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Review Psychometric Variables - Lecture Slides | ARE 346N, Study notes of Architecture

The University of Texas at AustinArchitecture
Prof. Jeffrey A. Siegel

Material Type: Notes; Professor: Siegel; Class: BUILDING ENVIRONMENTAL SYSTEMS; Subject: Architectural Engineering; University: University of Texas - Austin; Term: Unknown 1989;

Typology: Study notes

Pre 2010

Uploaded on 08/27/2009

koofers-user-m4z-1
koofers-user-m4z-1 🇺🇸

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bg1
Objectives
• Review psychrometric variables
• Draw psychrometric processes
• Relate psychometric variables to human
comfort
Differences
• What is the difference between dry and wet-
bulb temperature?
• Where are they on psychrometric chart?
• What is the difference between relative and
absolute humidity?
• Where are they on psychrometric chart?
• How do you use the psychrometric chart to get
dew-point temperature?
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Partial preview of the text

Download Review Psychometric Variables - Lecture Slides | ARE 346N and more Study notes Architecture in PDF only on Docsity!

Objectives

  • Review psychrometric variables
  • Draw psychrometric processes
  • Relate psychometric variables to human comfort

Differences

  • What is the difference between dry and wet- bulb temperature? - Where are they on psychrometric chart?
  • What is the difference between relative and absolute humidity? - Where are they on psychrometric chart?
  • How do you use the psychrometric chart to get dew-point temperature?

Examples

  • Is it more moist when it is 70% RH at 30 °F or 30% RH at 70 °F?
  • Air comes off a chilled water coil at 90% RH and 55 °F. How cold does the duct surface need to be for condensation to form? 4

Psychrometric Processes

  • Going from one state point to another
  • Typically drawn as a line on a psychrometric chart
  • Hint for problems involving psychrometric processes: - Think about what parameter is staying constant

Which of the following statements is

not true?

A) Adding steam to a room increases its relative humidity B) Placing a cup of ice water in a room decreases the humidity ratio of the room’s air C) Lowering the temperature of a room increases its humidity ratio D) Lowering the temperature while increasing the humidity ratio in a room will eventually cause the air in the room to become saturated

Example

  • 1 cup of room temperature water evaporating in a 20 ft × 20 ft × 20 ft room

Solution

  • What is a reasonable humidity ratio for an indoor environment?
  • What are the units on humidity ratio in the IP system?
  • How do we go from volume of water to mass of water? For air?
  • What is the humidity ratio in the room after evaporation?
  • What are some important assumptions that we made?

Example

  • Two ducts coming together
  • Duct 1: 1 lb/min, 100 °F RH = 20%
  • Duct 2: 4 lb/min, 40 °F, RH = 95%
  • What is mixed stream dew-point temperature?

Comfort temperatures

  • Mean Radiant Temperature ,^ tr
    • Temperature of an imaginary enclosure that would cause the same radiant heat loss as the present environment
  • Operative Temperature,^ to
    • Temperature that is weighted average of dry bulb and mean radiant temperature
  • Effective Temperature, ET*
    • Temperature at 50% RH that yields the same heat loss as for the actual environment
    • Environments at the same ET* should have the same comfort response

1989 ASHRAE Comfort Zone

Operative Temperature (°C) Wet Bulb Temperature (°C) Vapor Pressure (mm Hg)

1993 ASHRAE Comfort Zone

Figure 2.2 in your text

1997/2001 ASHRAE Comfort Zone

ASHRAE Comfort Zones

  • Still a matter of some debate
  • Implications of use in design:
    • Check to make sure that you are using the most recent version
    • Plausible deniability

Summary

  • Define all psychrometric variables and locate them on the psychrometric chart
  • Draw psychrometric processes
  • Establish whether a point is on the comfort zone
  • For Thursday
    • Quiz 1
    • Sections 2.4 & 2.