MET 150

1
Air Properties and Psychrometrics

• MET 150

2
Atmospheric and Gauge Pressure

• Atmospheric pressure
• Measure with a barometer
• Standard pressure (dry air at sea level)
• 14.7 psia
• 29.92 in. Hg. absolute
• Gauge pressure scale
• Conversion between atmospheric and gauge pressure

3
Pressure Conversions

• What is the absolute pressure if the gauge
  pressure reads 42 psig?
• What is the gauge pressure if the absolute
  pressure reads 27 psia?

4
Pressure in Inches of Water

• Common measure in air distribution systems
• 14.7 psi 406.9 water 33.9 water
• 1 psi 27.68 inches water
• Convert inches water to psi divide by 27.68
• Convert psi to inches water multiply by 27.68

5
Converting Between Inches Water and PSI

• What is the pressure in inches water if the gauge
  reading is 2.2 psi?
• What is the pressure in psi if the manometer
  reads 3.8 inches water?

6
Gas Laws

• Dry air is a gas
• Follows specific laws pertaining to relationships
  between pressure, temperature and volume
• Known as the gas laws

7
Daltons Law

• Total pressure of a mixture of confined gases is
  the sum of the pressures each gas would exert if
  it alone occupied the volume of the mixture at
  the same temperature.
• Each gas acts independently
• TP P1 P2 Pn

8
Boyles Law

• At constant temperature, the pressure on a given
  quantity of confined gas varies inversely with
  the volume of the gas.
• At constant temperature, the volume of a given
  quantity of gas varies inversely with the applied
  pressure.

9
Boyles Law

• Po x Vo Pn x Vn
• Vn Vo x Po / Pn
• Pn Po x Vo / Vn
• Where
• Po original absolute pressure (psia)
• Pn new absolute pressure (psia)
• Vo original volume (cubic feet)
• Vn new volume (cubic feet)

10
Boyles Law Examples

• What is the new volume of 3 cubic feet of gas at
  25 psig if it is compressed to 40 psig?
• 4 cubic feet of gas is expanded from 45 psig to
  10 psig. What is the new volume in cubic feet?

11
Charles Law

• With a constant pressure, the volume of a given
  quantity of confined gas varies directly with its
  absolute temperature
• With a constant volume of gas, the pressure
  varies directly with its absolute temperature
• Absolute temperature oF 460

12
Charles Law

• At constant pressure
• Vo x Tn Vn x To
• Vn Vo x Tn / To
• Tn Vn x To / Vo
• Where
• To original absolute temperature
• Tn new absolute temperature
• Vo original volume (cubic feet)
• Vn new volume (cubic feet)

13
Charles Law

• At constant volume
• Po x Tn Pn x To
• Tn Pn x To / Po
• Pn Po x Tn / To
• Where
• To original absolute temperature
• Tn new absolute temperature
• Po original absolute pressure (psia)
• Pn new absolute pressure (psia)

14
Charles Law Examples

• What is the new volume of 10 cubic feet of gas at
  50 degrees if the temperature is raised to 130
  degrees at constant pressure?
• What is the new pressure (in psig) of a quantity
  of gas in a cylinder whose pressure is 30 psig at
  40 degrees if the temperature is raised to 95
  degrees?

15
Psychrometrics

• Study of dry air and water vapor mixtures
• Condition of the air (temperature and humidity)
  affect human comfort
• Dry air a mixture of gases (nitrogen, oxygen and
  others)

16
Definitions

• Specific volume how much space 1 pound of dry
  air occupies.
• At 70oF and sea level 13.33 ft3/lb
• Specific density Weight of dry air per 1 ft3
• At 70oF and sea level 0.075 lb/ft3
• Specific heat ability to get hot
• At 70oF and sea level 0.24 Btu/lb/oF

17
Sensible Heat

• The amount of heat which, when added to air,
  causes a change in temperature with no change in
  the amount of moisture present
• Measured with a thermometer
• Sensible heat specific heat x specific density
  x 60 min/hr x CFM x DT (Btuh)
• Btuh 0.24 x.075 x 60 x CFM x DT
• Btuh 1.08 x CFM x DT

18
Humidity

• Amount of humidity affects rate at which
  perspiration evaporates, making skin cooler
• Comes from evaporation of earths ocean and other
  bodies into the atmosphere
• Inside, comes from cooking, showers, human
  respiration and perspiration

19
Humidity

• Moisture contained in the air
• Pounds moisture per pound of dry air
• Grains moisture per pound of dry air
• At 70oF and sea level, 7000 grains per pound of
  water
• Saturated air holds all the moisture it can at
  that temperature and pressure
• Saturated air at 70oF and sea level contains
  110.5 grains of water or 0.01579 pounds of water
  per pound of dry air

20
Relative Humidity

• Amount of water held by the air as a percent of
  what it can hold at that temperature and pressure
• Expressed as a percentage
• Warmer air will hold more water
• Relative humidity affects comfort
• Winter 67-76oF, 30 RH
• Summer 72-81oF, 40 RH

21
Comfort Zones

• Dressed in typical summer or winter clothing
• Engaged in sedentary activity
• Air motion in occupied zone does not exceed 30
  fpm (winter) or 50 fpm (summer)

ASHRAE Comfort Chart
22
Measuring Humidity

• Sling psychrometers
• Electronic psychrometers
• Hygrometers with thermometers
• Measure both air temperature and moisture content

23
Sling Psychrometer

• Two thermometers dry bulb and wet bulb
• Dry bulb measures sensible heat
• Wet bulb thermometer has wick saturated with
  distilled water, evaporation lowers temperature,
  taking into account moisture content of air

24
Sling Psychrometer

• Insure that wick is wet
• Whirl psychrometer with a steady motion through
  surrounding air (30 sec. minimum)
• Periodically check wet bulb readings
• Accept wet bulb reading when two consecutive
  readings the same
• Always read wet bulb before dry bulb
• Do not touch wick when taking reading

25
Sling Psychrometer

• Determine approximate relative humidity from
  sliding scale
• More precise readings can be made by plotting the
  dry bulb and wet bulb temperatures on the
  psychrometric chart (covered next week)

26
Digital Psychrometers and Hygrometers

• Measure dry bulb and wet bulb temperature
  directly or measure relative humidity directly
• Read out relative humidity

27
Dewpoint

• Temperature at which water vapor in the air
  becomes saturated with moisture and the moisture
  starts to condense into water droplets
• If relative humidity is 100, dewpoint, wet bulb
  and dry bulb temperatures are all the same (no
  water can be evaporated from the wet bulb
  thermometer)

28
Enthalpy

• Total heat content of air and water vapor mixture
• Measured from pre-determined base point
• Expressed in Btu/lb
• Find enthalpy of air stream by measuring dry and
  wet bulb temperature and plotting on a
  psychrometric chart

29
Equipment Capacity

• Btu/hr
• Enthalpy difference of air entering and leaving
  equipment x specific density of air x cfm x 60
  min/hr
• Btu/hr (Btuh) 4.5 x cfm x DH

30
Example

• Enthalpy of 10,000 cfm of air entering and
  leaving a cooling coil are as follows
• 41.5 btu/lb
• 22.3 Btu/lb
• What is capacity of unit in Btu/hr? Tons?
• Btu/hr 4.5 x 10000 x (41.5-22.3) 864,000
  Btu/hr
• 864,000 Btu/hr x 1ton/12,000 Btu/hr 72 T

31
Homework/Labs

• Read pages 11.1 11.7
• Answer Think about it in complete and
  thoughtful sentences and paragraphs. Watch
  spelling and grammar. Neatness counts!
• Worksheet on gas pressure relationships
• Lab on RH measurements