Hot rod technology is also called low temperature gravity heat pipe. Hot rod refrigeration technology is a kind of heat transfer system using liquid-vapor convection and circulation to realize heat transfer, and it is the most efficient device in passive cooling system. It has the characteristics of large heat transfer capacity, small heat transfer temperature difference, low start-up temperature, good uniform temperature performance, one-way heat transfer and safety and economy.

The hot rod consists of a sealed steel tube filled with liquid nitrogen, and the upper part of the steel tube is equipped with heat dissipation blades, called condensation section, which is placed in the atmosphere; the lower part of the steel tube is embedded in the permafrost of the foundation, which is called evaporation section. When there is a temperature difference between the evaporation section and the condensation section, the liquid nitrogen in the evaporation section absorbs heat and evaporates into gas. Under the action of pressure difference, the steam rises along the void in the tube to the condensation section, and contacts with the colder tube wall, releases the latent heat of vaporization and condenses into liquid. Under the action of gravity, condensed liquid nitrogen flows back to the evaporation section along the tube wall and then absorbs heat to evaporate. Such reciprocating circulation can transfer heat from stratum to atmosphere, thus lowering the ground temperature of permafrost, so as to prevent permafrost from melting, so as to improve the mechanical characteristics of subgrade soil.

The low-temperature hot rod is suitable for ground cooling in railway, highway, pipeline engineering, bridge pile foundation, culvert, tunnel, Airport runway, communication transmission line tower foundation and water conservancy engineering in permafrost area. Through the construction of hot rod, the frost heave and thaw settlement deformation of ground are prevented and the stability of frozen ground foundation is ensured.

Low-temperature hot rod products have the characteristics of high heat transfer coefficient, small heat transfer temperature difference, large heat transfer capacity, good uniform temperature, one-way heat transfer and long service life.

General Dimension Parameters of Low Temperature Hot Rod Structure

Nominal diameter DN(mm)	Total length of hot rod (pile) L(m)	Length of condensation section L1(m)	Fin specification h×δ(mm×mm)	Fin pitch t(mm)	Area of condensation section (m2)	Weight (kg)
50	6.3	2.5	21×1.9	12	2.4	55
65	6.3	2.5	21×1.9	12	2.6	80
	8	2.8	21×1.9	12	2.9	100
	10	3.15	25×1.9	12	4.0	120
80	6.3	2.5	25×1.9	12	4.1	105
	8	2.8	25×1.9	12	4.5	135
	10	3.15	25×1.9	12	5.2	165
	12.5	3.55	25×1.9	12	5.8	200
100	6.3	2.5	25×1.9	12	4.8	125
	8	2.8	25×1.9	12	5.4	165
	10	3.15	25×1.9	12	6.1	200
	12.5	3.55	25×1.9	12	6.9	240
125	6.3	2.5	25×1.9	12	5.8	320
	8	2.8	25×1.9	12	6.5	200
	10	3.15	25×1.9	12	7.3	240
	12.5	3.55	25×1.9	12	8.2	295
150	6.3	2.5	25×1.9	12	6.8	380
	8	2.8	25×1.9	12	7.6	235
	10	3.15	25×1.9	12	8.5	290
	12.5	3.55	25×1.9	12	9.6	350

Working Temperature of General Low Temperature Hot Rod

Working substance	Heat Pipe Working Fluid-a	Heat Pipe Working Fluid-a-1	Heat Pipe Working Fluid-a-2
working temperature (℃)	-60～+60	-40～+15	-50～+70