Presentation on theme: "PSYCHROMETRICS ...WITHOUT TEARS Professor Eugene Silberstein, CMHE"— Presentation transcript:
1PSYCHROMETRICS ...WITHOUT TEARS Professor Eugene Silberstein, CMHE SUFFOLK COUNTY COMMUNITY COLLEGE – BRENTWOOD, NYCENGAGE DELMAR LEARNING – CLIFTON PARK, NYHVAC EXCELLENCE INSTRUCTOR CONFERENCE LAS VEGAS, NEVADAMARCH 20-22, 2011
2What Makes Psychrometrics so Painful for our Students? Unfortunately, most of the time it’s us!
3How Do We Introduce the Topic? You guys are going to hate thisThis stuff is really difficultThis involves a ton of mathYou’re not going to understand this but it’s okay because I don’t eitherI hate it, so you will also
5TEACHING PSYCHROMETRICS IS A LOT LIKE COMMERCIAL FISHING...
6How Much Does the Air in this Room Weigh? 0 pounds? pounds? pounds?100 pounds? pounds?500 pounds? pounds? pounds?THE ANSWER MIGHT SURPRISE YOU...(I Hope It Does!)
7Room Dimensions... Length: 66 feet Width: 46 feet Ceiling Height: 20 feetRoom Volume: 66 x 46 x 20 = 60,720ft3Based on this volume, the air in this room weighs approximately:60,720 ft3 x lb/ft3 = 4,554 POUNDS
10The First Four Things... Dry-Bulb Temperature Wet-Bulb Temperature Absolute HumidityRelative Humidity
11TEMPERATURES: WET & DRY Are all temperatures created equal?Are all pressures created equal?What is the difference between psia and psig?How do we teach our students the difference?How are wet/dry bulb temperatures similar?How are wet/dry bulb temperatures different?Can we create visual examples?
12Dry Bulb Temperature Measured with a dry-bulb thermometer Measures the level of heat intensity of a substanceUsed to measure and calculate sensible heat and changes in sensible heat levelsDoes not take into account the latent heat aspectRoom thermostats measure the level of heat intensity in an occupied space
13DRY-BULB TEMPERATURE SCALE As we move up and down, the dry bulb temperature does not changeAs we move from left to right, the dry bulb temperature increasesAs we move from right to left, the dry bulb temperature decreasesDRY-BULB TEMPERATURE
14Wet Bulb Temperature Measured with a wet-bulb thermometer Temperature reading is affected by the moisture content of the airTakes the latent heat aspect into accountUsed in conjunction with the dry-bulb temperature reading to obtain relative humidity readings and other pertinent information regarding an air sample
15WET-BULB TEMPERATURE SCALE As we move up and down along a wet-bulb temperature line, the wet bulb temperature does not changeThe red arrow indicates an increase in the wet bulb temperature readingThe blue arrow indicates a decrease in the wet bulb temperature readingWET BULB TEMPERATURE
16WET-BULB, DRY-BULB COMBO WET BULB TEMPERATUREDRY-BULB TEMPERATURE
18657075100%7580%WET BULB TEMPERATUREWET BULB TEMPERATURE706860%65DRY BULB TEMPERATURE
19---- HUMIDITY ---- ABSOLUTELY RELATIVE There are two types of humidityABSOLUTERELATIVE“AH” and “RH” are not the sameCannot be used interchangeablyAll humidities are not created equal
20ABSOLUTE HUMIDITY Amount of moisture present in an air sample Measured in grains per pound of air7,000 grains of moisture = 1 pound60 GRAINS1 POUND
21The moisture scale on the right-hand side of the chart provides information regarding the absolute humidity of an air sample
22MOISTURE CONTENT SCALE As we move from side to side, the moisture content does not changeAs we move up, the moisture content increasesAs we move down, the moisture content decreasesMOISTURE CONTENT (BTU/LBAIR)
23WET-BULB, DRY BULB & MOISTURE CONTENT DRY-BULB TEMPERATUREWET BULB TEMPERATURE
24RELATIVE HUMIDITYAmount of moisture present in an air sample relative to the maximum moisture capacity of the air sampleExpressed as a percentageCan be described as the absolute humidity divided by the maximum moisture-holding capacity of the air
25RELATIVE HUMIDITY % FULL = 10 CARS 20 SPACES X 100% % FULL = # of CARS Example #1HOW FULL IS THE PARKING LOT?% FULL =10 CARS20 SPACESX100%% FULL =# of CARS# of SPACESX100%% FULL = 0.5 X 100%% FULL = 50%
28RELATIVE HUMIDITY Example #3 60 GRAINSIf capacity is 120 grains, then the relative humidity will be:RH = (60 grains ÷ 120 grains) x 100% = 50%
29RELATIVE HUMIDITY SCALE As we move along a relative humidity line, the relative humidity remains the sameAs we move up, the relative humidity increasesAs we move down, the relative humidity decreases
34SPECIFIC VOLUME & DENSITY Specific volume and density are reciprocals of each otherDensity = lb/ft3Specific volume = ft3/lbDensity x Specific Volume = 1Specific volume can be determined from the psychrometric chart, density muse be calculated
35LINES OF SPECIFIC VOLUME As we move along a line of constant specific volume, the specific volume remains unchangedAs we move to the right, the specific volume increasesAs we move to the right, the specific volume increasesft3/lb
42100 x 24 x 365 x 5280 x 12 x 2.54 x 10 mm/year, which is.... 100 MILES24 HOURSDAY365 DAYSYEAR5280 FEETMILEXXXHOUR100 x 24 x 365 x 5280 FEETYEAR12 INFT2.54 cmINCH10 mmcmXXXSo, my rate of speed was...100 x 24 x 365 x 5280 x 12 x 2.54 x 10 mm/year, which is....1,409,785,344,000 mm/year!
43Try These Ideas for Your Students If your car get 30 miles per gallon, how many inches per ounce will you be able to travel?If you earn $15/Hour, how many pennies per year will you earn in a year if you work 40 hours per week and 50 weeks per year?If air weight lb per cubic foot how many ounces per cubic inch is that?
44Let Students Take Ownership Ask the right questionsLet the students “create” a formulaLet students identify relevant factors that should be included in the formulaLet students identify relevant conversion factors that should be included
45Total Heat Formula We all know QT = 4.5 x CFM x Δh Where does the 4.5 come from?Work with the unitsQT (btu/hour)What factors will contribute to get this resultFactors must be relevant to sensible heatFor example, grains/pound is not a relevant term as it applies to latent heat
46Let the students “BUILD” the Sensible Heat Formula... Total Heat FormulaQT (btu/hour)= 4.5 x CFM x ΔhUnits on the right must be the same as the units on the leftLet the students “BUILD” the Sensible Heat Formula...
47Heat Formulae Variables So, ask your students what variables and factors will have an effect on the amount of heat transferred by the processΔW?60 MIN = 1 HOUR?CFM?ΔT?Δh?SPECIFIC VOLUME?SPECIFIC HEAT?
48Total Heat Formula We have btu/hour on the left... btu/hour = ? x ? x ? x ? x ?Which factor, Δh, ΔW, or ΔT, is associated with the total heat?btu/hour = Δh (btu/lbAIR) x ? x ? x ? x ?Which other factors are associated with the total heat?
49Total Heat Formula btu/hr = 60 x (btu x ft3)/hour x lbAIR x ? btu/hr = Δh (btu/lbAIR) x ? x ? x ? x ?Airflowbtu/hr = Δh (btu/lbAIR) x ft3/min x ? x ?btu/hr = Δh (btu/lbAIR) x ft3/min x 60 min/hrbtu/hr = 60 x (btu x ft3)/hour x lbAIR x ?
50Density btu/hr = 60 x (btu x ft3)/hour x lbAIR x ? We need to get rid of the ft3 in the numerator and the lbAIR in the denominator...What factor relating to air has ft3 in the denominator and lb in the denominator?Densitybtu/hr = 60 x (btu x ft3)/hour x lbAIR x lb/ft3
51Total Heat Formula btu/hr = 60 x 0.075 btu/hour Density = lb/ft3 at atmospheric conditionsbtu/hr = 60 x btu/hourQT (btu/hr) = 4.5 x Airflow x Δh
52Sensible Heat Formula We all know QS = 1.08 x CFM x ΔT Where does the 1.08 come from?Work with the unitsQS (btu/hour)What factors will contribute to get this resultFactors must be relevant to sensible heatFor example, grains/pound is not a relevant term as it applies to latent heat
53Sensible Heat Formula btu/hour = 4.5 x cfm x lb/hour x ? Specific Heat Which factor, Δh, ΔW, or ΔT, is associated with sensible heat?We already have some of our variables in placebtu/hour = cfm x 60 x x lb/hour x ?btu/hour = 4.5 x cfm x lb/hour x ?We need to add the “btu” to the right side and get rid of the “lb” on the right sideSpecific Heat
54Sensible Heat Formula btu/hour = 4.5 x lb/hour x 0.24 btu/lb The specific heat of air is 0.24 btu/lb/ºFbtu/hour = 4.5 x lb/hour x 0.24 btu/lbbtu/hour = 1.08 x btu/hourAdding in our other variable values gives us:QS (btu/hr) = 1.08 x Airflow x ΔT
55Challenges with the Sensible Heat Formula It doesn’t always give accurate resultsThe 1.08 is only an estimateThe lb/ft3 is not correct most of the timeThe density comes from the specific volumeSpecific volume must be determinedSpecific volume estimate is the average of the values before and after the heat transfer coil
56Latent Heat Formula We all know QL = 0.68 x CFM x ΔW Where does the 0.68 come from?Work with the unitsQL (btu/hour)What factors will contribute to get this resultFactors must be relevant to latent heatFor example, grains/pound is definitely a relevant term as it applies to latent heat
57Latent Heat Formula btu/hour = cfm x 60 x 0.075 x lb/hour x ? Which factor, Δh, ΔW, or ΔT, is associated with sensible heat?ΔW = Change in moisture in grains/lbAIRWe already have some of our variables in placebtu/hour = cfm x 60 x x lb/hour x ?btu/hour = 4.5 x cfm x lbAIR/hour x ?btu/hour = 4.5 x cfm x grains/hour x ?
58Latent Heat Formula 1 pound of water contains 7000 grains btu/hour = 4.5 x cfm x grains/hour x lb/7000 grainsbtu/hour = (4.5 ÷ 7000) x cfm x lb/hourWe need to add the “btu” to the right side and get rid of the “lb” on the right side
59SUPPLY AIRRETURN AIRWater Vapor at 75ºFWater at 50ºF
62Pertinent Enthalpy Information TEMP °FSaturated Vapor Btu/LbSaturated Liquid Btu/Lb
63Latent Heat Formula QL (btu/hr) = 0.68 x Airflow x ΔW btu/hour = (4.5 ÷ 7000) x cfm x lb/hourWe need to add the “btu” to the right side and get rid of the “lb” on the right sideFrom the steam table we get:1094 btu/lb - 18 btu/lb btu/lbbtu/hour = [(4.5 x 1076) ÷ 7000] x cfm x lb/hour x btu/lbQL (btu/hr) = 0.68 x Airflow x ΔW
64You can find automated steam tables at: Enter Temperature HereRead Cool Stuff Here
65MIXED AIR SYSTEMS Return air is mixed with outside air Heat transfer coil does not see return air from the occupied space exclusivelyPercentage of outside air changes with its heat contentProcess is governed by an enthalpy controlThe heat transfer coil sees only the mixture of the two air streams
66LAW OF THE TEE Also known as nodal analysis What goes into a tee, must go out!Electric circuit applicationsWater flow applicationsHot water heating applicationsMixed air applications
71(5 GPM x 100ºF) + (3 GPM x 140ºF) = (8 GPM x YºF) Here’s The Math...(5 GPM x 100ºF) + (3 GPM x 140ºF) = (8 GPM x YºF)= 8YºF920 = 8YºFY = 115ºF
72CLASSROOM DEMONSTRATION or EXPERIMENT LAW OF THE TEE FOR WATERCLASSROOM DEMONSTRATION or EXPERIMENT40ºF70ºF1 CUP CUPHave students predict final mixed temperature.... Then combine, mix, measure and confirm..... Then change the rules!
73CLASSROOM DEMONSTRATION or EXPERIMENT LAW OF THE TEE FOR WATERCLASSROOM DEMONSTRATION or EXPERIMENTTHE RESULTS:40ºF70ºF55ºF15ºF
74CLASSROOM DEMONSTRATION or EXPERIMENT LAW OF THE TEE FOR WATERCLASSROOM DEMONSTRATION or EXPERIMENT40ºF70ºF2 CUPS CUP
75CLASSROOM DEMONSTRATION or EXPERIMENT LAW OF THE TEE FOR WATERCLASSROOM DEMONSTRATION or EXPERIMENTTHE RESULTS:10ºF20ºF40ºF50ºF70ºF
76LAW OF THE TEE FOR MIXED AIR OUTSIDE AIRMIXED AIRAIR HANDLERRETURN AIR
77LAW OF THE TEE FOR MIXED AIR PERCENTAGE OF RETURN AIR + PERCENTAGE OF OUTSIDE AIR100% of MIXED AIROUTSIDERETURN
78LAW OF THE TEE FOR MIXED AIR SAMPLE PROBLEMAIR CONDITIONS: RETURN AIR (80%): 75ºFDB, 50%RHOUTSIDE AIR (20%): 85ºFDB, 60%RHMIXED AIR = 80% RETURN AIR + 20% OUTSIDE AIRMIXED AIR = (.80) RETURN AIR + (.20) OUTSIDE AIRMIXED AIR = (.80) (75ºFDB, 50%RH) + (.20) (85ºFDB, 60%RH)MIXED AIR = 60ºFDB, 40%RH + 17ºFDB, 12%RHMIXED AIR = 77ºFDB, 52%RH