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