miracledogg
Going by your numbers I would say you are looking for a
pump that has a minimum of 450 GPH at 2 m (6 feet) head height. But if it were me I always go bigger for two reasons. First, that's only a minimum requirement, and actual results may vary. Second, is it's quite simple to reduce water flow if the pump is to strong. You may also want to expand the system in the future, and/or use it for a different one all together. If 450 GPH is the goal at 6 feet high, I would be looking for a pump that could do twice that as a minimum.
With that said, it sounds like you plan to build quite a big system. That has me wondering how you came to your figures. If you have 24 m (75 feet) of gutters, and if they were spaced 1 foot apart, that would be 75 growing gutters. 450 gph divided by 75= 6 gph each. Then 6 gph divided by 60 minutes equals 0.1, or about a cup and half (12 oz) per minute water flow through the NFT gutters. I have never done a NFT system but that seems kind of low to me, but may be fine.
I also don't know if the spacing is what you have planed. If your growing lettuce, and you offset the plant spacing in the gutters, they can be be spaced much closer. Like about 6 inches, doubling the amount of gutters in the same space. But at the same time cutting the water flow in half to each from 12 oz a min, to 6 oz a min. So I have no idea how you came to the figure 450 GPH. But that dosen't mean its wrong. Just that I cant compare what I would do in the same situation, simply because I don't really don't know the specifics of your situation.
I should also mention that if you plan to run such a long water line, you'll want to make sure you don't loose water pressure from one side to the other. That can be done a few ways. But if it's not taken into account, the gutters first in line will get most of the water flow (to high), and those at the end will get very little water flow (to low).
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how can i calculate the oxygen pump,the air flow for these specs
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You cant. Again there are quite a lot of variables to consider. First and foremost is your system design. Second is your water temperatures. I also have no idea if you are trying to pump air to each NFT gutter, or just aerate the reservoir. Not to mention how many plants you plan, what type they are, or even how big they will/can get. All of witch will play factors in the oxygen saturation point of the water in the first place, as well as how fast the plants will use it up in the second place. But the best rule of thumb is get as much oxygen to the roots as you can, there's no such thing as too much oxygen to the roots in my opinion.