Here's how to use any of the getXXXXByName methods, which all return an OptionalXXXX object that you can call empty? (or is_initialized) on, and once you made sure it did find it, you can .get it:

air_loop_name = "my air loop"
# This returns an optional air loop
air_loop = model.getAirLoopByName(air_loop_name)

# If you couldn't find it
if air_loop.empty?
  puts "Couldn't match an AirLoopHVAC with the name #{air_loop_name}"
# Otherwise, get it (OptionalAirLoopHVAC > AirLoopHVAC)
else
  air_loop = air_loop.get
 // Do Stuff, eg:
 puts air_loop.name.to_s
end

Similarly, air_loop.name returns an OptionalString, meaning you need to call either .get or .to_s on it in order to get a String object.

For example, to list all the AirLoopHVAC names in your model:

model.getAirLoopHVACs.each do |loop|
  puts loop.name.to_s
end