smartt1982 From United Kingdom, joined Nov 2007, 225 posts, RR: 0 Posted (1 year 8 months 3 weeks 6 days 2 hours ago) and read 3305 times:
PACK LIGHT 737 NG
In regards to the PACK LIGHT annunciation on the 737 NG, I need some clarification as I believe I am completely confusing myself.
In the situation that the PACK Light illuminates as a result of PACK overheat (so does not extinguish after Master Caution reset) As per the QRH you select a warmer temperature on ALL the TEMP selectors in order so that AFTER Trip Reset you reduce the load on the PACK (as would be the case on a HOT day and the pack was working too hard to cool it down)
I have read that BOTH air mix valves are driven to full cold and TEMP selectors are disabled.
My questions/what I need clarifying is
When it says BOTH air mix valves. Does each of the TWO Packs have two air mix valves each or are these TWO air mix valves common between BOTH Packs. What is confusing me is that if it is a case that each PACK has TWO air mix valves and in this case due to an overheat the PACK shuts down and its two air mix valves drive full cold WHY then could I not still use the TEMP Selectors for control over the other pack that is still operating.
yeelep From United States of America, joined Apr 2011, 661 posts, RR: 0
Reply 6, posted (1 year 8 months 2 weeks 4 days 12 hours ago) and read 2529 times:
Where the -700 has a air-mix valve and low limit valve, the -800 uses a TCV (temperature control valve) and standby TCV. The air-mix valve has a cold and hot valve. On the -800 the cold valve is completely eliminated and the hot valve is basically the TCV, but the air re-enters the system at the ACM turbine discharge (mix-muff) instead of the mix chamber. The StbyTCV is in parallel with the TCV and under normal operation prevents icing in the condenser, although the method of control is different than the low limit valve.
On the -700, the air entering the pack goes through the FCSOV (flow control and shutoff valve) then divides. The majority of the air goes through the primary heat exchanger, the cold side of the air-mix valve, the ACM (air cycle machine) compressor, the secondary heat exchanger, ACM turbine, mix-muff, water separator, conditioned air check valve to the mix chamber and on to the distribution manifold and ducts..
The air that travels the other route goes through the hot side of the air-mix valve to the mix chamber.
There is also the low limit valve that tees into the duct after the cold side of the air-mix valve and goes to the mix-muff at the exit of the ACM turbine to prevent icing.
On the -800 the air entering the pack goes through the FCSOV then divides into four different ducts that I'll number 1-4. Number 1 goes through the primary heat exchanger, the ACM compressor, secondary heat exchanger, hot side of the reheater and condenser, water separator, cold side of the reheater, ACM turbine, mix-muff, cold side of the condensor, conditioned air check valve and on to the distribution manifold and ducts.
Number 2 and 3 are in parallel with a TCV in one and a standby TCV in the other, they then join together and go to the mix-muff at the ACM turbine exit.
Number 4 is for the trim air system that is supplied by both packs. Each packs number 4 duct first has a check valve to prevent any back flow back into either pack. Then they merge into a single duct and go through the trim air PRSOV (pressure regulating and shut off valve), then divide into three ducts with a trim air valve, one each for the flight compartment, forward cabin and aft cabin zones and then join the distribution ducting.
So basically the 737 has two different pack systems, a two zone and a three zone type. The -100, -200, -300, -500, -600, -700 are two zone. The -400, -800, -900, -900ER are three zone.