Friday, December 10, 2010

Sizing an Air Compressor for My Asparagus Harvester Invention

Well, I’m starting to get orders for my asparagus harvester invention. In fact, I have an order to build one right now from a grower in Australia.

I’ve had the harvester in mothballs for the last few years waiting for the asparagus growers to get brave enough to try mechanical harvesting. It looks as though that is now happening.

Asparagus growers to not all grow their asparagus using the same cultural practices. Different growers use different row widths and different center-to-center spacings for the beds. That means I have to basically custom design the harvester for each grower to match the row spacing’s and bed widths.

Without knowing the cutting width and row centers of the harvester I could not establish a number of needed parameters for the machine, like how much compressed air I would need. Sizing the air compressor that powers the air cylinders that cut the spears is one of the design parameters I need to address.

Here is how I have gone about sizing the air compressor for my harvesting machine.

I went to Google of course, and researched the yields of asparagus. I found studies by the University of California and others which included yields for a number of asparagus varieties. The studies even provided the number of spears per acre that were produced.

The largest number of spears per acre was about 70,000 spears. I decided to use 70,000 spears per acre as the basis for my calculations.

My machine uses air cylinders with a one inch bore and a 24 inch stroke to cut the spears. There are a number of these cylinders side by side across the bed. Each cylinder has a blade 2 inches wide. If a spear is tall enough to cut, the appropriate cylinder is selected and fired at the right moment to sever the spear as it is grasped by a set of rollers.

If a spear is lined up between two blades then both blades are triggered to be sure and cut the spear completely. I anticipate that about 25% of the time two blades will fire.

Anything above the cutting height of the spears and located on the bed will trigger the blades to fire. Hand crews cut down the culls but don’t pick them up. The machine may or may not pick up a cull, but it will fire at. I figure that will be result in another 20% of blade firings.

Adding the valid cuts, culls and weeds, and double blade firings I come out with about 100,000 cylinder actuations per acre per season.

Early in the season when it is still pretty cold the spears only need to be harvested every 2nd or 3rd day and as the temperature rises you have to harvest more often until you are harvesting every day. A spear of asparagus can grow over 7 inches in a day. A typical harvest can result in anywhere from 45 cutting days to 60 cutting days. So the 100,000 strokes need to be spread out over the number of cutting days.

I am going to figure on 50 cutting days. So that 100,000 cuts per acre per season becomes 2,000 cuts per acre per day. The machine I will be building is a one row harvester and will cut at a rate of about 1.25 acres per hour at top speed. This of course means the machine will be cutting at a rate of 2,500 cuts per hour, or about 42 cuts per minute.

The air cylinders that do the cutting consume 0.9 cubic feet of air per stroke at 100 psi. Multiplying the .9 cubic feet per cut times the 42 cuts per minute gives us 37 cubic feet per minute.

Now I know that I need an air compressor that can deliver right around 35 – 40 cubic feet per minute of compressed air at 100 psi.