As the Roots pump is a non-vacuum inside the compression, the compression ratio is usually very low, so high, medium and vacuum pumps need to pump before.

由于罗茨泵是一种无内压缩的真空泵，通常压缩比很低，故高、中真空泵需要前级泵。

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But there are some shortage of ordinary single-stage heat pump in low-temperature environment, such as high compression ratio, devoid heating power and low efficiency.

但普通单级空气源热泵热水器在低温环境下运行中存在压缩比过大、制热量不足和制热效率太低等问题。

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