Homeroasters - Home Roasting Coffee Community » BUILDING A ROASTER » Electric and Gas Heat Sources
Who is here? 1 guest(s)
 Print Thread
electric elements - raising the ohms to lower amps
jberks
Top  #1 Print Post
Posted on 07/11/2014 9:47 AM
Homeroasters,

My roaster:
7 amp domel two stage blower
8 ohm master appliance heat element

I'm running this on a standard 15amp circuit.

Issue: the element at 8 ohms will draw a theoretical 15 amps. I currently feather both of them back to keep the breaker from blowing.

I was thinking of getting an element with 15 amps to lower it to 8 amps. Thus I could run both at full at the same time.

Does anyone see a problem with this?

Another thing I thought of, would the element get hotter with the more surface area of more nichrome, or would it weaker because there is less amps running through it?

Thanks

Jamie
 
allenb
Top  #2 Print Post
Posted on 07/11/2014 11:19 AM
Something that will make figuring out the heating potential of any typical electric heating element in any combination of series/parallel is the good ole Ohms Law formulas.

Using the formula Power (in watts) = Volts squared / Ohms will give you the answer regardless of surface area, how many cfm of airflow or phase of the moon. One thing to remember is regardless of how we configure our heating element/elements whether single or multiples, watts will determine how much heat is being transferred to the air.
allenb attached the following image:
ohms-watts_law.jpg
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
allenb
Top  #3 Print Post
Posted on 07/11/2014 3:30 PM
I forgot to include this one
allenb attached the following image:
series_parallel_resistors.gif
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
jberks
Top  #4 Print Post
Posted on 07/16/2014 2:53 PM
Excellent.

Has anyone used a heat cartridge for before?

www.mcmaster.com/...rs/=sv613t

Wondering if it has enough surface area to be effective. Especially at the 1000 watt range.
Edited by allenb on 07/16/2014 6:04 PM
 
oldgearhead
Top  #5 Print Post
Posted on 07/17/2014 3:03 PM
Those look pretty 'wimpy' compared to Master Appliance
heat gun elements in the 800-1500 watts range. You might try
3 of them.
 
coffeeroastersclub
Top  #6 Print Post
Posted on 07/17/2014 6:35 PM

Quote

jberks wrote:

Excellent.

Has anyone used a heat cartridge for before?

www.mcmaster.com/...rs/=sv613t

Wondering if it has enough surface area to be effective. Especially at the 1000 watt range.


I went to the mcmaster page you reference. I don't know which you are focusing on, the cartidge ones or the immersion ones; I suggest not using the immersion ones as they will blow out if not in water. thumbdown I speak from experience with some experimenting I had done in the past with them & roasting coffee.

Len
"If this is coffee, please bring me some tea but if this is tea, please bring me some coffee." ~Abraham Lincoln
 
Web
allenb
Top  #7 Print Post
Posted on 07/17/2014 7:15 PM
Cartridge heaters were developed primarily for heating metal things by inserting them into a very close fitting hole within the mass of the metal. By doing this you're removing heat from the sheath of the cartridge heater very rapidly which allows designing for a much higher watt density without the sheath melting down. The way to determine if a particular cartridge heater will work is to calculate the heaters outer surface area in square inches. Divide the elements wattage by the number of square inches of surface area. If you don't go over the maximum allowable watt density for the particular cartridge heater in a given environment then you're ok but you'll most likely end up with a fairly large cartridge heater body to keep the watt density in the allowable range. Make sure you find the advertised watt density for still air and not for one immersed in liquid or into a metal heat sink.

Allen
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
 
Jump to Forum: