A person who economises; someone who spends carefully and avoids waste. Also, a device that reduces waste or improves efficiency.
From 'economise' plus the agent suffix '-er' (one who does something). British English spelling; American English is 'economizer.' Can refer to either a person or a mechanical device.
In engineering, an economiser is a specific device that captures waste heat from boilers and chimneys—it's literally money in the bank, which is why the word applies perfectly to both thrifty people and efficient machines.
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