Symbol dictionary
ThermoDraw 1.1.0

A place in the network that has a temperature, with nothing holding it there. Its temperature is set by the heat arriving and leaving: at steady state the two must be equal, and that is the condition you solve for it. This is usually the temperature you are trying to find. Most nodes are free ones, and it is the default, so leaving kind out gives you this.

Example: The outside of a mug of tea. Nothing fixes its temperature; it takes the value that balances heat in from the tea against heat out to the room.

{
  "id": "mug",
  "kind": "free",
  "label": "Mug surface",
  "sub": "s",
  "value": "48"
}

A temperature imposed from outside, which nothing the network does can change. The wire runs down into a hatched wall, the drafting mark for a boundary: heat may cross it in either direction, at any rate, without changing its temperature. The wall faces down unless wall turns it up, left or right, for a boundary that holds the thing from above or beside it.

Example: The air in the room. One mug of tea cannot warm it up, so its temperature is a given rather than an answer.

{
  "id": "room",
  "kind": "fixed",
  "label": "Room air",
  "sub": "air",
  "value": "20"
}

A boundary the network touches mechanically but not thermally. It is drawn like a fixed node with its connecting stub taken away, and that gap is what tells the two apart: nothing crosses here. With no heat path there is usually no temperature worth stating, so most break nodes carry a label and no value. Like a fixed node, its wall faces down unless wall turns it.

Example: The rubber feet under a laptop. They hold it to the desk without letting heat into it.

{
  "id": "desk",
  "kind": "break",
  "label": "Rubber feet"
}

A temperature held constant by a change of phase rather than by a boundary. It takes the constant-temperature marking, two short rules beneath, and no wall. While the phase change lasts, heat crosses it with no temperature rise at all.

Example: A pan of boiling water. It sits at 100 °C however far you turn the hob up; the extra heat makes steam instead.

{
  "id": "boil",
  "kind": "phase",
  "label": "Boiling water",
  "sub": "boil",
  "value": "100"
}

Heat crossing solid material. Section hatching is the drafting convention for solid material, which is what the interior of the box is naming. The thicker the material and the worse it conducts, the larger the resistance.

Example: The wall of a mug, between the tea inside and your hand outside.

{
  "from": "tea",
  "to": "mug",
  "kind": "cond",
  "label": "Mug wall",
  "value": "0.35"
}

Heat leaving a surface into a moving fluid. The streamlines inside the box stand for that fluid passing over the surface. The faster it moves and the more surface it touches, the smaller the resistance.

Example: A hot drink cooling into the air around it, and cooling faster when you blow across the top.

{
  "from": "mug",
  "to": "room",
  "kind": "conv",
  "label": "Mug → Room air",
  "value": "1.80"
}

Heat leaving a surface as thermal radiation. The box is empty because radiation needs no material to travel through, so the wave arrows cross it on their own. The outline is dashed as a warning: this is the one path whose heat flow is not proportional to the temperature difference. It goes as the difference between the fourth powers of the two temperatures, so one resistance value only holds near the pair of temperatures it was worked out at.

Example: The warmth on your face from a fire across the room, which reaches you without heating the air in between.

{
  "from": "fire",
  "to": "you",
  "kind": "rad",
  "label": "Fire → Face",
  "value": "6.40"
}

The resistance of two solids pressed together. No two surfaces are perfectly flat, so they touch only in places. The area actually carrying heat is a fraction of the area you can measure, which is what makes the resistance, and there is a step in temperature across the join. Each half is hatched in the opposite direction with a seam between them, which is the drafting convention for two parts meeting in section. Hatched the same way, it would read as one solid block.

Example: A saucepan sitting on a hotplate. The two never quite meet, and the gap costs you a temperature drop.

{
  "from": "hob",
  "to": "pan",
  "kind": "contact",
  "label": "Pan on hotplate",
  "value": "0.15"
}

Conduction into a cross-section that grows as the heat goes: a small source on a much larger sheet. The hatching fans from a point rather than running parallel, because the area available increases along the path. Drawn as plain conduction it would assert one-dimensional flow, which is exactly what spreading is not.

Example: A small gas flame under a wide frying pan. The heat has to fan out sideways through the base before it reaches the edges.

{
  "from": "flame",
  "to": "pan",
  "kind": "spread",
  "label": "Flame → Pan base",
  "value": "0.15"
}

A path of such low resistance that both ends sit at nearly the same temperature. Inside it a fluid boils at the hot end and condenses at the cold one, which is what the opposed arrows show: vapour out along one face, liquid back along the other. It is still drawn as a box with a resistance, because a real one does have a small resistance. The arrows say the heat is carried by boiling and condensing rather than by conduction.

Example: The flattened copper tube inside a laptop, carrying heat from the chip out to the fan with almost no temperature drop.

{
  "from": "chip",
  "to": "fins",
  "kind": "pipe",
  "label": "Heat pipe",
  "value": "0.10"
}

One number covering more than one mechanism, or a mechanism you do not wish to name. The box is left empty. Since the pattern inside a box is what names the mechanism, an empty box means the mechanism is deliberately not stated, not that it was forgotten. It is the only path whose subscript you set, because it is the only one the library cannot name for you.

Example: A double-glazed window, sold as a single figure that already has conduction and convection rolled together.

{
  "from": "inside",
  "to": "outside",
  "kind": "mixed",
  "sub": "window",
  "label": "Double glazing",
  "value": "0.31"
}

Thermal mass: how much heat it takes to raise something one degree, in joules per kelvin. It stores heat rather than passing it on, so it is hung from a node down to the reference rail rather than carrying heat to a second place. It matters only while things are changing. Once everything has settled it carries nothing.

Example: A cast-iron pan: slow to heat up, and just as slow to cool down again.

{
  "from": "pan",
  "to": "rail",
  "kind": "cap",
  "label": "Cast-iron pan",
  "sub": "pan",
  "value": "900"
}

Heat moved from one place to another because a fluid is moving and carrying it. It states a rate, not a resistance. It is drawn as chevrons in the line rather than as a box because nothing is being crossed: the fluid itself is what travels. It is the only path with a direction, so from and to are the way the heat goes, and angle is refused on one.

Example: Hot water pumped from a boiler to a radiator. The heat travels because the water does.

{
  "from": "boiler",
  "to": "radiator",
  "kind": "flow",
  "label": "Central heating",
  "value": "1.5"
}

A mechanical connection that carries no heat, drawn as an open circuit. Deliberately neither a plain wire, which would say heat flows, nor a resistance, which would say how much. It names no quantity, so it takes no value and gets no second line of text.

Example: The plastic handle on a saucepan. It is bolted on to hold the pan, and made of something that conducts too badly to carry heat worth counting.

{
  "from": "pan",
  "to": "handle",
  "kind": "break",
  "label": "Plastic handle"
}

Two nodes that are one place, drawn twice because the reader needs both names. A plain wire, which is precisely what a thermal break declines to be: a wire says heat flows, and here it flows with nothing in the way. It names no quantity, so it takes no value and no rate. It is a claim rather than a decoration: the checker merges the two ends before balancing anything, and says so if they state different temperatures.

Example: A baseplate bolted hard to the face it sits on. The reader counts two parts; the network has one temperature.

{
  "from": "face",
  "to": "baseplate",
  "kind": "link",
  "label": "Bolted flange"
}

A medium passing through, carrying heat away with it. It is a path rather than a place because it has two ends: it arrives at the temperature of one node and leaves at the temperature of the other, and the difference is what it took. You state the mass flow and the specific heat; the heat itself is worked out and drawn beneath them. Streamlines say the medium is going, the same fill a convection box has, and the chevron says which way. Directed, so which end is the inlet is from and to.

Example: Steel strip through a reheat furnace: in cold, out hot, and what the furnace has to supply is exactly the difference. Add the heat in stages by drawing the run as two streams with a node between them.

{
  "from": "in",
  "to": "out",
  "kind": "stream",
  "label": "Steel strip",
  "mdot": "2.5",
  "cp": "0.665"
}

Heat released at a place because energy in another form is being converted there, usually electrical work. It is an arrow rather than a box, because that heat does not travel to the place from anywhere else; it is produced there, so the arrow always points inward.

Example: A light bulb, which turns most of the power it draws straight into heat.

{
  "to": "bulb",
  "kind": "diss",
  "label": "Bulb power",
  "sub": "in",
  "value": "45"
}

Radiation arriving from outside the network, drawn with the same wave arrow the radiation path uses. Reach for it where the incoming radiation is a number you already know rather than something to be worked out. Being radiation arriving, it also always points inward.

Example: Sunlight falling on a parked car.

{
  "to": "roof",
  "kind": "radin",
  "label": "Sunlight",
  "sub": "sun",
  "value": "600"
}

A stated heat rate going into or out of one place: an annotation for where you know the number and do not need to draw the path it took. Give it to for heat arriving or from for heat leaving, and the arrow follows what you said.

Example: The heat carried out of a room by an extractor fan, when the figure is all you need to say.

{
  "from": "room",
  "kind": "flow",
  "value": "38"
}

A heat rate per unit area going into or out of a surface. It is spread across that surface rather than concentrated on one line, which is why it is drawn as several arrows against a hatched band instead of as the single heat-flow arrow. It is measured in its own quantity, q″, and never shares units with a heat rate.

Example: Sunshine on a roof, given per unit of area rather than as one total for the whole roof.

{
  "to": "roof",
  "kind": "flux",
  "label": "Sun on roof",
  "value": "0.1"
}