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I am looking for some guidance on calculations for a natural condenser project. I am a Computer Engineering student, so this stuff is out of my range.

The fundamental question I am trying to answer is: what fan speed is required to push air through the following system to overcome head loss?

A basic mockup of the system is shown below. Some additional details of the design:

  • This will be installed in a greenhouse to reduce humidity and ideally collect some water for reuse.
  • The long, narrow rectangles represent PVC pipes of diameter 20cm (0.2m). Both pipes are identical, aside from length.
  • The storage tank on the bottom will collect any condensed water.

Condenser Schematic

Functional procedure of the design:

  1. Humid air (90% relative humidity, 30 deg C) will enter the pipe at stage 1. A fan will be installed to move air down.
  2. The air will travel 2m down into the ground (denoted by the rectangle) to a storage tank at (12.7 deg C). The humid air will condense into liquid in the cooler temperature and collect in the storage tank.
  3. The air will travel 2.5m up and return into the greenhouse (unknown humidity and temperature).

In our small-scale testing, we were able to achieve condensation using a computer fan and 1.5" diameter PVC pipe. However, we'd like to calculate the required fan speed for the full scale prototype.

We attempted to calculate head loss using equations in MATLAB, but weren't sure what the pressures would be at each stage. I understand this is a lot of info, but any guidance would be much appreciated!

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Not an expert with fluids but maybe this will help narrow down stuff-

1) Fan rpm should not be the first priority.

2) You should rather calculate the total pressure drop across your path (straight pipe, rectangular box which is approximately a U-bend and the return straight pipe). The fan you select should provide a static pressure which is greater than this total pressure drop. For example if you calculate the pressure drop for your air path to be 2 inch of water column (one of the common units for fan static pressure measurement also written as I.W.C), then you need to select a fan that is capable of providing its rated volume of air at 2 I.W.C or more.

3) Next, you need to find out how much airflow you need. This will be limited by the heat transfer happening in the rectangular box. Let's assume some values. If your box is transferring heat to the ground at 2.5 Btu/second (Btu is a unit for measuring heat energy), then you can move a maximum of 1 pound of air through the box in 1 second, so that it's temperature will drop from 30 degree celcius to 20 degree Celsius (these values are calculated using properties of air which are easily available). So now we know that we can move 1 lb of air per second, which will give us our airflow rate for the fan. This is measured in cubic foot per minute or cfm.

4) To summarize, your process will be-

a) Find out pressure drop in the air path and select fan static pressure.

b) Find out maximum possible heat transfer in box and calculate maximum possible air flow.

c) Select a fan which will give you the required static pressure and airflow (you will have to select the fan from an available list and may not have a choice for rpm)

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