How can a sealed tube move heat without a pump?

A heat pipe can move energy away from a warm component without a mechanical pump. Its sealed tube contains a small amount of working fluid, a porous wick along the wall and a central space where vapor can travel. The warm and cool ends perform different parts of one continuous cycle.

At the warm end, liquid absorbs energy and evaporates. Vapor travels to the cooler end, condenses and releases energy there. Liquid returns through the wick: its narrow pores support curved liquid surfaces, and surface tension creates a capillary pressure difference that can draw liquid toward the evaporating region. The returning liquid keeps that region supplied for another trip.

A simple steady balance is heat-transfer power = mass-flow rate × latent heat. If the assumed latent heat is 2,000,000 joules per kilogram, carrying 60 watts by evaporation and condensation needs 0.00003 kilograms per second, or 30 milligrams per second. This example counts latent heat alone and does not calculate an operating temperature.

The wick can sustain only a limited capillary pressure difference. Returning liquid encounters resistance, and gravity can help or oppose the return depending on orientation. If liquid cannot reach the warm end fast enough, the evaporator can dry out and the cycle loses its effective liquid supply.

The fluid is reused rather than consumed, but the energy must still leave at the cooler end through a heat sink. A heat pipe transports energy to a place where it can be released; it does not eliminate the need for cooling.