ACtion Magazine - July 2013 - (Page 26)

How low can you go? What really happens when pulling a vacuum during service? by Jerry Jarosch A t several MACS Training Event, including the most recent event, I’ve noticed that many people have what I consider to be misconceptions and misunderstandings about what is actually achieved when pulling a vacuum on a refrigerant system during service. In MACS ACtion magazine’s May 2012 issue, an article titled “Vacuum: How Much is Enough?” was a good article about vacuum pumps, but I think it left some questions unanswered. Hopefully this article will give you a better understanding of what to expect when pulling a vacuum on a refrigerant system. the bottom (like a straw), and the other hole will have a fitting connected to an air compressor. Instead of reducing pressure in the tube, we’ll pump air into the container to force liquid up the tube. If we keep the tube vertical and pressurize the container to exactly 14.7 psig, how far up the tube will the liquid travel? Well, that depends on the weight of the liquid. If the liquid is mercury (Hg), then 14.7 psig will force the mercury exactly 29.92 inches (or 760 millimeters) up the tube. Now relieve the pressure, cap the fitting and connect a vacuum pump to the tube. When we start the pump 26 ACTION • July/August 2013 Photo: MACS Measuring units First let’s define the terms we use to express pressure and vacuum. This is important; read it carefully. If you’ve ever gone to the bottom at the deep end of a swimming pool, you’ve felt the weight of the water above you. Because you’re completely immersed, the water presses equally everywhere on your body. Atmospheric pressure is exactly the same thing; it’s the weight of the ocean of air surrounding our planet, pressing in from all sides just like water in the pool. Atmospheric pressure at sea level is 14.7 pounds per square inch (psi), but common pressure gauges are built to read ‘zero’ at normal atmospheric pressure. When you inflate a tire, the gauge starts at zero (atmospheric pressure) and increases as the pressure in the tire increases above atmospheric pressure. We call that reading “gauge pressure,” and you might see it written as “psig.” Now let’s look at lower-than-atmospheric pressure. When you drink through a straw, you reduce pressure in the straw below atmospheric pressure. At the same time, atmospheric pressure forces the liquid towards the lower pressure, so it moves up the straw. Now let’s seal the container and make two holes in the lid. In one hole we’ll install a clear tube that touches

Table of Contents for the Digital Edition of ACtion Magazine - July 2013

ACtion Magazine - July 2013
Contents
Outlook
Freeze Frame
Virtual View
Leonard’s Law
Cooling Corner
Member Profile
News & Updates
Motorhome Air Conditioning
How Low Can You Go?
Quick Check
Association News
Classified
New Products & Services
Last Watch

ACtion Magazine - July 2013