Mechanical systems also have condensers. Condensers are found underneath steam turbines. Air-conditioning systems have condensers too.

How this name came about is a mystery to me. Perhaps it is because condenser is place for converting vapor or gas into liquid - at least for mechanical systems.

So it is a kind of heat exchanger. What is cooling medium?

For air-conditioning systems, most common cooling medium is water and air. The vapor to be cooled is refrigerant. And you've guessed it... It changes hot refrigerant gas into liquid.

First, you have to remove sliding panel and stationary panel. The vast majority of doors out there have sliding panel on inside half of track, and stationary panel is on outside. Whatever case in your particular door, outside panel has to come out first. To remove fixed panel, you need to remove sill cap that snaps into bottom track and runs from fixed panel to side jamb where sliding panel locks. Force a screwdriver into crease and pry cap up. Then, look for screws on inside side jamb holding fixed panel in place. These screws prevent someone from prying up sill cap and pulling out fixed panel in order to gain entry into home. In addition to side jamb, be sure to check across top header and bottom track for screws there as well. Once all screws have been removed, you need to pull fixed panel away from side jamb. You can start by simply grabbing side rail of fixed panel and pulling as hard as you can. If you're lucky, panel will pop free. Then you can lift panel up as far as it will go into top channel and swing bottom away from track, and remove panel. Now, I said "if you're lucky", because in most cases panel will be stuck in side jamb, bottom track, or both. Usually, you're going to have to use a pry bar to loosen fixed panel before it will slide out of side jamb. Once fixed panel is out, removal of sliding panel is usually easier. Just lift up and swing bottom away from track, then remove panel. Sometimes bottom rollers will prevent bottom from swinging out of track. If that's case, you will need to find roller adjustment hole in bottom corners of sliding panel. Put a screwdriver into this hole and turn screw counterclockwise to bring roller as far up into bottom of sliding panel as possible. This should allow slider to come out.