Air Properties And Measurement |
Air conditioning systems can provide for all the following
-Sensible Heating
-Sensible Heating and Humidification
-Chemical De-hydration
-Sensible cooling
-Cooling + De-humidification
-Evaporative Cooling
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Psychrometrics Terminology for Air Properties |
-Dry Bulb Temp(DB): The temp of the air in °F or °C
-Wet Bulb Temp(WB): The temp of the air taking into consideration the amount of moisture it contains
-Sling Psychomotor: Instrument used to measure wet and dry bulb temperatures
-Relative Humidity(RH): Percentage of water vapor in the air I relation to the max it can hold at any given temp
-Specific Humidity(SP.H ): The moisture content of a given sample of air expressed in grains
-Specific Volume(SP.V): The amount of space in cubic feet occupied by 1 lb of air
-Dew Point: The temp at which moisture will start to condense out of a given sample of air
-Enthalpy(TH): Measurement of heat content of a given sample of air expressed in BTU/Lb
-Sensible Heat(SH): Amount of heat added or removed from a given sample of air expressed in BTU/Lb
- Latent Heat(LH): Amount of heat added or removed from the moisture present in a given air sample expressed in BTU/Lb
-Sensible Heat Factor(S.H.F): Amount of total heat used to change the temp of a given sample of air
Process Represented On The Chart
-Sensible Heat Processes: Represented by a horizontal line indicating a change in the temp but no change in specific humidity
-Latent Heat Process : Represented by a vertical line indicating a change in specific humidity but no change in temp
-Cooling Process : Represented by a horizontal line running from right to left
-Heating Process : Represented by a horizontal line running from left to right
-Cooling + Dehumidification : Represented by a diagonal line running from top to bottom
-Heating + Humidifying: Represented by a line (diagonal) running from bottom to top
-Dehumidifying process : Represented by a vertical line running from top to bottom
-Humidifying Process: Represented by a vertical line running from bottom to top
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Air Measuring Devices
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Manometers |
(Oil Filled) |
U-Tube
Inclined
Pitot Tube |
*Note: The above are used to measure duct pressure such as: Static Press, Velocity Press and Total Press |
*Note: The above are also used to measure the static pressure drop across filter banks which will indicate the need for replacement |
Magnehelic
-(No-oil filled)
-Used for direct measurement of pressure across filter banks
Anemometer or Velocity Meter
-Used for Direct measurement of velocity in FPM-(Feet per Minute)
Calculation of Velocity and Volumes
1. A single duct/ single zone A/C roof top unit is supplying air to the conditioned space by way of a 24” by 12” supply duct. Calculate the air velocity as well as the volume (CFM). A pitot tube manometer reads 0.06 inches water column.
| Area = |
L x W ÷ 144 = |
24 x 12 ÷ 144 |
= 2 SQ Feet |
| CFM= |
Area (SQ. FT) x Velocity (FPM) |
| 2 x 981 |
| 1962 CFM |
2. A single duct/ single zone A/C roof top unit is supplying air to the conditioned space by way of a 15” round supply duct. Calculate the air velocity as well as the volume (CFM). A pitot tube manometer reads 0.09 inches water column.
| Velocity= |
4005 x Square root of velocity press |
| 4005 x Square root of 0.09 |
| 1201 fpm |
| Area = |
Pie x R²÷ 144 |
3.14 x 7.5² ÷ 144 |
= 1.23 SQ Feet |
| CFM= |
Area (SQ. FT) x Velocity (FPM) |
| 1.23 x 1201 |
| 1473.1 CFM |
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Psychrometric Formulas
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| SHF= |
Sensible Heat ÷ Total Heat |
| Bypass Factor = |
(Leaving Db– Coil Temp)÷(Entering Db–Coil Temp) |
| Total Sensible Heat Formula = |
1.08 x CFM x Change in temp |
| Approx system Capacity= |
4.5 x CFM x (Change in Enthalpy) or (Total CFM ÷ 400) |
| Area of a rectangular Duct= |
L x W ÷ 144 |
| Area of a round Duct= |
Pie diameter squared ÷ 4 x 144 |
| Air Mixture Temp Formula |
(CFM or Air 1 x Temp of Air 1) + (CFM or Air 2 x Temp of Air 2)
Total CFM |
| % of Outdoor Air = |
Mixture temp – Return Air Temp
Outdoor Temp – Return Air Temp |
| Latent Heat Formula= |
0.68 x CFM x Delta Grains/Lb = BTU h |
| Total Heat Formula= |
4.5 x CFM x Delta BTU/lb (Enthalpy) |
| Sensible Heat Formula= |
1.08 x CFM x Temp D= BTU h |
| CFM = |
BTU h =
1.08 x TD |
Volts x Amps x BTU/watt
1.08 x TD |
| RPM2 ÷ RPM1= |
S.P.2 ÷ S.P.1 |
| (RPM2 ÷ RPM1)³ = |
B.H.P.2 ÷ B.H.P.1 |
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Derivation Of heat Formulas |
Sensible Heat Formula
-Density of standard air is 0.75 lbs/cu. ft.
-Specific heat of std. air is 0.24 BTU/lb/°F
| Qs = wt x sp .ht. x temp Diff |
| BTUh = cu.ft/min x 0.75 lbs/cu.ft x 0.24 btu/lb/°F x 60 min/hr x Temp Diff |
| Sensible Heat Formula= |
1.08 x CFM x Temp Diff = BTU h |
Latent Heat Formula
-Latent heat of vaporization of water at 70°F is approx 1055 BTU
| BTUh= |
1055 BTU/lb x 0.75lbs/cu.ft x 60 min
7000 Grains/lb
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÷ hr x CFM x Grains/Lb Diff |
| Latent Heat Formula= |
0.68 x CFM x ? Grains/Lb = BTU h |
Total Heat Formula
| Qt= 0.75 lbs/cu.ft x 60 min/ hr x CFM x ? BTU/lb |
| Total Heat Formula= |
4.5 x CFM x BTU/lb Diff (Enthalpy) |
1200 CFM of air enters an electric furnace at 60°F DB (Dry Bulb). It leaves at 105°F DB. How many BTU's were added?
| BTU's= |
1.08 x CFM x Temp Diff |
| 1.08 x 1200 x Temp Diff(105-60) |
| 58320 BTU's |
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Sensible + Latent + Total Heat from Psychrometric Charts |
Step#1
When two points are plotted on the chart join the lines. *Note: extend to the wet bulb line to receive apparatus dew point
Step#2
Next make a right triangle. In this case draw a line A to B. Do it horizontally to make the triangle
Step#3
Extend lines to the enthalpy section from points A, B and the 90° section of the triangle |
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Process Lines |
- When ever plotting make efforts to create process lines for better understanding and marks
-They always vary graph to graph but here are some examples |
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RA |
Return Air |
SA |
Supply Air |
CD |
Cold Deck = Cooling Coil |
HD |
Hot Deck = Heating Coil |
OA |
Out Door Air |
MA |
Mixed Air: Is given or extracted from formula |
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Psychrometric Chart Examples |
| #1 |
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| #2 |
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| #3 |
System:
-RA= 1000CFM
-Single duct terminal re-heat
-DX Coil
-Entering Coil= 70F DB @ 40% RH
-Leaving Coil 55F DB @ 60% RH
-1000 CFM Leaves Coil
-400 CFM = Zone #2 = Re-heat 105F
-Exhaust 350 CFM: No Pressurization |
Plot and Record the following:
-KW Rate of Zone #2 Heater
-Total Sensible Heat removed
-Total Latent heat removed
-Apparatus Dew Point
-Sensible Heat Factor
-Coil Bypass Factor
-System Capacity/ Total Enth
-*Note: 3412 BTU in a KW |
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-KW Rate of Zone #2 Heater: 1.08 x 400 x 50 = 216000BTU ÷ 3412 = 6.3 KW
-Total Sensible Heat removed: 1.08 x CFM x Temp Diff = BTU h
- 1.08 x 1000 x 15 = 16200 BTU
-Total Latent heat removed: Total heat(20250) - sensible heat(16200) = 4050
-Apparatus Dew Point: 35.5
-Sensible Heat Factor: Sensible heat(3.5) ÷ Total Heat(4.5) = 0.78
-Coil Bypass Factor: (Leaving Db– Coil Temp)÷(Entering Db–Coil Temp)
-(55-35.5)÷(70-35.5)= 0.565
-System Capacity/ Total Enth: 4.5 x CFM(1000) x BTU/lb Diff (4.5)= 20250
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| #4 |
System:
RA enters a DX coil of a single duct terminal re-heat A/C system at 76F DB @ 50% RH resulting in a saturation temp of 55F DB @ 80% RH
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Plot and Record the following:
-Total Heat removed
-Sensible Heat removed
-Latent heat removed
-Change in SP. Heat
-Change in SP. Volume
-Apparatus Dew Point
-Sensible Heat Factor
-Coil Bypass Factor
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7.6 BTU/lb
5.4 BTU/lb
2.2 BTU/lb
14 Grains
0.5625 cubic feet
45F
0.71
0.032 |
1. Plot 76 DB @ 50% RH
2. Plot 55 DB @ 80% RH
3. Having drawn just the two lines join the tops
4. This new line is represents cooling + Dehumidify
5. Plot the Wet bulb lines
6. Get the enthalpy numbers
7. Total heat equals larger(28.7) - smaller(21.1) = 7.6
8. Enthalpy: Make your Right triangle: (Latent heat: 28.7-26.5= 2.2) & (Sensible heat: 26.5- 21.1= 5.4)
9.
Sensible Heat Factor: Sensible heat(5.4) ÷ Total Heat(7.6) = 0.71
10. Coil Bypass Factor: (Leaving Db– Coil Temp)÷(Entering Db–Coil Temp)
(55-45)÷(76-45)= 0.032
11. Last: Change 55F air to 100F for zone 1
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| #5 |
System:
RA enters a DX coil of a single duct terminal re-heat A/C system at 74F DB @ 60% RH resulting in a saturation temp of 56F DB @ 90% RH. Then is re-heated at zone 3 to 101F
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Plot and Record the following:
-Total Heat removed
-Sensible Heat removed
-Latent heat removed
-Change in SP. Humid
-Change in SP. Volume
-Apparatus Dew Point
-Sensible Heat Factor
-Coil Bypass Factor
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7 BTU/lb
4.5 BTU/lb
2.5 BTU/lb
17 Grains
0.5 cubic feet
51F
0.643
0.0217 |
- Sensible Heat Factor: Sensible heat(4.5) ÷ Total Heat(7) = 0.643
- Coil Bypass Factor: (Leaving Db– Coil Temp)÷(Entering Db–Coil Temp)
(56-51)÷(74-51)= 0.0217
- Total Sensible Heat removed: 1.08 x CFM x Temp Diff = BTU h
1.08 x 1000 x (74-56) = 19440 BTU
-System or Capacity Total: 4.5 x CFM x Delta BTU/lb (Enthalpy)
4.5 x CFM(1000) x Delta(7) BTU/lb = 31500
-Zone#3: How many KW to Heat air in zone #1?
56 F to 101 F = 55F TD
1.08 x CFM x TD
1.08 x 400 x 55
=23760: Then divide by 3412 BTU/ KW
=6.96 KW to heat heat air in zone #3
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| #6: Question |
A dual path multi Zone AC Distribution System has 2000 CFM of return air at 75 DB and 60% mixing with 400 CFM of outdoor air at 68F DB @ 40% RH. 1200 of this CFM enters the hot deck and is heated to 105 DB, the cold deck is maintained at 47F DB and 80%. What are the resulting premixing box conditions?
Find: DB, WB, DP, RH, Sp H, Sp v, Total CFM
Also find: ADP, Coil Bypass Factor, SHF, App System Capacity
Find the discharge temp from Zone #5 terminal mixing box if 500 CFM of cold deck air is mixed with 550CFM of hot deck air.
Find: DB, WB, Total CFM
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| #7: Question |
A dual path multi Zone AC Distribution System has 1700 CFM of return air at 78 DB and 40% mixing with 1100 CFM of outdoor air at 60F DB @ 35% RH. The resulting mixture is divided equally thru both hot and cold decks. The hot deck and is maintained at 105 DB, the cold deck is maintained at 50F DB and 70%. What are the resulting premixing box conditions?
Find: DB, WB, DP, RH, Sp H, Sp v, Total CFM
Also find: ADP, Coil Bypass Factor, SHF, App System Capacity
Find the discharge temp from Zone #2 terminal mixing box if 650 CFM of hot deck air is mixed with 300CFM of cold deck air.
Find: DB, WB, Total CFM
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Things that affect Bypass Factor |
-Fin Spacing
-Depth of coil / Number of rows
-Type of fin being used
-Velocity
-Residential high latent load: 0.3 to 0.5
-Commercial applications: 0.2 to 0.3
-Operating / Hospital rooms: 0 to 0.1
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