B737-800 Recirculation fans and packs

[DongJoon Choi]

Question,

1. I do not know why a recirculation fan(s) affects operation of packs?

2. and Are packs operation only limited on the ground
(because there is only OAT)

Please respond to my question.

MEL say :

Left fan may be inoperative provided the left pack
is operating when OAT is above 100 degrees F (38
degrees C).

Right fan may be inoperative provided:
a) The left pack is operating when OAT is above
100 degrees F (38 degrees C), and
b) Flight is conducted pressurized.

both fans may be inoperative provided:
a) OAT remains below 100 degrees F (38
degrees C), and
b) Flight is conducted pressurized.

[Northbeach]

For our 737-800s MEL 21-21E regarding Recirculation Fan(s) Left Fan inop says the following:

“Left fan may be inoperative provided the left pack is operating when OAT is above 38C.”

I understand this to mean the following.

If the OAT is at or below 38C then two (2) recirculation fans are installed but only one (1) is required for flight…………..period…………no further restrictions or complications. Go fly & have fun, you do not need the left recirculating fan.

However if the OAT is ABOVE 38C then the left pack must be operating. If the left pack is operational then THERE IS NO FURTHER LIMITATIONS OR COMPLICATIONS. Go fly & have fun without the left recirculating fan.

But if the OAT is ABOVE 38C and the left pack is inoperative and the left recirculation fan is inoperative then it is not your day; you may not go flying (at least not under this MEL).

1. I do not know why a recirculation fan(s) affects operation of packs?



Recirculation fan(s) and packs are all part of the air-conditioning pressurization system, but they are different components. Recirculation fans reduce the total demand on the packs, and the bleed air system, by recirculation air rather than 100% always demanding ambient air. By reducing the demand on the bleed air system it reserves more bleed air for developing thrust-increasing fuel efficiency because more of the high pressure air developed by the engines is used to drive the turbines producing thrust rather than being “robbed” for other uses (air-conditioning). I believe the difference is somewhere between 3%-5% fuel efficiency (from memory without doing further “research”).

2. and Are packs operation only limited on the ground
(because there is only OAT)



Sorry I do not understand your second question, my apologies. Your English is very good, my Korean is non-existent I am unable to “help” you with your second question.

POSITIVE RATE - Gear UP, Autopilot On, LNAV, VNAV "pass the chicken"!

[Denti]

POSITIVE RATE - Gear UP, Autopilot On, LNAV, VNAV "pass the chicken"!

Now, you can have that easier, just engage LNAV/VNAV during preflight preparation, then it is only POSITIVE RATE - Gear UP, Autopilot On, "pass the chicken"! ;)

[Northbeach]

Now, you can have that easier, just engage LNAV/VNAV during preflight preparation, then it is only POSITIVE RATE - Gear UP, Autopilot On, "pass the chicken"! http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/wink2.gif

Denti's right:ok:

[DongJoon Choi]

Hi Northbeach,

Thanks for your reponse.My question is why L/H pack must operate when either recirculation fan is inoperative. Why not R/H recirculation?

[KBPsen]

My question is why L/H pack must operate when either recirculation fan is inoperative. Why not R/H recirculationWhich pack supplies the cockpit? Think about cooling and ventilation.

[DongJoon Choi]

Hi KBPsen,

I appreciate you. Does your reply mean that cockpit is more important than cabin if either recirculation is inoperative.

Please explain in detail

[Northbeach]

My question is why L/H pack must operate when either recirculation fan is inoperative. Why not R/H recirculation?


In researching this question and MEL I have; read the MEL, reviewed the air-conditioning/pressurization system as well as the related schematics & diagrams, spoken to our maintenance department and I have 2 emails out (one to the training department & the other to another contact in maintenance management). I am waiting for another response, but am fairly confident in my analysis. But pending that response I may need to amend my reasoning.

I did not talk to Boeing or the FAA; so I do not have any of their input. Together they came up with the Master MEL; I was not in on any of those discussions. Like many people I just fly these things, I am not in a position to make policy or design decisions.

Here are the important points, as I understand them, regarding this airplane B737-800 and this MEL (21-31E, 21-31F and 21-31G).

As long as the outside ambient temperature is at or less than 38C there is no issue with a failed recirculation fan-go fly.

The recirculation fan(s) pull air out of the E/E (Electrical & electronic) compartment prior to filtering where that air is later reintroduced into the mix manifold.

The E/E compartment generates heat when the systems contained in the E/E compartment are operational.

It is important to keep the E/E compartment from getting excessively hot. Extreme heat is an enemy of electronic components. Excessive heat leads to operational failures of electrical components.

The E/E compartment is a recipient of conditioned air under pressure. That cooling air flowing into the E/E compartment originates almost exclusively from the Flight Deck. Two equipment cooling fans {Not the recirculation fans. Switches on the forward overhead panel} blow it in and two different equipment cooling fans exhaust that air out from the E/E compartment through either the Overboard Exhaust Valve or (when that valve is closed) diffused to the lining of the forward cargo compartment. This is in addition to the recirculation fan(s) that also exhaust E/E compartment air.

Normally, the Flight Deck receives its air from the LEFT PAC directly as flight deck conditioned air does not go to the mix manifold (Left Pac operational). The LEFT Pac sends air to the flight deck, which then goes to the E/E bay/compartment. The Right Pac is not capable of supplying the flight deck without its discharge air first going to and through the mix manifold.

When the outside air temperature is above 38C; apparently it is not good enough (my analysis) just to pull air out of the E/E compartment with one operational recirculation fan. There must be a sufficient quantity of cool air being supplied into the E/E compartment- to meet the cooling requirements when the OAT is > 38C, and that air MUST come from the Left Pac (Hence the existence of this MEL requirement).

Therefore the Left Pac must be operational. It seems that the FAA/Boeing decided that the Right Pac may not be able to deliver enough cooling air due to the design of the system – IF THE OAT IS GREATER THAN 38C.

Remember: none of the above comes into the decision UNLESS the OAT is greater than 38C.

Summery: normally left Pac supplies conditioned air into the flight deck. The flight deck air is exhausted into the E/E compartment where that air is used to cool the heat generating electrical equipment located in the E/E compartment. Then that air is pulled out by the recirculation fans and/or otherwise discharged overboard.

In other words:Which pack supplies the cockpit? Think about cooling and ventilation. KBPsen was spot on and said the same thing with far fewer words.

Do you have any further questions?


Amendment #1.
I just received a preliminary briefing from maintenance. There may be more to do with this MEL than simply heat removal associated with the E/E bay, although that is part of the equation. The second piece of the puzzle has to do with smoke removal given a particular set of failures. More to follow……………standby.

[DongJoon Choi]

Dear Northbeach,

I greatly appreciate you answering my question. Your reponse give me better understanding of relation between packs and recirculation fans operation.

Sir, One more question.

WHY MUST THE PRESSURIZED FLIGHT BE REQUIRED WHEN ONE RECIRCULATION FAN IS INOPERATIVE?

[Northbeach]

You are asking excellent questions; “why” question, and not simply being content with reading and following the instructions. You are exactly the kind of pilot I would want to be in command of the flight if my family and I were on board. Reading and correctly following instructions is important, but those actions are only a basic knowledge level. Just about anybody could be taught to read a checklist and push a button or move a switch position. You wanting to discover the “why” behind a checklist item or in this case a MEL demonstrates a deeper commitment to learning.

I am digging further into this MEL myself; I have an idea that it has to do with smoke evacuation under certain combinations of failure. I am still researching this myself. There are tens of thousands of crews that have flown the B737; there are hundreds here on PPRuNe. There may be people on these threads who had their hand in certifying the B737 MEL. If so perhaps they would care to comment.

To make this even more interesting notice that on MEL 21-31E Left Fan Inopt there is NO requirement for the flight to be conducted pressurized, unlike 21-31F and 21-31G. As you know there is usually “more” to the answer in aviation, and there is always somebody out there a lot smarter than me (in fact there are many). In the meanwhile I will continue researching this MEL to determine the reason for the remarks and exceptions comments. There is a difference between posting what I “think” and getting the definitive answer from Boeing and/or the FAA. I am working on it.

[framer]

Thanks Northbeach, you're a champ.

[Northbeach]

L Pac/R Pac = ACM (Air Cycle Machine) a quick review.

I copied and posted the following text from Answer.com & wiki.answers.com – unable to credit the original author(s). The discussion accurately describes the 737 air conditioning Pack

Answers.com - How does A bootstrap air cycle machine in an aircraft work (http://wiki.answers.com/Q/How_does_A_bootstrap_air_cycle_machine_in_an_aircraft_work#i xzz1PzJFklay)


The air cycle machine (ACM) is driven by turbine engine bleed air that first passes through a primary heat exhanger (much like an intercooler for a turbocharged automotive engine) that significantly lowers the temperature of the 300-500 degree bleed air. After leaving the primary heat exchanger the air flows into a compression turbine that increases the pressure of the air and also increases the temperature.
After leaving the compression turbine, the air flows into a secondary heat exchanger that once again lowers the temperature of the air. At this point, the air is at extremely high pressure and relatively low temperature. From the secondary heat exchanger the air flows into the "expansion turbine" where this high pressure air is used to drive a turbine wheel that performs several functions:

1. It extracts energy from the high pressure air

2. Allows the pressurized air to expand

3. Drives the compression turbine

The extraction of energy from the air and subsequent expansion of the gases drastically lower the temperatures of the air. In fact, the air temperature is lowered so much that often times hot bleed air is bypassed around the ACM and ducted to the air coming out of the expansion turbine to raise the temperature above freezing.
The output air that is coming out of the ACM is then sent through a moisture removal system that essentially uses a fine mesh weave to collect small droplets of water. This removal of moisture helps to control the humidity of the air entering the cabin and it also prevents icing downstream in the system.

[DongJoon Choi]

Thanks a lot, Northbeach. I would like to tell you that you are the best.