpy. d. the heating capacity of the coil. Solution: From the IHVE Psychrometric Chart: On coil enthalpy = 23 kJ/kg (dry air) On coil specific volume = 0.809 m^3 /kg (dry air) On coil moisture content = 0.0049 kg/kg (dry air) On coil rh = 65% Off coil w-b = 19° C w-b Off coil specific volume = 0.894 m^3 /kg (dry air)
Fig. 3 shows the approximate effect of a steam injection type of humidifier, although in practice there is usually a small temperature change. In this theoretical example there is no change in temperature, so the humidification line is vertical; i.e. from A to B, and parallel to the dry-bulb temperature lines. Figure 3. Humidification Example 2: A steam injection type humidifier is used in a ventilation system which handles 4.72 m^3 /s of air entering at 25°C d-b, 13°C w-b. The steam is generated at 40 kPa by an electric generator (98% efficient) and the makeup water is at 10°C. Using the psychrometric chart, find: a. The seven properties of the air flow before and after the humidifier. b. The mass of air flowing. c. The volume of air leaving the apparatus. d. The amount of moisture added. e. The amount of steam required. f. The electrical power required. Solution: a. Plot the given entering and leaving conditions on a psychrometric chart as indicated by points A & B on Fig. 3. Tabulate the properties as shown in Table 2. Table 2 Humidification Chart
From this table it can be seen that 0.006 kg (0.010 - 0.004) of water is required to raise the relative humidity of 1 kg of the air from 20% to 50%, and that the enthalpy of the air has increased by 14.5 kJ/kg (50.5 - 36), mainly due to the latent heat required to evaporate the additional moisture. It is interesting to note that six out of the seven properties are increased when using a steam injection humidifier. Only the dry-bulb temperature remains constant.
seldom take place separately. Usually the air is subjected to two processes simultaneously. For example, during the winter season the dry-bulb temperature of the air in a building has to be maintained at the desired comfort level. This means that sensible heat must be supplied to replace the heat lost from the building, and to warm the air drawn in from outside for ventilation purposes. However, a supply of sensible heat alone reduces the relative humidity, making the air drier. If comfortable conditions are to be maintained, it will be necessary to add moisture to the air at the same time, and this requires adding latent heat. Theoretically, this combined heating - humidification process could be plotted on the psychrometric chart as a combination of the two fundamental plots for heating and humidification, as shown in Fig. 4. Assume here that the air is heated first to the required dry-bulb temperature by passing it through a heating coil, and that the air is then humidified by passing it through water sprays. The changes are then shown on the chart as straight horizontal and vertical lines. Figure 4. Theoretical Heating-Humidification Diagram In this example, air of 10°C dry-bulb temperature and 50% relative humidity (point A), is drawn into the system. This air must be supplied to the various areas at 21°C dry-bulb and 50% relative humidity (point C). In the heating coil, the dry-bulb temperature is raised from 10°C to 21°C, as indicated by the horizontal dashed line A-B (sensible heat added). At point B, however, the relative humidity has dropped to 25%. Now the air passes through a steam injection humidifier to raise the humidity. The dry-bulb temperature remains constant but the relative humidity rises to 50% as indicated by the dashed line B-C (latent heat added). In actual practice, however, when two fundamental processes are combined, the changes in conditions caused by both processes will take place almost simultaneously and the psychrometric pattern on the chart will not follow that of each individual process. In the above example, the change in conditions should be plotted along the solid line A-C. A few examples of plotting combined processes on the psychrometric chart are given below.
Heating and Humidification
Air can be heated and humidified at the same time by passing it through water sprays. This is done in air washers which will be discussed in more detail in the next chapter. The basic diagram of this system is shown in Fig. 5, together with a section of the psychrometric chart showing the change in conditions. In this case, the spray water is heated and recirculated. Figure 5. Heating and Humidification When this air passes through the air washer (line A-B), its dry-bulb temperature is raised as it picks up sensible heat. This means that in this process the spray water must be heated so that its temperature is higher than the dry-bulb temperature of the air. The water also adds latent heat to the air in the form of water vapour, raising the dewpoint of the air and increasing the relative humidity. The wet-bulb temperature is also increased. Usually the dry-bulb temperature of the air, after leaving the air washer, is still too low for discharge into air conditioned spaces, while the relative humidity is often too high. By passing the air through a heating coil (line B-C), the dry-bulb temperature will be raised to the required value which, in turn, will reduce the relative humidity to a lower, more desirable percentage. This process is shown in Fig. 5.
Cooling and Humidification
Cooling and humidification is also known as evaporative cooling. The air is again passed through an air washer. The water for the sprays is continuously circulated without any additional cooling or heating and it will assume the wet-bulb temperature of the air. This system is illustrated in Fig. 6. The spray water is recirculated with no additional heating or cooling. Figure 6. Cooling and Humidification
Cooling and Dehumidification (Cooling Coils)
Cooling and dehumidification can also be accomplished by passing the air through cooling coils, as in Fig. 8. The temperature of the coil should be kept well below the air’s initial dewpoint. Figure 8. Cooling and Dehumidification by Cooling Coil The plotted line (A-B) on the chart shows that initially only sensible heat is removed, hence the horizontal part of the line. When the water vapour begins to condense out, latent heat is removed, and the line curves downward. Here again the dry-bulb, wet-bulb, and dewpoint temperatures are lowered when the air passes through the coil, while the relative humidity will approach saturation.
Mixtures of Air
In most air conditioning systems, a certain amount of outdoor air must be introduced into the system to meet the ventilation requirements to "flush out" fumes, odours, and contaminants. The amount of ventilation air also provides a means to minimize air infiltration into the building through cracks and doors by maintaining the building at a positive pressure relative to the outdoors. Fig. 9 shows a schematic duct layout in which a certain amount of room air "A" is recirculated and mixed with outdoor air "B" and the resulting mixture "C" goes to the air conditioning equipment. Note that the mixing dampers are interlocked by a linkage so that they operate together. If the recirculated damper is fully open then the outside air damper is fully closed, and vice versa, over the whole range. Figure 9. Air Mixtures
As illustrated on Fig. 9, the condition of the air mixture can be easily plotted on the psychrometric chart. Point A represents the condition of the recirculated air and point B the condition of the outdoor air. If a line is drawn between point A and B, point C, representing the condition of the air mixture, will fall on this line. The exact position of point C depends on the proportions of the two air streams that form the mixture. For example, if the mixture consists of 50% recirculated air and 50% outdoor air, point C will fall halfway between A and B. If more than half of the mixture consists of recirculated room air, the condition of the mixture will fall closer to point A than to point B.
Objective Five
When you complete this objective you will be able to… Discuss what is meant by "comfort conditions", with respect to the psychrometric chart.
Learning Material
Comfort Conditions
The human body may be compared to a furnace using food as a fuel. For proper functioning (i.e. for comfort), it must be kept at an essentially constant internal temperature of 37°C (98.6°F) by a delicate temperature regulating mechanism within the body. This mechanism allows continuous transfer of heat by conduction, convection, radiation, and evaporation.
The chart is somewhat similar to the psychrometric chart, since it too shows the relationship between dry-bulb temperature, wet-bulb temperature, and humidity. The vertical lines represent dry-bulb temperatures; the horizontal lines, wet-bulb temperatures. The lines slanting from the lower left corner to the upper right corner show the relative humidities, and the lines slanting down from left to right the effective temperatures. If, for example, with the use of a psychrometer it is found that the dry-bulb temperature of the air in a room is 21°C (70°F) and the wet-bulb temperature is 14.5°C (58°F), the relative humidity of the air can be found at the point on the chart where the respective temperature lines intersect. In this case it will be 50%. The effective temperature, read at the same point of the chart, will then be 19°C (66°F). The chart shows that the same feeling of comfort is also experienced with a dry-bulb temperature of 22°C (72°F) and a 30% relative humidity, or with a dry-bulb temperature
of 20.5°C (69°F) and a 60% relative humidity, since in both cases the effective temperature remains 19°C (66°F). A central zone is outlined on the chart consisting of two overlapping areas, ABCD and EFGH. It was found that during the winter the majority of the people felt comfortable when the effective temperature fell within area ABCD; that is, between 17.2° and 21.7°C (63° and 71°F) ET, with 97% of the subjects preferring a 19°C (66°F) ET (line EH). These tests also showed that in summer, due to the need for acclimatization, the effective temperature should be higher and should fall in area EFGH; that is, between 19° and 24°C (66° and 75°F), with 21.7°C (71°F) ET (line BC) being the most satisfactory to 98% of the people. During summer as well as winter, the relative humidity should be kept between 30% and 70%. Above 70% the air will feel muggy regardless of the temperature; below 30% the air becomes too dry, affecting nasal membranes. In summer, ideal conditions would be a 24.4°C (76°F) dry-bulb temperature and 50% relative humidity, the conditions at the centre of the 21.7°C (71°F) ET line. However, it will not always be possible to maintain the relative humidity at that percentage, especially in humid coastal or very dry desert areas. If it is necessary to supply air with a relative humidity higher or lower than 50%, the dry- bulb temperature should be adjusted in order to maintain the effective temperature. Within certain limits, a rise or a drop of 10% relative humidity can be balanced by a decrease or increase of 0.55°C (1°F) in dry-bulb temperature. In winter, a 21°C (70°F) dry-bulb temperature and a 50% relative humidity, 19°C (66°F) ET, will be comfortable for most people. However, the relative humidity will have to be reduced considerably during very cold weather in order to avoid condensation problems. For example, when the outdoor temperature drops to -20°C (-4°F), the relative humidity will have to be reduced to 30%. In order to maintain a 19°C ET, the dry-bulb temperature should be raised to 22°C (72°F).
Variations in the Use of the Comfort Chart
The comfort chart in Fig. 10 is applicable to the northern part of the United States and the most southern part of Canada. It has been proven that upward adjustments should be made for regions farther south and downward adjustments may be necessary for northern latitudes. It has also been found that women, as a group, prefer temperatures one degree higher than those preferred by men. People over 40 prefer temperatures one degree above those chosen by younger people. Since radiant heat cannot be plotted on the comfort chart, the figures on the chart must be lowered for buildings heated by radiant heating. For the same reason, the temperature in areas containing a large number of people should be kept lower since a person receives a certain amount of heat radiated by the people located nearby. The physical activity of the occupants also plays a big role; temperatures should be lowered when activity increases.
the casual visitor. It is also good practice to allow the inside temperature to rise slowly to approach the conditions in Table 3 at the end of the working day, if the outside temperature is still high. Shock effect is not as apparent in winter. Normally, a person puts on extra clothing when leaving a warm building and takes this clothing off again when entering a warm space. This, of course, makes adjustment to different temperatures much easier.
Effect of Air Motion
Air motion has a considerable effect on comfort. An increased air flow increases the heat loss of the body by conduction, convection, and evaporation. This helps the body to keep comfortable when the room temperature is higher than normal. A good example of cooling by increasing the air flow is the use of an electric fan. On the other hand, a higher than normal air flow over the body when the room temperature is normal will make a person uncomfortable. An air flow of 15-25 feet per minute as used on the comfort chart is considered to be relatively still air, but air moving at 65 fpm would be considered a draft by most people. The chart in Fig. 11 shows how the effective temperature, thus the feeling of comfort, drops when the air flow is increased above 20 fpm. Figure 11. Effective Temperature Chart
This chart is applicable under the following conditions: a. Customary indoor clothing is being worn. b. Activity is sedentary or light muscular work. c. The heating method is either steam, hot water, or warm air. Also, the temperatures apply to inhabitants of the United States; for Canada some modification may be necessary. The example given on the chart shows that air with a dry-bulb temperature of 24.4°C (76°F) and a wet-bulb temperature of 16.6°C (62°F) would give an effective temperature of 21°C (70°F) when the air velocity is 20 fpm. When, however, the air velocity increases to 100 fpm, the effective temperature drops to 20.6°C (69°F). If a line is drawn from 16.6°C (62°F) wet-bulb temperature through the point indicating 21°C (70°F) ET on the 100 fpm velocity line, we see that it will be necessary to raise the dry-bulb temperature to 25.6°C (78°F) to offset the effect of the increased air velocity. In air conditioned buildings, the air is usually moved through the ducts at fairly high velocities, often up to 500 fpm. When this air is introduced into the rooms, it should be thoroughly mixed with the room air and its velocity reduced to a low level before it reaches the occupied area of the rooms, so that drafts are prevented.
Summary
Conclusion
From the above discussion, the student should be aware that maintaining a comfortable atmosphere that pleases the majority of the occupants in a building is no easy matter. However, a good understanding of the factors that affect comfort and the variations that may be required will put the building operator in a better position to deal with complaints. The following points should be always kept in mind:
- Know the comfort chart and how it should be varied for the geographical location of the building.
- Consider the variations between temperature desired by men and women or by different age groups.
- Consider the physical activities of the occupants.
- Remember the effects of radiant heat. Rooms on the sunny side of the building with warm walls or sun shining through the windows should be kept cooler. Rooms on the north side or exposed to the wind should be kept warmer in winter. A full auditorium, meeting room, theatre, or crowded department store should be kept at a lower temperature than a room that is sparsely occupied.
- Keep the shock effect in mind for summer air conditioning. Keep the effective temperature higher when most occupants only spend a short time in the building.
