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PDHonline Course M226 (5 PDH)Psychrometric Chart Fundamentals andits application to HVAC TroubleshootingInstructor: Timothy D. Blackburn, MBA, PE2012PDH Online PDH Center5272 Meadow Estates DriveFairfax, VA 22030-6658Phone & Fax: 703-988-0088www.PDHonline.orgwww.PDHcenter.comAn Approved Continuing Education Provider

www.PDHcenter.comPDH Course M226www.PDHonline.orgPsychrometric Chart Fundamentals and its application to HVAC TroubleshootingBy: Timothy D. Blackburn, PE, MBACourse ContentIntroductionIf HVAC problems and challenges are to be properly diagnosed and designed, it is essential thatthe Psychrometric chart and psychrometrics in general be clearly understood. This course will review theessential elements of psychrometrics (the behavior of mixtures of air and water vapor under varyingconditions of heat) and the chart that represents it. With this knowledge, you will be able to understandmany of the HVAC challenges that are certain to arise, as well as anticipate problems before they occurand incorporate in your design.Like many self-study courses, you will only get as much from it as you put into it. Plot each stepon your own psychrometric chart. Be certain you understand the basics before studying the examples. Tryto solve the examples for yourself. This is not the type of course you can just read through and take theQuiz – it will take effort. But if you do your best, you should be able to pass the Quiz with ease – whetherbeginner or advanced.Disclaimer – Nothing in this course should be considered consulting engineering for your specificapplication – each situation requires individual analysis.Definitions and ConceptsIt is important to understand the primary concepts and definitions before we begin our study. For someof you, this will be a refresher, and for others an introduction. There are other courses that cover thefundamentals in more detail, but the following are the minimum essentials.Page 1 of 38

www.PDHcenter.comPDH Course M226www.PDHonline.org HVAC – Heating, Ventilation, and Air conditioning Psychrometrics –The behavior of mixtures of air and water vapor under varying conditions of heat Enthalpy Total heat in the air Sensible plus Latent heat Sensible Heat – Changes in temperature that do not alter the moisture content of air Latent Heat – Related to level of moisture in the airBTU (British Thermal Unit) – The amount of heat that must be added to or subtracted from a pound ofwater at 60oF to affect a temperature change of 1oF BTUH or BH – BTU’s per hour MBH – 1000 BTUHTon 1 Ton equals the amount of heat needed to melt 1 ton of ice in one day 12,000 BTUH’sDrybulb Temperature – The temperature reading given by a dry thermometer that gives a directindication as to the sensible heat content of air Wetbulb Temperature – The temperature reading from a wetted bulb that gives a direct indication as tothe total heat content of air Dew Point Temperature – Temperature at which air will begin to release moisture. Relative Humidity (RH) The actual amount of moisture in the air expressed as a percentage of the amount of moisture the airis capable of holding. More technically: The amount of water vapor in the air divided by the amount of water vapor the air can hold (atthe same temperature and pressure.)Page 2 of 38

www.PDHcenter.com PDH Course M226www.PDHonline.orgThe ratio of the air's vapor pressure to its saturation vapor pressure.Example: An air sample that is at 50% RH is holding half the moisture it is capable of holding at thesame temperature (at dew point or saturated.) RH is inversely relational to temperature for the same moisture level (grains of moisture per poundof dry air) – warm air can hold more moisture RH is what we sense High RH: Sticking, mold Low RH Affects electronics, promotes static Low RH air is seeking saturation, absorbing moisture wherever it canSpecific Humidity or Humidity Ratio The weight of the water vapor in each pound of dry air Typically grains of moisture/pound of dry air Grain 1/7000 pound Density – Unit weight of dry air at a given temperature and moisture content, #/ft3 Specific Volume – Space occupied by dry air at a given temperature and moisture content (thereciprocal of density), ft3/#The Psychrometric Chart TemplatePause and print off the next page. Make multiple copies, as you will need them in the course. As notedpreviously, if you are to benefit from the course, you must plot each step for yourself. There are noshortcuts in learning this material. The smaller example charts are not intended for you to plot on, butrather instruction – use a clean larger copy to plot each example.Page 3 of 38

www.PDHcenter.comPDH Course M226PSYCHRO M ETRIC CHART7570Sea Levelwww.PDHonline.org8085903001.9BARO METRIC PRESSURE 29.921 inches of .6240BULB90TEAIR50DR Y- FREAT UPE RT EMIONRATSATU9 0%70.975.870.665606070.5.4506510706545%30%75IDH UMT IV E80IT Y20108590956050.34035.2302520.155100-40100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 18050DRY BULB TEMPERATURE - FLinric Company Psychrometric Chart, 4050403510150851.3DEW POINT - FOFNDPOUER- BTUPEN THA 10- lb OF6515-15E³/ME .0801.5220R1908530-5PE854025M9045902301520Page 4 of 382530354045ENTHALPY - BTU PER POUND OF DRY AIRETVAPOR PRESSURE - INCHES OF MERCURYWHUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR9555

www.PDHcenter.comPDH Course M226www.PDHonline.orgThe Psychrometric Chart – an OverviewBefore we proceed further in our study, let’s learn or refresh regarding the Psychrometric chart. Thefollowing is a summary of the major elements on the chart. Do not continue to the next section until youcan find the following on the chart. Constant Drybulb Temperature: Vertical Lines Constant Dew Point and Humidity Ratio: Horizontal Lines Constant Wetbulb temperature: Upward left sloping lines Relative humidity: Curving lines (100% line is the saturation curve or correlates with Dew Point) Constant specific volume, ft3/# of dry air: Nearly-vertical sloping lines Enthalpy or total heat, BTU/pound of air: Staggered scale left of saturation curve and left slopinglines Humidity Ratio: Right hand scale, grains of moisture/pound of dry air Saturation Curve: 100% RH Curve (or the point at which an air mixture can hold no additionalmoisture at a given temperature); temperature on the curve is the Dew PointThe following is a chart with the above noted (note: this is a standard curve is at Sea Level).Page 5 of 38

www.PDHcenter.comPDH Course M226PSYCHROMETRIC CHART7570Sea Levelwww.PDHonline.org8085903001.9Heavy Text Bullet Slide SampleBAROMETRIC PRESSURE 29.921 inches of E HU8545.34035.2302520.155100-40100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 18050DRY BULB TEMPERATURE - FLinric Company Psychrometric Chart, www.linric.com10157065606017575.70%VELATI10% R401DEW POINT - e 6 of 382530354045ENTHALPY - BTU PER POUND OF DRY 001607065555-517011080%6545-10 -5-25 -20 -15807555-30 -25 -20 -15 -108751807050108030105190 FTEMPERATURESATURA TION635200VAPOR PRESSURE - INCHES OF MERCURYAIRDRYOFOUNDERPY-BTUP40901.4210- F85851.5220RATURE904590230TEMPEHUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR901515BULB5060-5240IRRY AOF D8.WETE ft³/lbLU 14.0Constant dry bulb temperatureConstant Dew Point andHumidity RatioConstant wet bulb temperatureRelative humidity (100% line isthe saturation curve or DewPoint)Constant specific volume, ft3/#of dry airEnthalpy or total heat,BTU/pound of airHumidity ratio or grains ofmoisture per pound of dry airSaturation curve – 100%humidity line or the point atwhich an air mixture can holdno additional moisture at agiven temperature;25temperature on the curve is20the dewpointENTH1.2.17.016.0601.8270

www.PDHcenter.comPDH Course M226www.PDHonline.orgLatent versus Sensible ChangesIn the next graph, let’s look at changes in Latent versus Sensible changes. Latent changes move inthe “Y” axis (associated with moisture content changes), and Sensible changes move in the “X” axis(associated with temperature but not moisture content changes.)PSYCHROMETRIC CHART7570Sea Level808590300Heavy Text Bullet Slide SampleBAROMETRIC PRESSURE 29.921 inches of 501.6240ET90NDOFDRYAIRTURE- FPOUPERERABTUIONTY-EMPAIRDRYSATURATlb OFEN 00 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 18050DRY BULB TEMPERATURE - FLinric Company Psychrometric Chart, www.linric.com75656070YMIDITVE HUELATI10% R6080.78020%551.21.1DEW POINT - F153013.050-5-25 -20 -15 0505-30 -25 -20 -15 IC757065180SPE807525Latent20015.080901.4210- 0Page 7 of 382530354045ENTHALPY - BTU PER POUND OF DRY AIRWVAPOR PRESSURE - INCHES OF MERCURY9555HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIRSensible versusLatent: Sensiblechanges arehorizontal, andLatent changes arevertical

www.PDHcenter.comPDH Course M226www.PDHonline.orgRelationship of Dewpoint, RH, Wetbulb, and DrybulbThe following chart provides an example to determine Dewpoint, RH, Wetbulb, or Drybulb if onlytwo of the criteria are known. Drybulb is read with a typical thermometer. When I was in engineeringschool, we determined Wetbulb by using a sling psychrometer. In it was a Drybulb, plus anotherthermometer with a wet gauze. The Wetbulb reading was affected by the moisture content in the air; thelower the air moisture content, the faster the gauze evaporated and the cooler the temperature reading.Today, electronic instrumentation is generally used.PSYCHROMETRIC CHART7570Sea Level8085903001.9Heavy Text Bullet Slide SampleBAROMETRIC PRESSURE 29.921 inches of Mercury29065100280Relationship ofDewpoint, RH,Wetbulb, andDrybulb: Example:What is the Wetbulb temperature of70oF air at 50% TIV% 55100-40100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180DB50DRY BULB TEMPERATURE - FLinric Company Psychrometric Chart, www.linric.com10157055%757565605020%6080.7DEW POINT - 180RH55352560508520Page 8 of 382530354045ENTHALPY - BTU PER POUND OF DRY AIRIR120851.3VAPOR PRESSURE - INCHES OF MERCURY FERATURTEMPEIONRATSATUADRY130HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY 5515040353025016014.0WBDP00807565400170lb OF205-5807545-5-25 -20 -15 -10180ft³/UMEVOL3555-30 -25 -20 -15 -10200CIFICSPE40901.4210- B9030155WET9025-59550What is the DewPoint? 17.016.0601.8270

www.PDHcenter.comPDH Course M226www.PDHonline.orgImpact to Relative Humidity from Sensible ChangesThe following chart illustrates the impact to Relative Humidity from changes in temperature (whenmoisture content remains constant.) You will note that Temperature and Relative Humidity are inverselyrelational – the higher the temperature, the lower the RH. The lower the temperature, the higher the RH.PSYCHROMETRIC CHART7570Sea Level808590300Heavy Text Bullet Slide SampleBAROMETRIC PRESSURE 29.921 inches of 501.6120 556065.6.56055.4%30201010YMIDITE HULATIV% RE7580859095605045.34035.2302520.155100-40100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 18050DRY BULB TEMPERATURE - FLinric Company Psychrometric Chart, www.linric.com107065606020%70757050%6580.7DEW POINT - F30358080%20251.11520Page 9 of 382530354045ENTHALPY - BTU PER POUND OF DRY AIR1301.2VAPOR PRESSURE - INCHES OF MERCURYAIRDRYPOUPERY-BTUENTHALPSATURATIRRY AOF 001106560550FIFICPEC6510-575901.4210- 7015-10 -5-25 -20 -15807525-30 -25 -20 -15 015BULB9030Answer: 84.85%1024090What if we cool to55oF instead?5WET50Answer: 25.98%-59555HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIRSensible Changes:Example: If we heat70oF at 50% RH to90oF without addingmoisture, what is thenew RH?

www.PDHcenter.comPDH Course M226www.PDHonline.orgDehumidificationAs we saw in the previous section, when the temperature cools the RH increases. What happenswhen the RH is 100%? It rains. As cools, it eventually reaches its dewpoint and moisture begins toappear. That is what happens when you have a glass of iced tea, and there is moisture on the outside – theair in the immediate vicinity of the glass cools to the point it is fully saturated, and out comes moisture. Inthe following example, we continue on with the previous example to examine what happens when wecontinue to cool.PSYCHROMETRIC CHART7570Sea Level808590300Heavy Text Bullet Slide SampleBAROMETRIC PRESSURE 29.921 inches of 501.6240BULB90TEAIR50DRYOF190RATURE- RATIONALP200lb OFENTHFft³/UMEVOL35- %6560801.975.8706545-501050-55151012.00-5-25 -20 -15 -101052015152020252530303540453013.03525-30 -25 -20 -15 -