Measuring a vacuum evacuating a system requires the

This preview shows page 60 - 63 out of 106 pages.

We have textbook solutions for you!
The document you are viewing contains questions related to this textbook.
Human Physiology: From Cells to Systems
The document you are viewing contains questions related to this textbook.
Chapter 17 / Exercise 13
Human Physiology: From Cells to Systems
Sherwood
Expert Verified
Measuring a VacuumEvacuating a system requires the pressure inside the refriger-ation system to be drawn lower than atmospheric pressure, orin other words, to pull a vacuum. A vacuum can be expressedin inches of mercury(in. Hg). The theoretical perfect vacuumequates to 29.92 in. Hg. Let’s take a moment to review somecommon measurement acronyms and terms that you need tobecome familiar with:nmm—millimetersnin.—inchnHg—mercurynpsia—pounds per square inch absolute (atmosphericpressure + gauge pressure [psig])npsig—pounds per square inch gaugen“0” psig—atmospheric pressuren1 micron—1/1000 of a millimeter Hgn1,000 microns—1 mm HgnCompound gauge—an instrument that indicates pressure above and below atmospheric pressureSafety Note: Before evacuating a refrigeration system,all refrigerant must be removed in accordance withEnvironmentalProtection Agency(EPA) regulations.
We have textbook solutions for you!
The document you are viewing contains questions related to this textbook.
Human Physiology: From Cells to Systems
The document you are viewing contains questions related to this textbook.
Chapter 17 / Exercise 13
Human Physiology: From Cells to Systems
Sherwood
Expert Verified
Lesson 359To accurately measure a vacuum, refrigeration techniciansuse several different types of instruments. One instrument iscalled an electronic vacuum gauge.The display of this gaugecan be analog, digital, or light emitting diode (LED).The elec-tronic vacuum gauge responds quickly to pressure changeswithin the system. Here are a few important points to rememberwhen using an electronic vacuum gauge:1. Never expose the sensor portion of the instrument to system pressure.2. Always install the sensor vertically to avoid system oilfrom contacting it.3. Install a valve between the system and the sensor so youcan isolate it from the system when required.4. Allow the vacuum pump to draw a vacuum of at least 25 inches Hg before opening the valve between the sys-tem and the sensor.5. Be sure that a sustained vacuum of at least 250 microns(indicating no leaks in the system) is indicated.A variation of the electronic sensor uses a thermistor.A ther-mistoris a temperature-sensitive resistor, whose electricalresistance changes with temperature variations. Another typeof vacuum gauge is the U-tube manometer, which consists of a continuous dual column of mercury. One end of the tube issealed, while the other side of the instrument is connected tothe vacuum pump. This type of gauge has limitations, bothin total range and in the ability of the human eye to read thescale accurately.Using a Vacuum to Remove MoistureTwo kinds of moisture exist within a refrigeration system: vaporand liquid. Moisture in a liquid state is much more difficult toremove from a refrigeration system than it is in a vapor state.As pressure is reduced, liquid moisture boils off to a vapor. Butthere are limitations with this approach to removing moisture.The first is that as water boils, the amount of vapor created inrelation to the original amount of liquid is dramatic; causingthe evacuation time to be lengthy if the system has a large
Refrigeration Systems60amount of liquid water. A second problem arises since low-pressure moisture can freeze, making it difficult to remove.

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture