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Information about 0.16, the current stable version of Factorio, can be found on stable.wiki.factorio.com.
Heat exchanger
Heat exchanger 
Recipe 

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Total raw 

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Recipe 

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Total raw 

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Map color 

Health 
200 
Resistances 
Explosion: 0/30% 
Stack size 
50 
Dimensions 
2×3 
Energy consumption 
10 MW 
Maximum temperature 
1000 °C 
Mining time 
0.1 
Prototype type 
boiler 
Internal name 
heatexchanger 
Required technologies 

Produced by 

The heat exchanger exchanges heat between a heat connection and water to produce steam.
Heat exchangers produce ~103 steam with a temperature of 500°C every second.
Heat exchangers will not produce steam until they reach 500°C. The steam produced is exactly 500°C hot, even if the exchanger is hotter. Heat exchangers have a heat capacity of 1 MJ/°C. Thus, they can buffer 500 MJ of heat energy across their working range of 500°C to 1000°C, and require 485 MJ of energy to warm up from 15°C to 500°C when initially placed.
Calculating steam production rate
Heat exchangers produce 103 steam/second.This can be calculated by relying on steam turbine data: A steam turbine consumes 60 steam/second and produces 5.82MW (assuming 500°C steam). This means a single unit of 500°C steam has 5.8MW / 60/s = 0.097 MJ
of energy. A heat exchanger produces 10 MJ a second, therefore it produces 10MJ / 0.097MJ = 103.0927835
steam per second.
The steam production rate can also be calculated using the energy consumption: 1 Heat exchanger consumes 10MW, so it's putting 10,000,000 joule of energy into heating water/steam per second. To heat up 1 unit of water 1 degree, 200 joules are needed, so the heat exchanger is heating up water by 50,000°C in total. But the water only gets heated up from 15°C to 500°C, so by 485°C. So the 50,000°C are enough to heat up 103 units of steam per second, since 50,000 / 485 = 103.09
.
History
 0.15.0:
 Introduced
 Doubled the heat capacity of water from 0.1kJ per degree per liter to 0.2kJ