| Primary can be hot exhaust, air, liquid, solid surface. |
 |
| Secondary can be a gas, water, thermal oil, glycol, R245fa and various others. |
|
| Working fluid chosen according to temperatures involved - water, ammonia, acetone, Dowtherm, etc. |
|
| Pipe materials chosen according performance requirement of fluid types and temperatures: carbon steel, stainless steel, aluminium, copper, etc. |
|
|
