• Roughing out the cooling design

    Laz • 01/28/2015 at 00:56 • 0 comments

    So, the genesis of the idea was to make a more advanced meat curing chamber. The first thing to do was to rough out the design ideas. I wanted to cure meat, so that helped set the chamber design conditions. We starting hanging meats to dry a long time ago in caves. Some caves were better than others for the job. Over time, we gained experience about what conditions were the best. Different products might have slightly different conditions for preferred drying, but they cluster pretty tightly around 55F (~13C) and 70-80% humidity.

    During the drying process, the temperature in the curing chamber will be relatively stable. Once the meat is at the same temperature as the setpoint for the curing chamber, then the only sensible heating or cooling that will be required is what conducts through the walls of the chamber, or to heat or cool any air that leaks in. That perspective points out the necessity of doing a good job of sealing and insulating the curing chamber. From that perspective, a refrigerator is not a bad place to start. They are sealed up, and relatively well insulated.

    A design temperature of 55F means that the chamber is going to need to be colder than most rooms. That means that unless this thing is kept outside, or in an unconditioned space it's going to need more cooling than heating. Identifying the cooling source is going to be key to the design.

    A compressor-based refrigeration cycle is what is used in most refrigerators. It's a very efficient way to do cooling. Unfortunately, it's very expensive, and not terribly flexible. There's a compressor, multiple coils, expansion valves, lots of copper tubing, and refrigerant that (probably) requires a license to be able to deal with. The pressure of the refrigerant inside the system drives the temperature that it operates at. We'd have to add or remove refrigerant, or otherwise modify the compressor in order to reach a specific cooling temperature with this type of system. That's not an easily controllable system, and it's the reason that a standard refrigerator is not a good tool for a meat curing chamber.

    The pressures that most consumer refrigerators run at produce a cold-side temperature that's close to freezing. The problem with that is that it produces surface temperatures that are too cold. Moisture will condense out of the air inside the refrigerator, just like it condenses on the sides of a glass of ice water. This brings down the humidity inside the refrigerator below what is needed for meat curing, which is why all the refrigerator-based systems require using a humidifier to add moisture back to the air, even though the meats are releasing pounds of moisture into the air,

  • Genesis

    Laz • 01/28/2015 at 00:14 • 0 comments

    Before beginning this project, I had built a smoker. It's good for cold smoking, and I've done lots of bacon and cheese. These things are awesome. I've wanted to take the next step into cured and dried meat products. Thoughts of pancetta dance in my head.

    I did some preliminary research to find out what's been done in the DIY space. The examples that I've managed to come up with are all terrible. The common the answer seems to be start with an old refrigerator and put a temperature controller on it so that it doesn't get too cold. However, that doesn't do anything to change the fact that the refrigerator has surface temperatures that are too low, which drives the humidity down. So the common answer is to then stick a humidifier into the refrigerator to make up for that. That leads to very uneven distribution temperatures and humidity, so then we need to stick a fan inside the thing to mix everything up. I've seen designs that also include light bulbs as auxiliary heating devices.

    This design obviously works, because it has been repeated multiple times. From an engineering and design standpoint, though, it's a terrible mess of several systems that are operating out of their design range and fighting each other constantly. These leads to a terribly inefficient system of simultaneous heating and cooling with no consideration of control.

    I wanted to put together a better design that would be more efficient, enable modular controls, and possibly data logging to help provide actual data in the event of a failure. So with that idea, it was off to the races.