View Full Version : 733 Hot weather operations and recirculation fan??

3rd Jul 2001, 22:28
I see some different techniques for cooling the 733 cabin on the ground.

Some of our 733’s only have a recirculation fan and no gasper fan. I would like to meet the person that came up with that bright idea.

IMO, The best cooling seems to come from running the Left pack in High on the APU, left engine bleed valve and isolation valve closed, and the Right pack in High.

But some captains claim the recirculation fan cuts out when one of the packs is put in High on the ground. That doesn’t seem to be true in the above configuration.

What controls the auto cutout of the recirculation fan on the ground?

4th Jul 2001, 10:55
L5 yes it does cutout with the configuration you presented. I assume you have No 2 engine running?
On the ground:
The packs operate in either AUTO or HIGH. In AUTO the pack valve will modulate a lower VOLUME of air than HIGH (normal pack flow rate) from the bleed duct. For pack-loading relief the recirc fan will make up extra air going into the mix manifold. This air is derived from the cabin via the forward cargo bay which in theory is supposed to be coolish. Because its a fan its pushing air through the system which is why it feels better. I agree its a dumb system in high OATs.
When you select HIGH you are restoring normal pack flow rate. What your probabley feeling is a rush of normal VOLUME pack air. The recirc fan is of no value anymore. Indeed if the recirc was operating youd have coolish air mixing with full cold pack air, which you dont want.
The switching you describe doesnt fool the recirc logic in this case unfortunatley.

4th Jul 2001, 17:18
Thanks Slasher.

4th Jul 2001, 22:23
...And not to forget to be sure to have the empty center tank pump turned Off, ...for obvious reasons.