INTRODUCTION: A few weeks ago my daughter had a cold and she did not want me to switch on the main evaporative cooler which is a relatively cheap and effective device to cool houses in dry and desert like climates like Tehran, so while I was feeling terrible because of hot weather inside my room I had to work, so that even my small fan which I made it to cool me as a spot cooler did not helped and I was sweating like a hell, suddenly a glimpse of an idea came in to my mind which was " WHY SHOULDN'T I MAKE A SMALL DESK TOP COOLER ?" and make myself independent from others especially while others do not like global cooling in our surroundings. So I started to prepare software and hardware to make such cooler. My first step was to draw it roughly and see what I needed, and after drawing it I decided to make it as small as possible so that even It could fit on my desk or next to my desk. It took a month for me to complete the design and needed material while I bought electronic components from internal market and use my junk box for other parts I was stuck because the kind of pump I needed was not available and most sites were run out of it until one supplier informed me about adding it to their scope of supply. So every thing was ready for starting up making it although I have already prepared most of mechanical part. In what follows I have included the following steps:
1- Theory of evaporative cooling
2 - Explanation of my design
3 - Electronic schematic circuits and software
4 - Bill of materials and the price list
5 - Tools needed
6 - How to make it
7 - Measurements and calculations
8 - Conclusions and Remarks
Step 1: Theory of Evaporative Cooling
Evaporative Air Cooling Equipment Commonly called air washers or evaporative coolers, this equipment can be used to provide sensible cooling of air by the direct evaporation of water in the supply airstream. Either sprays or primary wetted surfaces are used to achieve this direct contact between the circulating water and the supply air. The water is constantly recirculated from a basin or sump with a small makeup flow being added to compensate for the water lost by evaporation and blow down. This water recirculation results in the water temperature being equal to the wet bulb temperature of the entering air. Evaporative air cooling equipment is generally classified by the way in which the water is introduced into the supply air. Air washers use water sprays, sometimes in conjunction with media. Included in this category are spray-type washers and cell-type washers. Evaporative coolers use a wetted media. Included in this category are wetted pad-type coolers, slinger coolers and rotary coolers. Capacities of this equipment usually are given in terms of the quantity of air flowing (cfm). The cooling effect is determined by how closely the leaving dry-bulb temperature of this air approaches the entering air wet bulb temperature-variously called saturation effectiveness, saturation efficiency or performance factor.
Performance factor = 100 *(tin – tout)/(tin – twb)
e.g. if the dry bulb temperature of the air is 100oF and its dry wet bulb is 65oF and we use an air washer which produce the outlet dry bulb of 70oF then the performance factor or the effectiveness of this equipment would be:
P.F. = 100 * (100 – 70) / (100-65) = 85.7%
Values for this effectiveness depend on the particular designs of individual pieces of equipment and must be obtained from the various manufacturers. It is recommended that the determination of the cooling effect for this equipment be based on the 2.5 percent value of the ASHRAE-recommended summer design wet-bulb temperatures. When evaporative air cooling is selected for air cooling air washers will be the likely choice for the cooling equipment. They are available in the capacities associated with the large airflows required for evaporative cooling systems. They can be furnished as separate modules or as packaged units, complete with fans and circulating pumps, as required to suit the application. The spray-type air washer consists of a housing in which atomizing nozzles spray water into the air stream. An eliminator assembly is provided in the air discharge to remove entrained moisture. A basin or sump collects the spray water, which falls by gravity through the flowing air. A pump recirculates this water. Air velocities through the washer generally range from 300 fpm to 700 fpm. Air handling assemblies (fan, drives and casings) can be provided to match the air washers. In the smaller capacities (up to approximately 45,000 cfm), packaged units with integral fans, but without basins or pumps, are available. These units operate at air velocities as high as1, 500 fpm with a resulting savings in equipment weight and space requirements. The cell-type air washer consists of a housing in which the airstream flows through tiers of cells packed with fiberglass or metallic media, which are wetted by spray water. An eliminator assembly is provided in the air discharge to remove entrained moisture. A basin or sump collects the water as it drains from the cells, and a pump recirculates this water. Air velocities through the washer generally range from 300 fpm to 900 fpm, depending on the cell arrangement and materials and on the inclination of the cells with respect to the airflow. In the smaller capacities (up to approximately 30,000 cfm), these washers can be provided with fans, drives and pumps as completely packaged units. Generally, spray-type washers have lower capital and maintenance costs than cell-type washers. The drop in air pressure through the sprays normally is also lower .Cell-type washers generally have a higher saturation effectiveness, which results in a slightly lower leaving-air dry-bulb temperature, but a higher relative humidity, than comparable capacity spray-type washers. The final selection of a type of washer should be based on an economic evaluation of both the installation (including equipment rooms) and operating costs for each type.
EVAPORATIVE COOLING AS READ ON THE PSYCHOMETRIC CHART: Evaporative cooling takes place along lines of constant wet bulb temperature or enthalpy. This is because there is no change in the amount of energy in the air. The energy is merely converted from sensible energy to latent energy. The moisture content of the air increases as the water is evaporated which results in an increase in relative humidity along a line of constant wet bulb temperature. By taking a set of conditions and applying the process of evaporative cooling to them we can get a clearer picture of how this process happens.
Step 2: Explanation of My Design
My design was based on two part 1- mechanical and thermodynamics and 2 - electrical and electronics
1-Mechanical and Thermodynamic: As far as these topics are concerned I tried to make this as simple as possible, i.e. use the smallest dimensions in order the device can easily be put on a desk or table so the dimensions are 20* 30 centimeters and the height 30 centimeters. the arrangement of the system is logical i.e. air is drawn inside and goes through wet pads and then get cool by evaporation and then after decrease in sensible heat of that the dry temperature of it decreases, the body of the lower part are perforated so it helps air goes inside the cooler and the diameter of the holes are 3 centimeters for least amount of pressure drop, the upper part contains water and the bottom of that has many small holes these holes are located so that the distribution of water happens evenly and drops on the wet pads while the extra water which is collected on the bottom of the lower compartment is pumped to the upper container until the whole water is evaporated and the user pours water in to the upper container. the performance factor of this evaporative cooler later will be tested and calculated to see the effectiveness of this design. the material of the body is poly-carbonate sheet with 6 mm thickness because firstly it is resistant to water secondly it can be cut easily with the cutter and with the use of glue could be stick to each other permanently with good structural stability and strength plus the fact that these sheets are pretty and neat. for structural and aesthetic reasons I use 1 centimeter electrical ducts without its cover as a kind of frame for these parts as is seen in the photos. I used sliding design for the connection of upper container to the lower one in order to facilitate separating these two containers without the use of screws and screw driver, the only exception is that I used plastic sheet for the bottom of the lower container to make it sealed because my attempt to seal it with poly- carbonate sheet was unsuccessful and despite using a lot of silicone glue there was still some leakage.
The thermodynamic part of this design is fulfilled and realized by placing the sensor in a way(explained underneath) in order to read the temperature and relative humidity in two locations and by using psychometric chart for my location (Tehran) and finding the wet bulb temperature of the incoming air and then by measuring the conditions of the outgoing air could calculate the performance of this device, another reason to incorporate the temperature and relative humidity sensor is to measure the room condition even when the device is switched off and this is a good thermodynamic indices for the person in his /her room. The last and not the least is the sensor could help to increase the performance of this cooler by trial and error i.e changing the wet pad location and the distribution of water droplets etc etc.
2 - Electrical and Electronics: As far as these parts are concerned, the electrical part is very simple the fan is a 10 cm axial fan used for computer cooling and a pump which is used for solar energy projects or small aquariums. As far as electronics is concerned since I am only an electronics hobbyist so I could not design custom made circuits and only I used the status quo circuits and adapt them to my case with some minor changes especially the software for the controller which is completely copied from the Internet sources but were tested and applied by myself so these circuits and the software are tested and safe and correct to be used by anybody who can program a controller and has the programmer. Another thing related to the electronics is the place of temperature and relative humidity sensor which I decided to put it on a hinge for two readings i.e. room reading and output air (conditioned air) reading, this may be an innovation with respect to the known project in the Internet.
Step 3: Electronic Schematic Circuits and Software
1 - I have divided the circuit for the measuring temperature and relative humidity in to three parts and call it a) the power supply b) microcontroller and sensor circuits and c) seven segment and its driver, the reason is I have used small perforated boards not PCB so I had to segregate these parts for ease of making and soldering then the connection between each of these three boards were by breadboard jumper wires or breadboarding wires which are good for later trouble shooting of each circuit and their connection are as good as soldering.
A brief explanation of each circuit follows:
The power supply circuit consist of LM7805 regulator IC to produce +5V voltage from 12V input voltage and to distribute this input voltage to fan and pump, LED1 in that circuit is an indicator of power- on status.
The second circuit consist of a microcontroller (PIC16F688) and DHT11 temperature and humidity sensor and the photocell. DHT11 is low cost measuring sensor in the range of 0 - 50% with + or - 2 degrees centigrade and relative humidity ranging 20 - 95% (non-condensing) with an accuracy of +/- 5%, the sensor provides fully calibrated digital outputs and has its own proprietary 1-wire protocol for communication. The PIC16F688 uses RC4 I/O pin to read the DHT11 output data. The photocell is behaving as a voltage divider in the circuit , the voltage across R4 increases proportionally with the amount of light falling on the photocell. The resistance of a typical photocell is less than 1 K Ohm under the bright lighting condition. Its resistance could go up to several hundred K under extremely dark condition, so for the present setup the voltage across the R4 resistor can vary from 0.1 V (in very dark condition) to over 4.0 V (in very bright condition). The PIC16F688 microcontroller reads this analog voltage through RA2 channel to determine the surrounding illumination level.
The third circuit i.e. the seven segment and its driver circuit consists of a MAX7219 chip that can directly drive up to eight 7-segment LED display(common cathode type). through 3-wire serial interface. Included in the chip a BCD decoder, multiplex scan circuitry, segment and digit drivers, and a 8*8 static RAM to store the digit values. In this circuit the RC0, RC1 and RC2 pins of the microcontroller are used to drive the DIN, LOAD and CLK signal lines of MAX7219 chip.
The last circuit is a circuit for pump level control, I could use just relays to achieve that but it needed level switches and it was not available in the present miniature scale so by using timer 555 and two BC548 transistors and a relay the problem solved and just the end of breadboarding wires were enough to achieve the water level control in the upper tank.
The hex file of the software for PC16F688 is included here and can be copied and directly feed in this controller to achieve the assigned function.
Step 4: Bill of Materials and Price List
Here the bill of materials and the price of them is explained, of course the prices are made equivalent to the American $ to enable the large audience in North America to asses the price of this project.
1 - Polly carbonate sheet with the thickness of 6 mm ,1 m by 1 m (including the wastage ) : price = 6$
2 - Electrical duct with 10 mm width, 10 m : price = 5 $
3 - Pads ( should be tailored for this usage so I bought one pack which includes 3 pads and I cut one of them according to my dimensions), price = 1$
4 - 25 cm of a transparent tubing which has the internal diameter equal to external diameter of the pump output nozzle (in my case 11.5 mm, price = 1$
5 - Computer case cooling fan with the rated voltage of 12 V and rated current of 0.25 A with the power of 3 W, noise of that = 36 dBA and the air pressure = 3.65 mm H2O , cfm = 92.5, price = 4 $
6 - Submersible pump, 12 V DC, head = 0.8 - 6 m, diameter 33 mm, power 14.5 W, noise = 45 dBA, price = 9 $
7 - Breadboarding wires with different lengths, price = 0.5 $
8 - One MAX7219 chip, price = 1.5 $
9 - One IC socket 24 pin
10 - One IC socket 14 pin
11 - One DHT11 temp.& humidity sensor, price = 1.5 $
12 - One PIC16F688 micro_controller price = 2$
13 - One 5 mm photocell
14 - One IC timer 555
15 - Two BC548 transistors
16 - Two 1N4004 diodes
17 - One IC 7805 (voltage regulator)
18 - Four Small toggle switches
19 - 12 V DC relay
20 - One 12 V female socket
21 - Resistors : 100 Ohm (2), 1 K (1), 4.7 K (1), 10 K(4), 12 K (1)
22 - One LED
23 - Capacitors: 100 nF(1), 0.1 uF(1), 3.2 uF(1), 10 uF(1), 100 uF(1)
24 - Four of 2 pins Printed Circuit Board Connector Block Screw Terminals
24 - glue including silicone glue and PVC glue etc.
25 - A piece of fine wire mesh screen to use as pump inlet filter
26 - a few small screws
27 - Some plastic junks I found in my junk box
Note : All prices which are not mentioned are less very less than 1$ each but collectively are: price = 4.5 $
The total price is equal : 36 $
Step 5: Tools Needed
Actually the tools to make such cooler are very simple and probably many people have these in their homes even if they are not hobbyists, but the name of them are listed as follows:
1- A drill with stand and drill bits and a circle cutter of 3 cm diameter.
2 - A small drill (dremel) to enlarge holes of perforated board for some components.
3 - A good cutter for cutting poly- carbonate sheets and electrical ducts
4 - A screw driver
5 - Soldering Iron( 20 W)
6 - A soldering station with magnifying glass stand with crocodile clips
7 - A glue gun for silicone glue
8 - A pair of strong scissors to cut pads or other things
9 - A wire cutter
10 - A long nose pair of pliers
11 - A small manual drill bit
12 - bread board
13 - 12 V power supply
14 - PIC16F688 programmer
Step 6: How to Make It
For making this cooler the steps are as follows:
A) MECHANICAL PARTS:
1 - prepare the lower and upper tank or container shells by cutting the poly- carbonate sheet in to suitable sizes in my case 30*20, 30*10, 20*20, 20*10 etc (all in centimeters)
2 - Using drill and drill stand make 3 cm diameter holes on three faces i.e. two 30*20 and one 20*20
3 - Make a hole equal to the diameter of computer cooling fan in one 20*20 sheets which is for the front of the cooler.
4 - Cut electrical duct in to suitable lengths i.e 30 cm, 20 cm and 10 cm
5 - Insert the edges of poly-carbonate pieces (as above) in to the relevant duct and glue it before and after insertion.
6 - Make the lower container by gluing all the above said parts and configure it as a rectangular cube without the top face.
7 - Connect the fan to the front face of the lower container with four small screws but in order to prevent the ingress of wood debris from the pads a wire mesh should be inserted between fan and the lower housing.
8 - Glue the upper tank and make it as rectangle and use electrical duct to shape a rail to attach this two tanks for ease of repair(instead of screws) i.e. sliding base.
9 - Make the upper face and attach a handle to it as shown in the photos ( I used a scrap handle from our old kitchen cabinet doors) and make it sliding as well for ease of filling up water.
10 - Cut the pads in to two 30*20 and one 20*20 piece and use needle and plastic strings to sew them and make them bound together.
11 - Use wire mesh sheet and form it a cylinder for pump inlet in order to protect the pump from ingress of debris of pads.
12 - Attach the tubing to the pump and insert it in to its place in the back of the lower tank of the cooler and position it in to its final position by two wire straps.
13 - Connect the tubing via a piece of plastic which I found it in my junk box it is part of the head of a foaming hand washing liquid container, it looks like a nozzle or a enlarge fitting, this firstly decrease the speed of water coming from the pump secondly produces friction and loss (the length of tubing is 25 cm and need more loss to match pump head), thirdly it connect the tubing to the upper tank firmly.
B) ELECTRONIC PARTS:
1- Program the PIC16F688 micro-controller by using programmer and the hex file provided above.
2 - Use bread board to make the first part i.e the 5 V power supply and 12 V distribution unit then test it if it works use a perforated board to assemble all components and solder them up, be careful to use all safety precautions when you soldering especially ventilation and protective goggle, use magnifying glass and extra hand to do a neat soldering.
2 - Use bread board to make the second unit i.e. the micro -controller and temp.& humidity sensor unit. use the programmed PIC16F688 and assemble other components if the result was successful i.e. enough indication of a correct hookup then use the second small perforated board to solder them in place, use IC socket for PIC micro - controller, while soldering the PIC16F688 observe extreme caution not to attach neighboring pins. Do not solder sensor to the perf. board and use suitable sockets on the board to later connect them with breadboarding wires also do not solder switch S1 in the relevant diagram to let it be assemble on the face of the device for resetting purposes and later use continuity tester to test the outcome for a neat work.
3 - Assemble the third unit i.e. the seven segment and it's driver i.e. MAX7219, at first on the bread board and then after test and being certain of its functionality start soldering this unit carefully but seven segment should not be soldered to the perf. board and by using breadboarding wires it should be fixed on a small box made for these 3 units to be fix in that. MAX7219 should be installed on an IC socket for future repair or trouble shootings.
4 - Make a small box from poly- carbonate(16*7*5 cm*cm*cm) to contain all these three units as shown in photos and fix the seven segment and S1 on its front face and the LED and a switch and the female 12 V jack on its side face, then glue this box to the front face of the upper tank.
5 - Now start to make the last circuit i.e.pump level control, by first assembling its components on the breadboard to test it I used a small strip of LED instead of the pump and a small cup of water to see its proper function when it worked, then use perf.board and solder the components to it and three level electrodes i.e. VCC, lower and higher level electrodes should be connect to the board by breadboarding wires in order to be inserted via a small hole on the upper tank in to it as level control electrodes.
6 - Make a small box in order to fix the level control unit in that and glue it to the back face of the upper tank.
7 - Connect fan, pump and front unit to each other.
8 - In order to enable measuring and reading the room and fan outlet temperatures and relative humidities I have used a hinge by which the temperature and humidity sensors can turn either directions one is straight on to measure room air condition and then by tilting it and bringing it close to the outlet flow of fan to measure fan outlet air condition.
Step 7: Measurements and Calculations
Now we have reached the stage in which we can asses the performance of this evaporative cooler and its effectiveness, first of all we measure the temperature and relative humidity of the room and the by turning the sensor to ward the fan outlet we wait for a few minutes to have steady conditions and then reading the display, since both of these readings are in the same situation so the errors and accuracies are the same and no need to incorporate it in to our calculations, the results are:
Room( cooler inlet condition) : temperature = 27 C relative humidity = 29%
Fan outlet : temperature = 19 C relative humidity = 60%
Since my location is Tehran(1200 - 1400 m above sea level, 1300 m is taken in to account) by using relevant psychometric chart or psychometric software the wet bulb temperature of the room would be found = 15 C
Now we substitute the above quantities in the the formula which was described in the theory of evaporative coolers i.e. Cooler effectiveness = 100*(tin - tout)/(tin - twb) = 100*( 27 - 19)/(27 - 15) = 67%
I think for the small size and extreme compactness of this device this is a reasonable value.
Now to find the water consumption we embark upon the calculations as follows:
Fan volume flow rate = 92.5 cfm ( 0.04365514 m3/s)
Fan mass flow rate = 0.04365514 * 0.9936(air density kg/m3) = 0.043375 kg/s
humidity ratio of the room air = 7.5154 g/kg(dry air)
humidity ratio of fan outlet air = 9.6116 kg/kg(dry air)
water consumed = 0.043375 * (9.6116 - 7. 5154) = 0.09 g/s
Or 324 gr/ h, which is 324 cubic centimeter /hr i.e. you need a jar with 1 liter volume next to the cooler to pour water occasionally when it runs dry.
Step 8: Conclusions and Remarks
The results of the measurements and calculations are encouraging, and it shows this project at least fulfills the spot cooling of its maker, also it shows the best idea is self independence as far as cooling or heating is concerned, when other people in the house do not need cooling but you feel overheated then you switch on the personal cooler especially in a hot day in front of your personal computer when you need spot cooling, this apply to all sorts of energy, we should stop using so much energy for a big house when you can get that energy in a spot i.e. your own place, either this energy is cooling or illumination or else, I can claim this project is a green project and low carbon dioxide project and can be harnessed in remote places with solar power.
Thank you for your kind attention