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A look at the science of psychrometrics, and a real life example of how it can be used in the field to deliver lasting comfort to customers. BY CAMERON TAYLOR, CM Images courtesy of Fieldpiece Instruments Inc, unless otherwise noted. sychrometrics is simply defined as the measurement of temperature and water vapor mixtures in a given sample Pof air. It is a subject that nearly all HVAC students are taught, and many often struggle to master. It is also almost always taught in an HVAC design con- text; less so from a perspective of field analysis and trouble- shooting. Students who go on to become service technicians often wonder why they were taught psychrometrics when they perceive how seldom they use it in the field. This should not Sensible heat transfer methods (conduction, convection be so, as psychrometrics is the very foundation of HVAC, and radiation) explained visually. Image credit: Nate Adams, both in terms of design/engineering, and in field analysis. “The Home Comfort Book”. The reason we have HVAC in buildings is for human comfort, and the basis for proper HVAC design and function is psychrometrics. Understanding what is required to make Four forms of heat transfer humans comfortable indoors is also foundational for effectively The human body relies on two basic forms of heat transfer designing, installing and servicing HVAC systems. for comfort: sensible and latent heat. Within the sensible transfer form are three methods: convection, which is the Basis for human comfort transfer of heat by a fluid (such as air); conduction, which is The human body creates more heat than it needs, therefore the transfer of heat via solid objects; and radiation, which is the body will always reject this excess heat, regardless of the a transfer of heat via electromagnetic waves, such as the sun environment that surrounds it. It then comes down to warming our planet. Providing comfort by radiation and con- controlling the speed at which body heat is rejected. If it duction is largely a function of the environment surrounding occurs too quickly we will feel cold; too slowly and we feel the body, such as walls, floors, windows and objects within a hot. And if humidity levels are high, the terms “sticky” or room, whereas the HVAC system is primarily concerned with “muggy” are often added to the term “hot.” controlling the sensible and latent heat levels in the air that HVAC is the act of providing an indoor environment ideal surrounds the body. HVAC systems can, however, indirectly for the body to reject heat at a neutral rate, where one is control the rate of radiant and conductive heat losses from not conscious of a need for greater heating or cooling of the the body by conditioned air in contact with surfaces within environment. To achieve this ideal, HVAC designers need a a room. Warmer surfaces will slow body heat loss, whereas thorough understanding of psychrometrics. For the service cooler surfaces will speed up the loss rate. technician, knowing how HVAC equipment can deliver Latent heat transfer from the human body occurs via this ideal involves grasping what psychrometrics, as applied evaporation of perspiration (sweat) from the body’s skin. The to HVAC systems, looks like in the field. evaporative process requires sensible heat, which the body 22 RSES Journal JULY 2018 www.rsesjournal.com An HVACR tech uses a psychrometer to take wet bulb and dry bulb measurements. readily provides through its metabolism. Perspiration evapo- Service technicians, when responding to comfort problems rating from skin will effectively cool the skin’s surface, re- within a building, are often trained to evaluate the HVAC sulting in increased heat loss from the body. The amount of system’s performance by observing system refrigerant pres- moisture that is in the air, however, has a direct bearing on sures, the temperature difference between return and supply how effective evaporative cooling can assist in heat loss from air at the air handler or furnace, and other parameters, such the body. Humid air, or air with high levels of water vapor, as compressor amp draw and indoor blower amp draw. These cannot accept as much moisture evaporating from the skin as are practical measurements that can yield considerable infor- dry air can. Excessively dry air, however, can not only lead to mation about the system at the moment the measurements excessive evaporation from the skin (overcooling), but also to are taken. Often missing are concurrent psychrometric mea- dry skin itself, bringing on discomfort. surements, which would minimally include the return and supply air wet bulb readings along with the more commonly Psychrometrics and HVAC measured dry bulb readings. system performance Electronic devices are now available that can measure re- Precisely creating, delivering and sustaining a comfortable turn and supply conditions simultaneously and report these indoor environment for people, aside from special applica- readings to a smartphone or tablet. This is superior to the tions for industry, etc., is what HVAC is about. The HVAC former method of a mercury filled sling psychrometer, which designer is tasked with knowing how to size a system to meet few technicians would use due to its time consuming method comfort needs year round, whereas the HVAC technician is of delivering information, often with inconsistent results. charged with ensuring the system continues to meet design There are also many apps available to technicians that can expectations over its lifetime of operation. Many times, how- perform airside analysis via psychrometric readings, along ever, the technician was not involved in the design process, with refrigeration system analysis. yet is expected to understand the designer’s intentions in order to restore a system to service that has either broken Psychrometrics in the field down or is experiencing performance shortfalls. Today’s technician, therefore, is well positioned to take advan- If a technician can understand basic psychrometric tage of these advances in data gathering and analysis to bolster parameters preferable for human comfort, he or she will be confidence in analyzing HVAC system performance from a psy- better equipped to evaluate how well a given HVAC system chrometric standpoint. Psychrometric knowledge can be useful is meeting these parameters. ASHRAE Standard 55 defines in HVAC system analysis and troubleshooting in the field. indoor thermal comfort as establishing conditions likely to Psychrometric heat transfer equations—The first step be acceptable to at least 80% of the adult occupants in a for optimizing system performance involves a little math. space. In practice, many residential and commercial systems are While there are three common heat transfer equations based designed for indoor summer conditions to be maintained at on whether one is measuring differences in temperature, 75°F dry bulb at 50% relative humidity, and winter conditions moisture or a combination of both, the most useful ones for to be held at 70°F dry bulb, often with no specific relative field work, based on measured airflow enthalpy in real time, humidity target. are the following: www.rsesjournal.com JULY 2018 RSES Journal 23 A techni- cian checks a supply duct to analyze and troubleshoot an issue. 1. Total heat (sensible + latent): in his city. The customer complaint was that the system cools 4.5 x cfm x ∆E = Btuh total the house but the air inside always feels “sticky” to her, no matter how long the HVAC system runs on a hot day. Wes Btuh = British thermal units per hour heat added or removed has been onsite awhile, gathering readings he knows he needs cfm = cubic feet per minute of air moving through air handler or to determine the three-ton system’s superheat, subcooling, and furnace refrigerant pressures in the evaporator and condenser. He has ∆E = enthalpy difference (example: between return and supply air) entered the following readings into a popular HVAC refrigera- 4.5 = a “factor” derived from time, weight, volume, and thermal tion performance analysis app installed on his tablet: properties of air gReturn air wet bulb (measured at ceiling return air grille 2. Sensible heat: adjacent to attic): 68°F 1.08 x cfm x ∆T = Btuh sensible gReturn air dry bulb (at same location): 75°F gSupply air dry bulb (measured at nearest ceiling supply ∆T = temperature difference between return and supply air streams outlet to evaporator): 64°F 1.08 = a “factor” derived from time, weight, volume, and gOutdoor air dry bulb: 95°F thermal properties of air gSuction line pressure (R-22): 69 psig 3. Sensible heat ratio: gSuction superheat: 13.5°F Sensible heat Btuh/total heat Btuh gLiquid line pressure (R-22): 228 psig gLiquid subcooling: 10.5°F Psychrometrics as an analysis gMeasured cfm (obtained via TESP and blower chart): 1160 cfm and troubleshooting tool In the field several digital psychrometer instruments on the Wes then brings up the diagnostics screen on his app, which market can provide enthalpy values easily by inserting a probe states that superheat, subcooling and airflow are all “okay.” He in the supply and return air streams of a running system. Cfm is dismayed, as he is certain all of his readings are accurate. Yet can be obtained by first measuring total external static pressure even he remarked to the customer that the house felt humid to (TESP) and then finding how much air the system’s blower him after spending time inside gathering his readings. is expected to move with the measured TESP. Often this He begins doubting his supply air reading, however, so he not chart is attached to the air handler, or can be looked up only takes it again, but this time in the attic where the system’s air online if the make and model number are available.

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