Practical Applications of Psychrometrics
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Service Application Manual SAM Chapter 650-002 Section 3A PRACTICAL APPLICATIONS OF PSYCHROMETRICS by Harlan Krepcik, CMS Associate Professor of HVACR, Tidewater Community College, Portsmouth, VA Energy used to change air temperature is defined as BASIC PRINCIPLES sensible heat. Psychrometrics is the science of the physical properties of air and its associated water vapor. All The next most common physical property of air HVACR professionals must have a good command described by customers is relative humidity. of the basic principles that support this science. We Complaints about humidity range from air being too select, install, and service mechanical systems that damp to air being too dry. Relative humidity (RH) is are designed to manipulate the physical properties a value used to communicate the ratio of water vapor of air for the purpose of controlling its temperature, in an air sample to the maximum amount of water humidity level, and cleanliness. This is done to vapor that the air sample can hold at a given dry- provide improved comfort within a conditioned space, bulb temperature. The vapor pressure of the air as well as for other reasons—to protect construction sample is compared to the vapor pressure of that materials used in the building, for example, or to same air sample when saturated with water vapor. preserve food products. The following paragraphs The resulting ratio is converted to a percentage that review eight physical properties of air: we call relative humidity. þ dry-bulb temperature For example, air at a dry-bulb (db) temperature of þ wet-bulb temperature 75°F has a vapor pressure equal to 0.88 inches of þ relative humidity mercury (in. Hg) when saturated with water vapor— þ specific humidity that is, when its relative humidity is 100%. The same þ dew point air sample would have a relative humidity value of þ enthalpy 50% when the water vapor content of the air creates þ vapor pressure vapor pressure equal to 0.44 in. Hg: þ density. The physical property of air most commonly described by customers is dry-bulb temperature. The temperature value is measured in degrees, Air at 50% RH can accept more water vapor. Any either Celsius or Fahrenheit. Heat intensity, not additional water vapor added to air that is already heat quantity, is measured in units of temperature. completely saturated will fail to dissolve into the air, Warmer molecules vibrate more rapidly than do and fog will appear. cooler molecules, and this rate of vibration is called heat intensity. However, a temperature value does The relative humidity of air is important not only not tell you the quantity of heat present in a system. to comfort, but also to providing a healthy indoor Heat quantity is measured in British thermal units, environment. Flu viruses, for example, thrive in air or Btu. The temperature value of an air sample is that is too dry. On the other hand, dust mites cannot only one measurement of several that must be taken survive in dry air. These tiny creatures lack the to calculate the heat quantity present in the sample. ability to drink water. They must absorb water from © 2010 by the Refrigeration Service Engineers Society, Des Plaines, IL Supplement to the Refrigeration Service Engineers Society. 1 the surrounding air to live. Dust mites cannot absorb This smaller and denser “container” cannot hold as water from air when its relative humidity value is less much water mass in solution as the larger “container” than 50%, and as a result they dehydrate and die. at 75°F. At the lower temperature of 55.15°F, the The job of HVACR professionals is to provide indoor existing 64.9 grains of water in each pound of air environments that are both comfortable and healthy. (specific humidity) results in a relative humidity To do so, mechanical systems must be able to control equal to 100%. all physical properties of air. The reverse occurs when air is heated—the density Specific humidity (also referred to as humidity ratio) of the air decreases and its specific volume is the term used to describe a third physical property increases. As the “container” or volume of air of air. This value is a measure of the actual mass of expands, it can hold more water in solution. Consider water dissolved into an air sample. It is measured in the same air sample at 75°F db and 50% RH. When either pounds of water or grains of water that have heated to a temperature of 100°F db, the sample can been evaporated into one pound of air (7,000 grains hold 302.5 grains of water in each pound of air at equals 1 lb of water). As a reference of scale, one saturation (100% RH). At this new higher pint of water has a mass of 1.04 lbm (pounds mass). temperature, the specific volume of the sample Energy used to remove water mass from an air sample increases to 15.083 ft3/lb of air. The “ball” of air or to add water mass to an air sample is measured in increases to a diameter equal to 36.79 in. The units defined as latent heat. The quantity of heat existing 64.9 grains of water create only enough (Btu) required to change the moisture content of air vapor pressure to result in a relative humidity equal varies with the temperature of the air sample. For to 22.6%. This physical property explains why winter example, at a temperature equal to 212°F, heat air that infiltrates into a home and is heated to room quantity equal to 970 Btu is required to change the temperature has a lower relative humidity. phase of 1 lb of water from liquid to vapor, or vice versa. However, when the temperature of standard Clearly, as work is done on an air sample by a air is cooled to 60°F by an evaporator, heat quantity mechanical system, changes in the physical equal to approximately 1,060 Btu is required to properties of that air are significant. An HVACR change the phase of 1 lb of water. professional must be able to measure and calculate the impact of work done on an air sample for the When air is cooled, its specific volume decreases, purpose of predicting the acceptability of an airstream as does its ability to hold water in solution. This is for a given application. The psychrometric chart is a due to the increase in the density of air as it cools. useful tool that allows you to make those calculations Specific volume is the reciprocal of density—in without the need for higher-level mathematics. other words, the two properties are inversely related. Consider an air sample at 75°F db and 50% RH. Another term with which technicians must be The density of the air equals 0.073126 lb/ft3 and its familiar is wet-bulb temperature. The wet-bulb (wb) specific volume equals 13.675 ft3/lb (1 ÷ 0.073126). temperature is measured using a thermometer with Try to visualize this volume by picturing a ball of air its bulb covered with a water-saturated wick. Air 35.6 in. in diameter. At 50% RH, 1 lb of the air is caused to flow over the wick at approximately sample holds 64.9 grains of water in solution. When 900 ft/min, resulting in evaporation of water from saturated, 75°F air can hold 131.8 grains. Notice that the wick. The evaporation process draws heat from relative humidity is a ratio of the vapor pressures of the bulb of the thermometer, causing a decrease in the two air samples, not a ratio of the water mass temperature. When the temperature equalizes, the dissolved into each air sample. When this air sample wet-bulb temperature of the air has been measured. (50% RH at 75°F) is cooled to 55.15°F, its density This temperature is subtracted from the dry-bulb increases to 0.0759 lb/ft3. At this point, its specific temperature of the air, and the difference is called volume decreases to 13.175 ft3/lb. The “ball” of air the wet-bulb depression. A large wet-bulb depression in question now has a smaller diameter of 35.16 in. indicates that the air is dry and readily accepts water 2 from the wick, thus accelerating the evaporation rate. The last physical property of air to be considered A small wet-bulb depression indicates that the air here is enthalpy. Enthalpy is a measure of the total is damp and lacks ability to accept water from the heat content (measured in Btu/lb) of an air sample. wick, thus reducing the evaporation rate. Psychrometric charts do not use enthalpy values referenced to absolute zero (–460°F). Values used on Electronic instruments typically are used to measure the chart are referenced to the heat content of liquid the physical properties of an air sample, since they water at 32°F and dry air at 0°F. This method is afford greater accuracy than sling psychrometers. similar to that used on pressure gauges. Pressure A manually operated sling psychrometer that measurements are referenced to atmospheric includes a slide rule calculator is shown in Figure 1. pressure. A gauge reading of 0 psig does not mean An example of a digital psychrometer used more that the space is under no pressure. Rather, a reading commonly today is pictured in Figure 2. of 0 psig indicates that the space is at atmospheric pressure, which is 14.696 psi at sea level.