Heat Exchangers / Water/steam

Why You Should Consider A Thermocompressor

This unit offers potential opportunities for improving energy efficiency

By Alan Rossiter, Energy Saver Columnist

I first encountered a thermocompressor as a young process engineer working at a multiple-effect salt evaporation plant. The device sat at the back of the plant, blending medium-pressure steam (MPS) and low-pressure steam (LPS), to provide intermediate-pressure steam (IPS) for the evaporation train. In the years since then, I’ve found many other uses for thermocompressors. They are used commonly to satisfy a demand for steam at a pressure that’s slightly higher than one of the header pressures in the site’s central steam system — generally the low-pressure (LP) header. In most cases, the end user of the steam is a reboiler for a distillation column.

The thermocompressor allowed us to maximize power generation, and thus minimize net steam cost.

So, what is a thermocompressor, and how does it help us improve energy efficiency?

Bernoulli’s Theorem is essentially an energy balance for flowing fluids. In its simplest form, it can be written as: p+ρv2/2 = constant, where p is the pressure of the fluid, ρ is its density, and v is its velocity. In this form, the equation assumes the fluid is incompressible, which actually applies only to limited cases. However, it does illustrate the key principle of the thermocompressor: assuming no energy enters or leaves a fluid, its pressure falls when its velocity rises, and vice versa.

A thermocompressor consists of a metal casing with three main parts: a motive steam nozzle, a mixing chamber and a diffuser. With no moving parts, it’s generally a low-maintenance item.

In the evaporator example, the steam that drives the compression (MPS) is called “motive steam;” the steam that’s compressed (LPS) is called “suction steam,” and the combined effluent (IPS) is the “discharge steam.” The motive steam enters the thermocompressor through the motive steam nozzle, where it expands and accelerates into the mixing chamber. Due to its increased velocity, we know from Bernoulli’s Theorem that the motive steam’s pressure drops. It reaches a pressure below that of the suction steam, which is drawn into the mixing chamber. The two steam streams intermingle, and the combined flow enters the diffuser, where the velocity falls. The pressure therefore rises, and reaches its discharge value, which lies between the motive and suction pressures.

The same principles also apply to steam jet vacuum systems. These also use steam as the motive medium; however, in this case the suction load is usually either air or a process vapor, at a pressure below ambient.

The usefulness of thermocompressors is tied to the interface between central steam systems and process plants. For example, the utility plant for our salt evaporators also served several other plants, and it provided high-pressure steam (HPS), as well as MPS and LPS. The HPS came directly from the boilers — but the MPS and LPS were obtained by passing HPS through steam turbogenerators. These produced electricity, which reduced the amount of expensive power we had to import from the grid. Our salt evaporation system required a pressure between the MPS and LPS levels. We could have supplied this by passing MPS steam through a pressure-reducing valve, but this would have lost the very significant benefit of power generation between the MPS and LPS pressure levels. This thermocompressor allowed us to maximize power generation, and thus minimize net steam cost.

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The steam system at our site was well-balanced, and we didn’t often vent LPS. However, there are many sites where LPS venting is a constant problem. In such cases, the LPS used in a thermocompressor reduces the LPS vent. This is usually more valuable than increasing power generation.

Thermocompressors also can be used to compress low-pressure flash vapor. The vapor can be obtained by flashing steam condensate or by flashing an aqueous process stream (1). On its own, the flash vapor isn’t at a high enough pressure to be useful. However, the thermocompressor, can boost it to a pressure for use in process applications such as stripping or reboiling for distillation columns. In this way, “waste heat” can become “useful heat.”

However, the practical range of operability for thermocompressors is limited. The percentage of suction steam in the mix decreases as the discharge pressure increases. As a rule of thumb, a thermocompressor can only be justified if the motive:suction flow ratio less than 2:1. In a typical MPS/LPS thermocompressor, this would deliver a discharge pressure about 2 bar higher than the LPS (suction) pressure.


Alan Rossiter Formal crop copy7Alan Rossiter has more than 35 years of experience in process engineering and management, including eight years in plant technical support, design and research with Imperial Chemical Industries (ICI, United Kingdom) and nine years in energy efficiency and waste minimization consulting with Linnhoff March, before starting his own business. In 2019 he joined the University of Houston as Executive Director, External Relations for UH Energy. He is a chartered engineer (U.K.) and a registered professional engineer in the state of Texas. His latest book, Energy Management and Efficiency for the Process Industries, coauthored with Beth Jones, was published by John Wiley & Sons in 2015. He is a Fellow of the American Institute of Chemical Engineers and a Past Chair of the South Texas Section of the AIChE.

REFERENCE:
1. Alan P. Rossiter, Glenn T. Cunningham & Bruce Bremer, “Save Energy Without Investing Capital,” AIChE Spring Meeting and 12th Global Congress on Process Safety, Houston, TX, April 12, 2016, Paper No. 72a.

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