Heat transfer in concrete
Heat transfer in concrete
(OP)
I need to know how much heat in BTUs I will require and how long it will take to raise the temperature of a concrete core or cylinder 18" in diameter and 12 inches high to a depth of 1" all around, from 32ºF to 50ºF and the same for the corresponding concrete hole of similar size. We plan to use hot air as a heat source at a temperature between 150º-200ºF and the optimum time would be 30 - 45 minutes. The application is required to heat the concrete so as to allow cementatious grout used to rebond the cylinder back into the concrete substrate to set up and gain strength (20-30 minutes) before surface frost returns under winter conditions. Thanks





RE: Heat transfer in concrete
First question: is the heated air recirculated such as in an oven whereby the temperature under some velocity is somewhat constant let's at an average temp of 175 deg.F? Would the cylinder be exposed to such recirculation or would the air speed be created by a hot air blower whereby the heated air is blowing longitudinally? Is the concrete core already cure when the cylinder is to be heated?
RE: Heat transfer in concrete
These forums are not well suited for "free consultancy services" because: How will you document your work? If documentation and accuracy is not require - why worry?
Best regards
Morten
RE: Heat transfer in concrete
RE: Heat transfer in concrete
RE: Heat transfer in concrete
I think that makes more sense, since it allows you to ensure that the entire interface is controllably heated. You can still use the hot air blowers to do that.
TTFN
RE: Heat transfer in concrete
i do not know if i understood your description correctly...
i am not a theory guy... inclined to field fixes...
but if the following is correct:
____V__ 18in dia _______
| |
| |
| | 12in high
| |
-------------
pit in the concrete and a cylinder that fits in this hole...
i think the fastest way to heat hole and plug is with an immersion heater... in any case the grouting is water soluble.
fill the hole with water.
put immersion heater in.
use contact TC's to measure temp at the top (V) one inch away from the edge to detect the temp u need - connected to a thermostat that will control the immersion heater.
have a pit pump handy to empty the hole.
(in case you leave the water in the hole overnight... you will need to melt it with a flame, not the immersion heater)
build a bucket that will accomodate the plug, with some spikes at the bottom so the plug does not sit at the bottom
i would say that the ID of the bucket should be 30 in to have some water volume.
set the plug in the bucket
start the immersion heater
measure with contact tc's the temp 1" in away from the edge
the tc's should be connected to a thermostat in the immersion heater to keep the temperature at the desired level for as long as required.
add salt, pepper and serve with sprinkled parsley (it does sound like a food recipe, doesn't it?)
water has much better heat transmittion and retention characteristics than air.
to heat the pit, the air will be bouncing off the walls and bottom and the efficiency of the process cannot be very high. also the air will flow around the cylinder and most of the heat will dissipate into thin (and very cold) air.
as water retains heat very well, if there is a delay in the process it will keep the temperature right until the crew is ready to proceed. and the thermostats will help keep the electricity bill low...
i.e. remove the water from the hole and the cylinder from the water only when ready...
you will need all the typical electrical protections required per the applicable codes and standards.
the power required to heat up the volume of water in 45 min is approximately 1900 watt, i would say that a 2.5 KW immersion heater is quite enough for the pit
the bucket will need more power (more water) a 5 KW immersion heater should be plenty.
may be there are other considerations that will preclude using water as the heating medium... but if not i think this could be a good field solution...
what do you think?
suerte.
saludos.
a.
RE: Heat transfer in concrete
RE: Heat transfer in concrete
Temperature distribution at .1" increment and after one hour of heating the hole at 250 degF and 1000 degF. are:
250,151,101.2,67.7,48.5,38.8,34.5,32.8,32.3,32.0,32
1000,845.6,561.4,339.2,190.7,105.11,62.1,43.2,35.7,33.1,32.3
Above results shows concrete to be a good insulator even w/ 10% moisture content.
Heating the concrete w/ a torch will probably spall your concrete surface.