THE DIRTY P WORD!©
by Bill Smith, president of Elite Software
Most contractors hate to hear the P word, one of the dirtiest words that can ever come up in hvac conversations. It's not just that the word is hard to spell, or even hard to say, it's because the mention of psychrometrics usually means problems, and not simple ones at that.
To quote the 1989 ASHRAE Handbook of Fundamentals, "Psychrometrics deals with the thermodynamic properties of moist air and uses these properties to analyze conditions and processes involving moist air." When an hvac system isn't working properly, it can be guaranteed that the psychrometric processes aren't being performed properly on the conditioned air. Whether from a problem with the air distribution system or the refrigerant components, psychrometric processes and thermodynamic properties of air have to be measured and analyzed.
The traditional way to evaluate psychrometric processes is with the use of a "psych" chart (Figure 3) showing thermodynamic properties of air at a sea level barometric pressure of 29.921 inches of mercury. To the novice, a psych chart seems a dizzying maze of lines and curves going every which way. This appearance stems from the fact that a psych chart conveys an amazing amount of information about air. For any given point on a psych chart, one can read the dry and wet bulb temperatures, relative humidity, humidity ratio, enthalpy, and specific volume. Once a few fundamental things are understood about the psych chart, it is not really that difficult to understand.
Most technical people have no problem reading simple graphs that have an X and Y axis crossing at an origin labeled at zero. A pump curve (Figure 1) showing pressure on the vertical axis and flow rate on the horizontal axis is an example of a simple graph. A psych chart is a little different in that it is actually a "window" of a simple graph. This is why a psych chart doesn't show the real origin of the graph, and it draws the vertical axis to the right of the graph instead of at the left.
Figure 2 shows how a psych chart relates to the complete graph that it is a part of. Notice that a psych chart is actually only a small portion of a graph with temperature as the horizontal axis and humidity ratio as the vertical axis. The humidity ratio is commonly measured as grains of moisture per pound of dry air or sometimes just pounds of moisture per pound of dry air. Since virtually all psychrometric air processes involving hvac design occur within a 10 F and 105 F range, most psych charts only show this range as a practical measure.
Most psych charts use the Fahrenheit scale for temperature on the horizontal axis. However, the Fahrenheit scale is not an absolute temperature scale in which the lowest possible temperature is zero degrees. Rather, the lowest possible Fahrenheit temperature is minus 459.67 degrees. In order to show a psych chart graph with a zero origin an absolute temperature scale such as the Rankine scale must be used. Hence, Figure 2 as a representation of what a full range psych chart looks like, was drawn with temperature in degrees Rankine.
Once it is understood that a psych chart is just a small window of a much larger underlying graph, there are only a few more concepts needed for a complete understanding. The next point to be aware of is that a psych chart is really an energy or heat graph. Any point on the psych chart represents air in a specific condition containing a certain amount of heat. The further a point is located away from the lower left corner of the chart, the more heat the air has in terms of btu's per pound of dry air. Move a point either up or to the right on a psych chart, and the air gains heat or increases enthalpy. Enthalpy is the thermodynamic term for the heat content of air.
Since air can gain heat with either an increase in temperature or moisture content, the terms sensible heat and latent heat are used to distinguish how air has gained heat. Sensible heat can be thought of as "dry" heat since a sensible heat gain involves gaining heat with no increase in the moisture content of the air. On the other hand, latent heat can be thought of as "wet" heat since a purely latent heat gain results from adding moisture to the air with no increase in the air temperature. Putting water in the air (humidifying) requires energy, and the more water that air has in it, the higher the heat content of the air.
Besides realizing that a psych chart is basically a graph showing the heat content of air for various conditions, it is important to understand that a point can't be located just anywhere on the psych chart. The bold curved line in Figure 2 represents the condition where air is completely saturated with water for any given temperature. All valid air state points must fall to the right of this line.
For any given temperature of air, there is a maximum amount of water the air can hold in suspension. The hotter the air, the more water that can be held. At cold temperatures, air can hold very little water as indicated by the steeply sloping saturation line. This is the reason that winter months have so much precipitation. Cold weather promotes dry air because low temperatures cause whatever moisture is in the air to fall out as rain, sleet, or snow. However, the main point to remember is that a point cannot be located to the left of the saturation line on the psych chart. Attempting to take air into this region causes precipitation, and thus air can never go beyond the saturation line.
As mentioned previously, the psych chart reveals a great deal of information about air for any given condition. Because of this, there are many ways to "enter" the psych chart and locate a point on it. The most common way an hvac contractor uses the psych chart is with dry bulb and wet bulb temperatures. The dry bulb temperature is a simple concept in that it is the temperature measured by a stationary non-wetted thermometer. The dry bulb temperature is basically a measurement of dry heat or sensible heat.
The wet bulb temperature is very dependent on the moisture content of the air. It is measured by wetting a thermometer and placing it in a moving air stream. What happens in a wet bulb temperature measurement is that water begins evaporating from the bulb of the thermometer. The evaporation of water causes a cooling effect on the thermometer bulb, and the temperature measured by the thermometer drops. If evaporation occurs easily, as with very dry air, there is a significant temperature drop. Conversely, if it is difficult for water to evaporate, as in high humidity conditions, the temperature measured on a wetted thermometer bulb may drop only slightly or not at all. When the wet bulb temperature equals the dry bulb temperature, air is totally saturated with water and no further evaporation can occur.
The most common psychrometric analysis made by hvac contractors involves measuring the dry and wet bulb temperatures of air entering and leaving a cooling coil. If these temperatures are known along with the cfm air flow rate through the coil, the cooling capacity of a unit can be verified. Using the dry and wet bulb temperature information, two points can be located on a psych chart and the corresponding enthalpy values read for them. The total btuh cooling capacity can then be determined by multiplying 4.5 times the cfm value times the enthalpy difference of the two air state points. Contractors often have to perform this calculation to prove that their equipment is working properly on a problem job.
Computer software cannot do anything to help the temperature measuring aspect of psychrometric analysis. Somebody still has to access the air handler and take temperature readings. However, software can greatly aid in the analysis and handling of psychrometric data. With a conventional psych chart, the contractor must use a straight edge and pencil to plot lines and read data. The whole affair is very imprecise as the width of a pencil lead can cause significant error. In addition, most psych charts are printed for sea level barometric pressure. If a project is located thousands of feet above sea level, the standard psych chart cannot be reliably used. Barometric pressure affects the density of air and hence, the thermodynamic properties of air. High altitude psychrometric analysis requires a special psych chart or lengthy manual calculations.
There is actually a wide range of psychrometric software available. Some of it does nothing more than list all the properties of air when given any two conditions about a single state point. Nice, but not that useful. The next level of analysis involves the ability to perform mixed air psychrometrics where two state points are defined along with corresponding cfm values. An ideal application for this type of analysis is the mixing of return air with outside air in the determination of entering cooling coil air conditions. These types of psychrometric software tend to be non-graphical and print only tabular reports.
The highest level of computerized psychrometric analysis involves the ability to display a complete psych chart on the computer screen. Not only are state point properties automatically calculated, but any psychrometric process desired can be simulated. Points can be labeled as necessary, and lines connecting points are automatically drawn as required by the selected process. Additional advantages include perfect accuracy, the ability to automatically adjust for any elevation, and the ability to print psych charts in full graphic detail. Such sophisticated analysis renders conventional psych charts obsolete. Solutions are instantly displayed and "what if" scenarios are quickly analyzed.
Some psychrometric software is actually free of charge. In the March 1985 issue of Heating, Piping, and Air Conditioning magazine, an article entitled Psychrometric Analysis By Computer: A Follow-up listed a program that could be typed in by the user. Another such program was listed in the May 1984 issue of the ASHRAE Journal in an article entitled PSYCHEN.
Commercial sources of psychrometric software include the Carrier Corporation (315-432-6000), the Trane Company (608-787-3445), York International (717-771-6130), UPC Inc. (617-754-6501), and Elite Software (1-800-648-9523). Carrier, Trane, and York have their psychrometric software as parts of a larger software package. Carrier calls their software HVAC Utility programs and Trane calls theirs Applications Engineering Toolbox. York refers to their package as York Tools, while UPC calls their program PSYC. Elite Software offers two psychrometric programs. One is called HVAC Tools and the other more graphic oriented program is called PSYCHART.
Even though software can take the drudgery out of performing psychrometric calculations, a fundamental understanding of psychrometrics is still required before such software can be profitably used. Unfortunately, the majority of hvac service technicians do not fully understand psychrometrics. However, most contractors agree that it is highly desirable for all service people to know psychrometrics.
Besides helping to solve real problems, psychrometric software also helps to educate. Service personnel can quickly test their understanding of psychrometrics by attempting to predict solutions offered by software. By just playing with psychrometric software, technicians can gain a practical "feel" for how psychrometrics works.
Although psychrometrics is often known as that dirty P word, it doesn't have to be that way. A thorough understanding of psychrometrics helps in initial design as well as trouble shooting. By understanding and using psychrometric software, many potential problems can be prevented and profits enhanced. Think of the P word as profits, not problems.
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