Idle throttle and Carb Ice !?

[NewBloke]

Sorry if this is one of the less interesting questions ;) but why is the carburettor more susceptible to icing when closed ?!? I hope it's not too obvious, or a silly question (I'm new to this game!), but it's been puzzling me. I thought the major contibutor to icing was due to the venturii effect - so how is this related (if it is) a closed throttle and ice !? Or is it simply due to a cooler engine at low RPM. Many books state this without explaining !

Cheers,

NB.

[GayTangle]

When the throttle is nearly closed at idle or descent power, the air intake is more restricted than at fuller throttle settings. Air passing through the intake therefore suffers a greater temperature drop.

Also, downstream of the restriction caused by the throat and the butterfly valve the liquid fuel is introduced, where it will vaporise; when it vaporises it removes heat from the air (latent heat of vaporisation) and cools the air even more.

The temp drop can be greater than 30 degrees C - meaning warm, or even hot days of quite normal levels of relative humidity can be expected to produce carb icing at idle/descent power.

:(

[NewBloke]

Cheers GT.... cleared that one up !