Vapor-compression refrigeration

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Vapor-compression refrigeration
Vapor-compression refrigeration and its
representation on T-s diagram (Thermodynamics 4th
ed., Cengel and Boles, Figure 10-3)
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Question to Ponder

• You know that you have a saturated R12 mixture
  at the exit of the expansion valve. You have both
  a temperature and a pressure gauge at that point.
  If your pressure gauge reads 400 kPa when the
  actual pressure is 390 kPa (only a small error),
  what type of substance would it look like you
  have at this point subcooled liquid, saturated
  mixture, or superheated vapor? Watch out for
  this problem whenever you have a mixture.

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Hilton Air Conditioning Unit

• Steam is added at the inlet.
• Air is preheated before sending it through the
  evaporator. If the temperature and relative
  humidity is right, liquid will condense as the
  air passes through the evaporator.
• Air is then reheated near the end.
• A normal air conditioning unit would not include
  all of these processes.
• Air flow rate is measured at the exit of the unit
  with an orifice plate and manometer.

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Basic Equations

• 1st law, no condensation or evaporation
• Where is the only place in this system where
  the power input term would be kept?
• 1st law with condensation
• If we dont measure the temperature of the
  condensate, what would be a good estimate of the
  temperature?

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Measuring Humidity Approaches

• Glass thermometer with wet wick will measure
  wet-bulb temperature, which can be used with
  dry-bulb temperature to determine humidity and
  state. (psychrometer)
• Cellulose sponge-type material expands with
  changes in relative humidity, which can be
  calibrated or used with dial indicator.
  (hygrometer)
• Electronic Capacitance or resistance changes
  with relative humidity can be calibrated.
  Digital read-out common. (electronic hygrometer)

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Humidity Meters
DMM/thermometer/ relative humidity
meters (source www.omega.com)
Sling psychrometer (becoming obsolete) Source
sciencekit.com
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Measuring Mass Flow Rate With Pressure Drop

• Orifice plates, flow nozzles, and venturi tubes
  are common.
• The greater the air flow rate, the greater the
  pressure drop.
• What general equation is used to calculate
  velocity?

Flow nozzle Source White, F.M., Fluid Mechanics
2nd Ed.
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Examples
Orifice plate
Venturi
Rotameter (used for R12 measurement in this
experiment)
Manufacturer Airflow Measurement Systems
Source http//lambdasquare.com/