The HVAC Process: Changing the Properties of Air

Transcription

1 The HVAC Process: Changing the Properties of Air Alexander Delli Paoli, Jr. ABSTRACT This discussion addresses the physical properties of air and water vapor mixtures. The components of the heating, ventilation, and air conditioning (HVAC) process that alter the moisture content of air to achieve desired levels in the facility or manufacturing process being served are described. Topics discussed can be thought of as the wet side of HVAC. The wet side includes all aspects of removing and adding moisture to air along with removing water condensed out of the airstream from the air handler. This paper continues a previous discussion describing HVAC systems (1). This and future discussions will provide a working knowledge of the HVAC process. A future paper will address the dry side of HVAC, which includes filters, fans, ductwork, controls, and safety components in the system. INTRODUCTION Psychrometrics is the study of air and water vapor mixtures. It is important to understand this topic and how those properties influence and affect the environment being controlled. A psychrometric chart is a graphic representation of all the possible conditions of air/water vapor mixtures (see Figure 1). Most psych charts are limited to certain areas of the total mixture spectrum. Practical limitations result in the creation of unique psych charts for sea level and differing elevations as well as charts portraying low, normal, and high temperature ranges. It is evident when a chart is not of the proper temperature range. It is unfortunately too easy to use a chart of the wrong elevation. The consequences of this error can result in HVAC equipment components that are not suited for the application. The moisture content levels portrayed by the process cycle on a psychrometric chart translate to desired humidity ranges in the controlled environment. Low moisture content in air is desirable when the concern is control of microbial or mold growth. However, low moisture content is not favorable if static electricity is a concern. Low humidity can also accelerate evaporation of ambient moisture, which can be good or bad for different process environments. Moisture content can also be an important factor for personnel in an environment. High moisture content in air also has positives and negatives, although there are usually more negatives with high moisture content: people feel muggy, microbes and molds flourish, and paper products become soft and perform inadequately in high moisture environments. Positive aspects of high moisture levels include the absence of static electricity and lower evaporation rates of ambient moisture. An optimally higher moisture level also minimizes drying of mucous membranes and could promote a healthier environment. THE PROPERTIES OF AIR AND WATER VAPOR MIXTURES The following discussion is expressed in English units. International (SI) units could also be utilized. Familiarity with the topic is achieved regardless of the units used. This section refers to Figures 1-3 to define the significant parameters presented on the psychrometric chart. Dry-Bulb Temperature Dry-bulb temperature is located on the abscissa of the chart (Figure 1, DB, Point 1). This is the temperature that indicates the addition and subtraction of heat in air. There is no connection to moisture levels when ABOUT THE AUTHOR Alexander Delli Paoli Jr., P.E., is managing director of Engineered Strategic Visions, Inc., Libertyville, IL, USA. Engineered Strategic Visions is an engineering consulting firm specializing in project planning and For more Author information, go to management, manufacturing support, asset and energy management plus several other areas of expertise. gxpandjvt.com/bios[ Alex may be reached at alex@engineervisions.com and at gxpandjv t. com Journal of Validation Technology [SPRING 2012] 75

2 Figure 1: Reading the psychrometric chart. is an important factor toward getting an accurate reading. Impurities in the water change the evaporation rate and the evaporation temperature, delivering erroneous results. Dew-Point Temperature The dew-point temperature parameter (Figure 1, DP, Point 3) is located on the ordinate scale at the right side of the chart. Dew-point temperature is exactly as the name suggests. It is the temperature at which air becomes saturated with moisture at a specific dry-bulb temperature and begins to create dew. Dew is the result of moisture condensing to water and coming out of the air/ moisture mixture. If this condensation is sufficiently rapid, fog forms. Dew point always occurs along the edge of the wet-bulb temperature line (Figure 1, Point 2) at the left side of the chart. presenting dry-bulb temperature. The real-feel or wind-chill temperature reported by network weather people is dependent on wind intensity. A stronger wind will strip the human skin s boundary layer, allowing more heat removal from skin and clothing. Moisture can also play a part, but is usually a small factor if skin is substantially covered by dry clothing. Wet-Bulb Temperature The wet-bulb temperature scale (Figure 1, WB, Point 2) is located on the curved part of the chart running from lower left to upper right. The terms wet bulb and dry bulb have historical origins based on the way the readings were taken with the technology available at the time. Mercury-in-glass thermometers were the instruments of greatest accuracy. Two thermometers were mounted on a sling to be manually spun together by a person. The bulb of one of the thermometers was covered with a fitted sock. The sock was wetted with the purest water available, hence the description wet-bulb temperature. The sling assembly was then spun to cause evaporative cooling on the wetted sock, cooling the covered thermometer bulb, and thus, indicating wetbulb temperature. Once both wet-bulb and dry-bulb temperatures were known, the state of the air/moisture mixture was known. Using the purest water available Specific Volume The specific volume parameter is represented diagonally across the chart (Figure 1, Point 4) running diagonally from upper left to lower right. Specific volume is doubly significant because the reciprocal of the number is the density of the air/moisture content. It is amusing to note that while a pound of dry air is equal to a pound of wet air, a cubic foot of wet air is lighter than a cubic foot of dry air. Enthalpy This property of the air/moisture mixture (Figure 1, Point 5) runs roughly parallel to the wet-bulb temperature scale. Enthalpy is a measure of the heat content of the mixture. It is used to determine equipment capacities for processing the HVAC cycle. Relative Humidity Relative humidity is the most popular way of expressing the moisture content of air. Relative humidity (Figure 1, RH, Point 6) is plotted on the chart running diagonally from lower left to upper right. RH is calculated as a ratio of the partial pressure of the water vapor in the current sample of the air/water vapor mixture to the partial pressure of water vapor at saturation in the current sample. 76 Journal of Validation Technology [SPRING 2012] ivthome.com

3 A l e x a n d e r D e l l i Pa o l i, J r. Absolute Humidity and Vapor Pressure Absolute humidity is not shown on the chart. This parameter is used to determine the actual amount of water that must be added or removed for sizing of HVAC processing equipment. The scale is typically located on a second ordinate scale on the right side of the chart. It is the ratio of the actual mass of water vapor in a sample to the mass of dry air in that same sample. Vapor pressure and other measures specific to the HVAC industry are also available on the chart, but these are of lesser value to understanding the HVAC process cycle. Figure 2: HVAC process cycle warm climate. CASE STUDIES The following case studies describe representative HVAC process cycles at different extremes (see Figures 2 and 3). Case Study One Warm Climate Case study one describes a facility in a warm and humid climate (see Figure 2). The key operating parameters are as follows: Desired process operating conditions: dry bulb temperature: 72ºF relative humidity: 50% RH Ambient (outside) conditions: dry bulb temperature: 100ºF wet bulb temperature: 80 F relative humidity: 45% RH. Note that the ambient relative humidity starts to lose relevance for most people at higher temperatures, so the wet-bulb temperature is provided. The wet-bulb temperature or absolute humidity readings are important for design engineers. The HVAC process cycle on the psychrometric chart will start at the desired operating point (Figure 2, Point 1) in the facility. This is the point representing 72 F/50% RH. Air from the facility is returned via ductwork to the air handler (Figure 4) where it is mixed with outside air at the conditions of Point 2. The two airstreams mix to conditions at Point 3. The total air then passes on to the cooling coils where conditions change from Point 3 to Point 4. At Point 4 within the cooling coil, the air/moisture mixture reaches its dew-point temperature. Moisture begins condensing and continues until the air reaches the temperature at Point 5. The air then leaves the cooling coil and is heated by the air handler or by heating coils in the ductwork to the temperature at Point 6. This additional heating is necessary because the air was cooled below the temperature needed to maintain facility equilibrium. This sub-cooling was necessary to achieve the desired moisture content offered at Point 5. The air then moves on into the controlled facility at Point 1 where it acquires heat being generated within the facility to maintain the desired environmental conditions. This completes the HVAC process cycle. Case Study Two Cool Climate This case study describes a facility in a cooler climate (see Figure 3) operating at the same process operating conditions as the facility in case study one. Comparison of Figure 2 to Figure 3 offers a clear graphic indication of the extremes to which the HVAC process equipment must perform to maintain the desired facility control conditions. The examples portray potential seasonal variations to the HVAC process cycle of a facility located in the Northern Hemisphere. Key operating parameters are as follows: desired process operating conditions: dry bulb temperature: 72 F relative humidity: 50% RH gxpandjv t. com Journal of Validation Technology [SPRING 2012] 77

4 Figure 3: HVAC process cycle cool climate. Outside air taken in: 50% of total volume being circulated by the air handler Ambient (outside) conditions: dry bulb temperature: 34 F relative humidity: 80% RH. The HVAC process cycle on the chart will again start at the desired operating point (Figure 3, Point 1) in the facility. This is the point representing 72 F /50% RH. Air from the facility is returned via ductwork to the air handler (see Figure 4) where it is eventually mixes with air coming from the outside. Notice in this example the mixing occurs only after the outside air at the conditions of Point 2 is heated through the preheat coil in the air handler to a safe temperature at Point 3 that will not expose the other components in the air handler to a freezing hazard. The two airstreams of equal volume then mix to conditions at Point 4. The total air has not been sub-cooled in this example, but it is cooler than is necessary to maintain the facility environmental conditions. The total air passes through a heating coil in the air handler where it is heated to a temperature at Point 5 appropriate to still remove the heat being generated in the controlled environment. Also note that the absolute humidity is increasing between Point 5 and Point 1. This suggests that humidification is occuring to prevent the air from becoming too dry for process conditions. The air then moves on into the controlled facility at Point 1 to maintain the desired environmental conditions. This completes the HVAC process cycle. COILS FOR COOLING AND HEATING This discussion explores the characteristics of coils used in HVAC systems. The name is somewhat of a misnomer; it suggests copper tubing on a bootlegger s distillation unit. That is most likely the origin of the term, but the coils used today must fit efficiently in rectangular air handlers and rectangular ductwork. For this reason, most coils are themselves put in a frame of a rectangular geometry. There is usually copper pipe inside the frame, typically with aluminum heat transfer plates called fins attached to the pipe (finned tubing). Row after row of copper is run back and forth across the airflow surface called the coil face. There are enough rows to cover the entire air stream in the air handler or ductwork. Even that is not enough to ensure complete treatment of the airstream passing through the coil. Several other important design details must be addressed for maximum coil efficiency. Face Velocity The velocity entering a coil is important for three primary reasons: heat transfer, pressure drop, and carry-over. Heat transfer. Heat transfer is best in a velocity range specified by the manufacturer of the coil. If the velocity is too high, air does not spend residence time in the coil to pick-up or give-up enough heat to be effective. If the velocity is too slow, the boundary layer on the fins of the coil get thicker and less turbulent causing less heat transfer. Pressure drop. Pressure drop through a coil is important to the total static pressure needed to drive air through the HVAC system. This is usually a decision made during the system design phase. The energy consumption, noise, and possibly lower heat transfer efficiency of the coil will endure through the life of the facility. Carry-over. Carry-over refers to the moisture being condensed on cooling coils after the air goes below dew-point temperature and is giving up the excess 78 Journal of Validation Technology [SPRING 2012] ivthome.com

5 A l e x a n d e r D e l l i Pa o l i, J r. Figure 4: Schematic of the HVAC system. water in the saturated air. If the velocity of the air is too great, the water droplets will be carried-over back into the airstream and downstream of the cooling coil. This usually means the water lands outside of the coil drain pan and impinges at another point in the air handler. This can cause floods as water accumulates, or higher humidity than desired if the water re-evaporates back into the airstream. Rare instances of fog entering the controlled facility have also been observed. Flooding in unexpected places is the most common occurrence. Number of Rows The entire face of a coil is covered by finned tubing. This is not enough by itself because a percentage of the air bypasses the fins and is not treated. There is also not enough contact time with one row of heat transfer surface. Additional rows are needed to assure the air has enough residence time in the coil to transfer heat or condense moisture. Typical heating and preheat coils range from one to six rows of depth (coil depth). Cooling coils typically range from four to ten rows deep with occasional needs for cooling coils of up to 16 rows of depth. The selection is the resolution of many design constraints by the design engineer. Fins per Length of Pipe Fins per length of pipe, like face velocity, is an important design consideration for the same reasons: heat transfer, pressure drop, and carry-over. Heat transfer is enhanced if fins are spaced close enough together to keep air from getting in between them without being treated. Fins too close together can lead to excessive pressure drop in the airstream. Close fins can be bridged by water droplets condensing in the coil, effectively blocking the unit. This blocking could cause water carry-over and decreased airflow to the controlled environment: another set of constraints for the design engineer and coil manufacturer. Drain Pans Drain pans should be placed under all water-bearing components in air handlers and ductwork. It is a small price to pay at installation to ensure a material failure gxpandjv t. com Journal of Validation Technology [SPRING 2012] 79

6 during the life of a facility does not cause a major flood in sensitive areas below the coils. Most facility personnel think of cooling coil pans as mandatory. Placing drain pans under other coils is a good practice if their rupture could cause undue hardship or business interruption. Cooling coils endure condensation from seasonal moisture in the northern climates. In warmer and more tropical climates, the cooling coils could be wet all four seasons. In either case, it is important to perform periodic cleaning and sanitization of coils and drains to minimize the possibility of microbial contamination. Traps A trap is a down-and-up loop in a drain to keep air from passing through the pipe. Traps are present in all sanitary sewer systems to prevent offensive sewer gases from migrating up through use point such as sinks in bathrooms and kitchens. In the case of a cooling coil trap, the trap is connected to the drain pan. The difference in this case is the fact that the air handler is under negative pressure at this location. This can cause two undesirable issues: air contamination within the air handler from an uncontrolled environment and flooding of the air handler by condensate overflow. Within the air handler (Figure 4) the fan section is pulling air through all the upstream components. This creates an increasing negative air pressure as the air is drawn across each component. By the time air has past the cooling coil, it has been drawn through return ductwork, prefilters, heating coils, and the cooling coil. This negative pressure can amount to multiple inches of water column. To prevent the trap from being blown, it must have an elevation difference outlet-to-inlet of typically six inches or more so the negative pressure can pull the water in the trap up toward the drain pan discharge without allowing the water in the bottom of the trap to pass air. Once the trap is blown-out, the negative pressure causes high velocity air to pass through the trap entraining air and conveying it back into the cooling coil drain pan. The drain eventually fills up and overflows inside the air handler. The leaks that ensue are disruptive to most facilities. Seasonal changes can cause trouble even after the trap has been primed (filled with liquid) initially. During a season when dehumidification is required, condensation flowing out the drain keeps the trap primed. Once a dryer climate occurs, cooling coil condensation stops and slow evaporation of the liquid in the trap begins. When the trap can no longer support the column of water created in it by the negative pressure inside the air handler, the trap blows-out and uncontrolled air starts flowing. This can continue throughout the dry season until condensation begins again, presenting the likelihood of flooding. This could be an undesirable annual event unless preventive measures are in place. Mechanical devices to prevent undesirable airflow exist, but they should be inspected periodically much like the rest of the HVAC system. It is just as simple to make an inspection of the traps part of the overall system preventive maintenance work orders and not introduce more contraptions on the trap. Traps can be filled with a fluid of low evaporation rate such as glycerin during the dry seasons to minimize the threat of a missed inspection. This fluid will be purged from the system when condensate begins flowing again. DEHUMIDIFICATION SOURCES Moisture is usually removed from air in one or more of three ways: cooling coils, desiccants, or compressed air. Cooling Coils Cooling coils are the most prevalent and cost effective method of dehumidification within typical temperature and humidity ranges found in controlled environments. Because temperature and relative humidity have an inverse relationship, it is difficult to quote exact numbers. An example of an operating point is 70 F at 50% relative humidity (RH). Cooling coils are limited to the freezing point of water because the condensation forming cannot be allowed to freeze in the coil and block the airflow. The fluid in the coil could be circulated colder than the freezing point of water with the use of antifreeze, but any temperature below 34 F is risking the freeze hazard. Using refrigerant directly in the cooling coil (direct expansion) is common on many smaller HVAC systems. This eliminates the risk of the fluid inside the coil freezing, but the concern for the condensate on the outer coil surfaces still exists. Control of the temperature leaving a direct expansion cooling coil is also less precise across the operating range of the coil. Desiccant Dehumidification Desiccant dehumidification is used when significantly lower humidity is needed in a facility. The size of the HVAC system is typically limited to serve just the facility requiring the lower humidity. Facilities using this type of system are likely to contain hygroscopic powders or other materials sensitive to moisture pick up. The dehumidification method can use either solid or liquid desiccant materials. Dry desiccants are usually in the shape of a porous wheel. The wheel is rotated between two ducts, each covering half of the wheel. The first half 80 Journal of Validation Technology [SPRING 2012] ivthome.com

7 A l e x a n d e r D e l l i Pa o l i, J r. of the rotation is in a duct performing the moisture removal for the air serving the controlled environment. The moisture-laden side of the wheel then rotates into the air stream of the second duct where the moisture is removed by warm air (wheel regeneration). The warm air then moves on for heat recovery or is exhausted to atmosphere. The wheel continues around again into the controlled air stream. The liquid desiccant system is similar in operation except that the liquid is exposed to the controlled environment airstream in a chamber to pick up moisture. The desiccant liquid is then circulated back to the dehumidification equipment where it is warmed to drive out the moisture. The liquid is then ready for reuse in the circulation loop. Compressed Air Air can be compressed to its saturation point and below to achieve low humidity in very small applications. The compressed air is usually cooled to further enhance moisture removal. When the air is later expanded the amount of moisture in the air can be low. While this is an effective way to remove moisture, it is not cost effective or energy efficient on a large scale. This method of dehumidification should not be ruled-out for small-scale, temporary, experimental, or emergency operations. The capacity of a facility s central compressed air system should be investigated before proceeding with this type of dehumidification system (i.e., tied into the source of compressed air). HUMIDIFICATION SOURCES Moisture is usually added to air in one or more of three ways: evaporation, steam humidifiers, or water humidifiers. Evaporation Evaporation from people and processes operating within a controlled environment contribute to the total moisture in a controlled environment. This can be considered an advantage in dryer seasons and part of the overall cooling load during the seasons of higher humidity. While evaporation is a contributing source in an operating facility, it is not considered a controllable parameter. Steam or water can be infused into air environment in a controllable, metered fashion. Steam Humidifiers Steam humidifiers disperse vapor in air handlers, ductwork, or in the controlled facility directly. Steam is already a vapor, so it poses a minor risk of wetting or flooding the inside of air handlers and ductwork. The steam is distributed to a piping grid typically located across the downstream face of the cooling coil in the air handler. In ductwork, steam humidifiers also consist of a piping grid distributed across the area of the duct. It is good practice to locate a humidity sensor downstream of any internal humidifier to detect saturated air. Condensation and eventual flooding would be the likely outcome of a failure of the primary humidification controls. This condition could occur with a control valve malfunction when the steam is not throttled or turned off adequately. The downstream humidity sensor could be designed to override the normal control sensor and generate an alarm to alert maintenance personnel. A safety concern with steam humidifiers is the hot pipes of the distribution grid. It is important that maintenance personnel are trained to be aware of this potential burn hazard Water Humidifiers Water humidifiers also disperse a mist through a distribution grid, which is evaporated as it is mixed with passing dry air in an air stream. The evaporation of this mist causes cooling of the airstream. Misting humidifiers can also be installed in the controlled facility directly, but they may cause cool spots in rooms due to the evaporative cooling effect of the water droplets. This type of humidifier requires greater attention to location in air handlers and ductwork. If the mist impinges on objects within the equipment, the liquid can build up and cause flooding. This could happen with the air never reaching its saturation point. A high humidity sensor and alarm are also recommended for the same reasons as with steam humidifiers. SUMMARY This paper initiated a more complete discussion of design constraints and details concerning the removal of moisture (wet side) from the air stream in a controlled environment. Topics addressed included psychrometrics and associated parameters including dry-bulb temperature, wet-bulb temperature, dew-point temperature, specific volume, enthalpy, and relative humidity. Case studies described HVAC processes conducted at different extremes. Coils for cooling and heating, and associated design details are described. Methods of dehumidification are discussed including cooling coils, desiccants, or compressed air. Humidification sources including evaporation, steam humidifiers, and water humidifiers are described. REFERENCE 1. Delli Paoli Jr., Alexander, The HVAC Process, Journal of Validation Technology, Volume 17, #4, Autumn 2011.JVT gxpandjv t. com Journal of Validation Technology [SPRING 2012] 81