A heat pump uses energy to transfer heat from a lower temperature (source) to a higher temperature demand (sink) using additional energy. For industrial heat pumps, the heat source is process waste heat.
Mechanical vapour recompression (MVR) is an open heat pump system in which the pressure and temperature of the vapour, together with the corresponding saturation temperature, are increased by means of compression (Klop, 2015). Steam recompression is a specific variation of MVR. Low-pressure steam exhaust from industrial operations, such as evaporators or cookers, is usually vented to the atmosphere or condensed in a cooling tower, while other plant operations on the same site may require intermediate-pressure steam. Instead of expanding high pressure steam across a throttling valve to meet these needs, low-pressure waste steam can be mechanically compressed to a higher pressure so that it can be reused (US-DE, 2012). Steam recompression relies upon a mechanical compressor to increase the temperature of the latent heat in steam to render it usable for process duties (US-DE, 2012). The advantage lies in the fact that the required compression energy is very small compared to the amount of latent heat present in the recycled steam (Klop, 2015).
The energetic performance of steam recompression is expressed in the coefficient of performance (COP). The COP gives the ratio of the net recovered heat and the energy used by the compressor. In this case, the net heat is the steam production including the additional steam yield by water injection. Typical economical and energy-efficient applications have a minimum COP of 3.5. Some applications of MVR have a COP of 10 or even higher (Klop, 2015). The key elements for a high COP are the following (Klop, 2015):
• A low ratio of the absolute steam pressures. A guideline for the maximum ratio is 6, but in daily practice the ratio is about 3.
• A minimum capacity. A guideline is a minimum of one tonne of steam per hour.
• Water injection after compression.
Technology suppliers (not exhaustive list): AtlasCopco, MAN‐turbo, Howden, Siemens, Piller, Turbo Claw, SpiraxSarco, Spilling, Heliex.