a constant stream of hot air - Page 1
Forum section: electronics / projects

author: k8943
author_rank: Regular Contributor
timestamp: December 12, 2021, 10:48:08 am
content: I'd like to build something that produces a constant stream of hot air at modest temperatures such as 60C, 80C and 100C.- The air will actually be recirculating (so start temperature will gradually rise from ambient);- thought of using a PTC heater for safety reasons(resistance increases with temperature and elements only modestly hot eg 200C, which should be enough if air not circulating too fast);- have a boiler fan that can handle the heat (designed for exit gases from a gas boiler);Is there a simple PID solution that could manage this by controlling the heater with PWM? The air will be getting hotter and the heater resistance changing too, so it needs to be smart enough to keep adapting on the fly such that the output air is at the right temperature plus or minus. Notice there are lots of digital PID controllers sold with SSR relays for around 50 buck but not sure they PWM or that they will be smart enough? Or is this something to tackle ground up with a microcontroller?

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 12, 2021, 12:57:59 pm
content: It would be trivial to adapt the thermal control code for a 3D printer.

author: themadhippy
author_rank: Super Contributor
timestamp: December 12, 2021, 01:21:46 pm
content: QuoteHire a a politician

author: LaserSteve
author_rank: Super Contributor
timestamp: December 12, 2021, 01:39:04 pm
content: For smooth control of hot air sources your heater has to have either a large thermal mass and large surface area, or lots of very hot coiled wire. The later generally has the heater element placed in a bridge circuit, as the resistance of the metals used can change drastically leading to burn out.Depending on the amount of mass flow you need, this can become a very thorny control problem. So how much flow do you need?Steve

author: k8943
author_rank: Regular Contributor
timestamp: December 12, 2021, 04:39:05 pm
content: Hmm to tell you the truth I hadn't attempted to calculate flows and was going to take an experimental approach.Starting point experience is pointing a fan up at the bottom of a 750W 220v 3d printer bed @120C in an enclosed build chamber and achieving a stable 65-70C chamber temperature (depending on outside temperature).Initial objective is to do something similar - but controllable - to heat the inside of an insulated box in order to cure composite plastics that need to go through cycles such as 4hrs@@60C then 4hrs @80C etc.. Figured that a system with fast circulating air would be good because it will heat the mold (item in the box) more quickly to the next temperature level.AND then, based on experience gained (and system built) from the above, build a recirculating air jet at the level of a 3d printer platform, following the hints provided by the following patenthttps://patents.google.com/patent/US20190240969A1/en?q=Heated+air+system+for+3D+printer&oq=Heated+air+system+for+3D+printerRegarding air flow, the boiler blower I have is capable of a lot of flow and that seems good for the composite curing box project. For the 3d printer, in the patent they suggest keeping the flow slow (2.5m/s across the print surface).... though would have to calculate cross sectional areas etc.. But seeing as the box is supposed to be also serve as a prototype for the printer, could take a lower flow approach if problematic.Suppose need to use a naked thermistor (rather than something in a chunky case) so it picks up on temperature variations easily and liked the idea of using a large surface area heater at a lower temperature (like the 3d printer bed above) for safety reasons, but since have to buy it anyway not tied to any particular approach.

author: k8943
author_rank: Regular Contributor
timestamp: December 13, 2021, 10:17:32 am
content: An estimate of flow for the 3d printer application would be: 2.5m/s over an area of 30cm by 2.5cm= 2.5 x 0.3 x 0.025 = 0.019m3/s == 68m3/hr ~=40cfmOn the other hand the boiler fan (can't find specs for these fans that are sold as being "compatible" with whatever brand of boiler) fills a 30litre bin bag in about 2 secs => 15 litres/sec = 0.015m3/s == 54m3/hr ~= 32cfmThis is all very approximate but let's say correct within a factor of 2 i.e we're targeting somewhere between 15cfm and 80cfm in terms of airflow (to make the 3d printer application work and if we just use existing blower to generate flow in composite box oven).Is this doable? How/what do I need?

author: nali
author_rank: Frequent Contributor
timestamp: December 13, 2021, 10:43:59 am
content: If this is a hobby thing i.e. not a commercial product & you just want to "get it done", Arduino has a workable PID library which works OK. I used a Nano as part of my DIY reflow oven with a MAX31855 as the thermocouple interface (SPI) and it works just fine.

author: k8943
author_rank: Regular Contributor
timestamp: December 13, 2021, 11:15:11 am
content: Thanks, good info. Arduino nano might be more fun, more programmable and cheaper than buying a PID controller.We know that PID is going to control a standard "oven" situation where the heat goes on/off to try to keep a sensor in range. And putting a fan in the oven arguably changes nothing - bar improving the homogeneity of the temperature / eliminating gradients.But measuring the temperature of air as it flows off the heater such that it's neither too hot nor too cold (like the air coming out of a reflow heat gun) is maybe trickier. Not quite sure why. Heater response time / heat capacity? Turbulence - or lack of? Temperature gradient + conductivity of air?The Arduino PID library example uses an on/off time of 5000ms (5secs) but presumably it will work with eg 50ms, in other words be quite responsive.To connect them together we use "time proportioningcontrol" Tt's essentially a really slow version of PWM.First we decide on a window size (5000mS say.)https://playground.arduino.cc/Code/PIDLibraryRelayOutputExample/

author: ajb
author_rank: Super Contributor
timestamp: December 13, 2021, 04:28:27 pm
content: I'm not sure whyQuote from: k8943 on December 13, 2021, 11:15:11 amBut is the temperature of the air exiting the heater the parameter you actually care about? If you're using this to heat a chamber like for a 3D printer, then surely the chamber air temperature is the thing you really care about. Measuring the exit air temperature at the heater will inevitably result in a certain amount of error because the chamber will lose heat through its walls, but also gain heat from whatever's inside of it. Tricky to say which direction the error will be in without some analysis.Either way, regulating exit air temperature to your set point will slow your rise time quite a bit because that air at 60C or whatever is going to lose a fair bit of heat into the rest of the system (rest of the air volume as well as whatever it flows through). Measuring inlet air temperature would likely be more useful if you wanted the heating system to be self contained, although you'll want to limit the maximum temperature of the heater itself and possibly the exit air temperature for safety. Still better to measure the temperature wherever you most care about it, though. Ideally you would size the heater such that at maximum power it's not going to catch fire or something, wouldn't be a bad idea to include an mechanical thermal cutout switch as well. The control algorithm could be arbitrarily complex, if you wanted to minimize rise time it wouldn't be hard to run the heater at higher powers during startup to help get up to temperature faster. When you have high thermal mass that needs a big step change it can be helpful to start out in a simple on/off control mode and then switch to PID (or whatever) when you get closer to the set point. It's certainly possible to tune a PID loop to run the whole cycle and get decent results, but at the very least it's time consuming when you have to wait for the whole system to cool off before running another test/tune cycle.

author: k8943
author_rank: Regular Contributor
timestamp: December 13, 2021, 04:53:30 pm
content: In the details of the patent (cited Reply #4 above) they describe how they send a jet of hot air across the build chamber at the level where the printing is occurring. The air is sucked in on the opposing side of the chamber. So in fact temperature is only controlled at that height, the zone where the most recently extruded plastic is solidifying. They then mention that a temperature gradient naturally establishes itself down the chamber such that, as the part moves down it can further harden (and shrink) in a progressive sort of way, which apparently is advantageous. Until now I've simply heated the whole chamber by pointing a fan at the bottom of the bed and empirically that gets it to 65-70C which is pretty helpful in terms of preventing part warping (and also necessitates careful choice of printer parts to withstand the heat!). The ideal chamber temperature for ABS printing is however around 100C and aim to achieve this in a way similar to the patent (taking it on faith that these guys are the experts), concentrating and controlling the heat on the build plane.Since the hot air is jetting out towards the part, would like to keep it at a constant and precise temperature, optimised for the solidification of the plastic in question (at present ABS comes out of nozzle around 270C and have gleaned that 100C would be a good target). As you suggest could have a preparatory cycle in which air jet temperature exceeds target temperature for a period in order to warm up the whole machine before printing, though in practice I'm not sure the inners of my machine will be up to much more than 100C.