Dryer venting

[Practigal]

I noticed that all of the manufacturers recommend the shortest run for the dryer duct with the least bends possible but they almost all include a picture showing a dryer with the exit vent making an immediate 90° angle upwards as the duct leaves the dryer, the duct then continues up the wall a short distance to another 90° angle and then exits the house.
I have also read elsewhere that the best angle is for the duct to be sloped downwards so that any moisture is pulled down and out of the house. I see no mention of downward venting in any of the manufacturers' instructions and so far no pictures of that either. Any reason they are vented up, and not straight out (which would be about 1' of duct) or down?
Only one of the manufactures mentioned backdrafting and did not really explain the concept. How far apart should the dryer exit vent and the air intake of other another appliance (like the hot water heater) be? Does your answer change if the heater is outside and separated from the dryer vent by a galvanized steel cabinet that encloses the hot water heater? (local authorities are not helpful on this point)

 

[fisherpaykel]

Dryer vent 90 degrees

I think the typical illustration of exit vent you described is based on manufacturer assuming most dryers are [1] in a basement and [2] pushed back against a wall hence the vent must go up to a height above ground level and with a tight 90 degree exit bend from dryer. If your dryer is above grade you of course can vent straight out horizontally provided winter snow accumulation won't block vent-and I assume you being in Los Angeles that will not be a problem. The other clearance issue questions I will leave for those in the know to answer.