Volume 21, number 2, june 2007

Produce hot water with steam and still be efficient? Why not!

High-efficiency water heaters and boilers have generated many decentralization projects to save energy. These replacements are not always appropriate to heating needs and, in these cases, the expected savings do not materialize. The vertical water heater is a very interesting alternative.

The arrival on the market of high-efficiency (condensation) water heaters and boilers has led to several decentralization projects. The purpose was to save energy. Steam was systematically replaced by a hot water system. Unfortunately, these projects were not always adapted to the customers’ needs and the expected savings did not materialize. The vertical water heater is a very interesting alternative to decentralization.

But before presenting the economic advantages, let’s see how it works.

The two types of exchangers

Figure 1 illustrates a conventional arrangement for hot water production with a horizontal steam/water exchanger. At the boiler outlet, the steam pressure and temperature are often reduced by a combination of valves and vents. The steam then passes through the exchanger and transmits its latent heat to the liquid. At the exchanger outlet, another combination of hardware assures the return of the condensate to the boiler. Energy will be lost into the atmosphere along the way in flash steam through the system’s different vents.

Figure 1. Conventional method

The temperature of the fluid to be heated is controlled by the pressure of the steam admitted into the exchanger. In fact, the higher the steam pressure, the higher the steam temperature will be. Thus, when the fluid temperature sensor indicates a temperature above the setpoint, the steam pressure will be reduced (to lower the steam temperature). With this type of arrangement, it is difficult to maintain a constant fluid temperature.

A vertical water heater in fact is a steam/liquid exchanger, which can be flooded. Figure 2 illustrates such an arrangement. You will notice a decrease in the usual control hardware. Indeed, it is no longer necessary to lower the stream pressure at the exchanger inlet because it is designed to operate at higher pressure. The exchanger recovers the latent heat and a portion of the sensible heat of the steam. Thus, condensate can accumulate and cool in the exchanger before returning to the boiler. Finally, such a system no longer requires a condensate pumping station.

Figure 2. Arrangement with a vertical exchanger

Selection

A heat exchanger is selected by means of two equations:

With a vertical exchange, condensate accumulation and cooling allow the exchange area to vary according to the heat demand. The quantity of energy coming from steam is thus the sum of the latent heat and a portion of its sensible heat, as indicated above. This generally translates into a smaller vertical exchanger than a conventional exchanger.

Source of savings

The savings come from several sources. For the same quantity of energy required for the application, the boiler or boilers will produce less steam overall because the flash steam losses will be reduced. From then on, a vertical water heater arrangement involves a smaller steam side pipe and optimization of the number of vents and their dimensions. The make-up water flow will be lower because a greater portion of the steam flow entering the exchanger will be returned to the boiler in condensate. This water generally is preheated by the steam before it is admitted to the boiler.

Installation costs are also reduced because the pipes are smaller than with a standard setup. Indeed, the vertical water heater continues to be very efficient, even at high pressure (100 psig and over). Finally, refinement of the hardware reduces maintenance costs because there are less parts to maintain.

An example

A major hospital needed six exchangers for its heating, hot water and clean steam loads. The initial design included the following features:

  • Two parallel pressure reduction stations of 3 in. and 1 1/2 in.;
  • A 4 in. (welded) supply pipe and a vent on an 8 in. safety valve;
  • Two pumping stations with a 2 in. vent;
  • One sump tank with a 1 in. vent;
  • Two condensate return networks.

The new arrangement with vertical exchanges allowed the following modifications:

  • No condensate pumping station;
  • No sump tank for the clean steam generator;
  • A 2 1/2 in. (threaded) supply pipe;
  • 1/2 in. control valves on all steam equipment;
  • No pressure reduction station;
  • Only one condensate return line.

These changes allowed the customer to achieve savings on three levels;

  • $300,000 in construction costs;
  • $20,000 in maintenance costs;
  • $40,000 in energy savings.

This example clearly shows that it is possible to improve a steam system’s energy performance at low cost. To accomplish this, several technologies must be analyzed, including a vertical water heater in flooded mode, of course.

Marie-Joëlle Lainé, Eng.
Technical Advisor
Datech Group

This article was written with the collaborationof Patrick Lach of Maxi-Therm.