Analysis and Design
Proposed Sequence
We will first find the heat loss rate necessary to cool 75° F air to a 55° F standard, based off of previously laid out standards.
After this, we will be able to find the flow rate of cooling fluid necessary to do so based off of the amount of cooling data.
Next, we will calculate the power needed to drive the fan motor for one day. In this case, that will be 8 hours, as the unit operates mainly throughout times where there is sunshine.
Lastly, we will analyze the fluid friction losses throughout the system.
Our analyzed optimization will consist of variations of heat flow rate via experimentally changing both the hydronic flow rate and the air flow rate through the air handling unit and through the evaporator unit.
Design
The containment apparatus will house the heat exchanging element, and will have ducting going into and coming out of the unit. The fan motor will be attached to the ducting before entry into the apparatus. It will blow at 200 CFM, with 100 CFM going into the apparatus and the other 100 CFM going into the evaporative unit, which will then be exhausted into the outside air.
A box shaped device will be fine, as the fluid friction losses from such a design are calculable. A box shape is also easy to design and manufacture. We will have the side panel removable, with inlets cut into the side to accommodate the heat exchanger that is specified. This will allow for a tight fit, as the box is custom built for the heat exchanger. In a manufacturing sense, the design is simple. There are four bends on the main apparatus and one bend on each of the two required ducting connection pieces (See Appendix B for drawings and visuals). There will be spot welds on these pieces, and the gaps will be filled with caulk for insulation and producing a tight seal.
The ducting will be standard 6” aluminum accordion. There is a splitter with dampers to ensure that 100 CFM of air will reach the heat exchanger. We have also incorporated a motor speed controller (See appendix C), which will allow us to change the overall airflow, ensuring that the evaporator and the heat exchanger will each receive adequate circulation.
Calculated Parameters
Upon performing the initial calculation for the heat load removed (Appendix A, sheet 1), we found the amount of heat to be removed from the room at the outlined parameters to be 2140 BTU per hour [1] [2].
Looking to the text, we see that the average male who is seated and doing very light work will emit 450 btuh of heat. Also, it was found that a small computer, such as a laptop produces roughly 145 W of waste heat, or 495 btuh [3]. Combined, the total heat load comes to roughly 950 btuh. Since the initial calculated parameter of heat load that could be removed is 2140 btuh, we can then move towards calculating the hydronic flow rate. This is the amount of fluid at a certain temperature that must be passed through a coil to produce a change in the air temperature that is passing over it. Looking to the appendix for the calculation, we find that a hydronic flow rate of 0.241 gallons per minute will accommodate this volume of heat flow out of the room (see appendix A, sheet 2) [2].
As seen in appendix A, sheet 8, the sheet metal must have a bend radius that is large enough to allow the material movement without “orange peeling”, or opening of the grain [4]. Upon analysis, it is found that our bend radius for a 90° bend on 20 gauge sheet metal is 0.063 in. This corresponds with the output value in Solidworks, which uses the minimum bend radius as a standard.
Device Shape
The device will be a box-shaped apparatus, containing two heat exchanging elements. The apparatus will feature inlet and outlet holes for both the air flow over the heat exchanger and the fluid flow throughout the unit. The overall setup is to look like the following setup, although for testing purposes, we will be using one heat exchanger within the air handling unit. The following diagram represents a more realistic scenario with a fully-integrated system.
We had also discussed adding a 1” layer of foam insulation around the air handling unit, then encasing it again with additional sheet metal. This was deemed unnecessary for our purposes, as this would be more applicable to a real-world scenario and not a testing application. Also, this would raise concern regarding access to the inner workings of the unit. A slight redesign would be in order, and if we wanted to take our idea to a marketing standpoint, we would need to reconsider that option. However, as a proof of concept, the uninsulated air handling unit will serve the purpose.
Device Assembly
The containment apparatus will have a three-sided structure made from 20 gauge steel sheet metal, and also incorporates a side panel which can be entirely removed from the unit. Since there are no structural requirements of the box from a loading standpoint, 20 gauge sheet metal is a viable option. It is also inexpensive, readily available and easier to work and bend. [4] We plan to plasma cut the shapes out of stock 20 gauge sheet metal, so the fact that this is a thinner stock size makes for a cleaner cut as well as less warping from heat exposure both in welding and in plasma cutting.
The heat exchanger will be 12.5” x 14”, in a rectangular-shaped design (see Figures 3 & 4). There will be two panels that will cover the ends of the containment unit, each with a 6” hole cut into it for ducting. The fan will be connected to a splitting device, on which we will mount backdraft dampers. This will allow us to control the flow rate of air through both the evaporator and through the air-handling unit. All ducting will be standard 6” flexible aluminum.
Fabrication issues
The required tolerances will be 1/16” for the lengths and for the holes cut in the sheet metal. For the location of the holes to be drilled with the #31 drill, we will require a tolerance of 1/32”. After speaking with Tedman Bramble and Matt Burvee regarding common practices for sheet metal tolerancing, there is no standard as far as fabrication goes. The containment apparatus will be attached with the ducting connection side panels via spot welds and caulking. After this has been completed, the removable side panel will be located and then drilled while on the unit. This will allow for tight control and easy location of the screw holes.
The entirety of the system will not be in motion, leading to kinematic considerations to be ignored. Although vibrations will be produced by the fan motor, they will be of insignificant magnitude. The system is designed to be semi-permanent and immobile, yet easy to install and maintain.
For the ergonomic aspect of this project, the human interaction with this system will be minimal. As with any maintenance, there will be some human interaction. This will be mitigated by the availability to access the coil through the removable side panel.
Critical Failure Mode
There are no load bearing aspects of our system. With that being said, the critical failure that could occur with this system would be failure in the heat exchanger itself and would present itself as a leak in the working fluid. This would be easily amended, yet would undoubtedly affect the performance of the system negatively if neglected due to pressure losses in the heat exchanger [5].
