Steam to hot water applications using a vertical flooded steam heat exchanger design
Benefits of an instantaneous steam to hot water heaters using a vertical flooded heat exchanger
Two key factors have led to the growth of instantaneous steam hot water heater popularity. First, they are much smaller than conventional storage tank heaters. Secondly, testing and research has shown that the smaller reservoir and higher temperatures do not create a breeding ground for Legionella bacteria.
Size is the most frequent consideration. Floor space is at a premium in modern mechanical rooms. The difference in footprint between a 16 square foot instantaneous skid and a 2000 gallon storage tank can be worth thousands of dollars.
Retrofit steam to hot water applications are perfect for instantaneous units. Large, bulky storage tanks that were installed over the last 40 to 50 years are at the end of their useful life. Many times the facility is built up around these failing tanks. The only way to get a new tank in would entail major demolition.
Instantaneous packages can be wheeled through a doorway. They can be piped up with the existing tank in
place, causing minimal downtime. Sometimes the existing tank is encapsulated and left in place, or it can be drained and cut into pieces for removal.
The Maxi-Therm system is a new, patented form of instantaneous steam to water heater. Conventional piping components are utilized in every critical valve or pump. A facility can even specify pumps and valves for which they already stock parts.
Energy conservation is unparalleled. There is no unit on the market more energy efficient than a Maxi-Therm for steam to water heating. It squeezes every practical BTU out of the steam and condensate.
Normally when a system is energy efficient, it’s not the lowest cost to install. The Maxi-Therm is different. It
does not require a pressure reducing valve station, or a dedicated condensate pump. This can save up to 40% of the installed cost on a new job.
To explain how the dramatic energy conservation and cost savings are possible, let’s start at the beginning, with water…
Steam water heater fundamentals
When make-up water enters the boiler room or power plant, its first stop after treatment is the deaerator. Here it is sprayed overheated trays to drive out dissolved gases. The water is usually heated from 40F to 205F by adding sensible heat. It takes about 1 BTU to raise a pound (or pint) of water by 1F. These BTU’s are supplied by steam in the trays.
Smaller package boilers might not utilize a deaerator, in which case, the boiler provides this same sensible heat demand.
Here is one place where the Maxi-Therm system makes a difference. Why is make-up water needed in the first place? Some losses are due to boiler blow-down and some to make up for leaks. The other loss is from “flash steam”. That’s the steam puffing out of vent pipes all around the condensate return system.
The steam boiler takes water from the deaerator, heats it to the boiling point by adding more sensible heat, and then adds latent heat to vaporize the water into steam. How much sensible and latent heat are needed? That depends on the steam pressure desired.
A sophisticated system for a university, food processor, or chemical plant will generate steam at 600 psig or higher, because steam is used to generate power before distribution. The steam is normally distributed at 100 or 150 psig. Some industrial plants (usually with turbine drives) will distribute steam up to 600 psig.
Systems are designed around higher pressure distribution because of the steam volume.
High pressure distribution is used to minimize pipe sizes, and reduce the drop in pressure experienced by flowing steam. Intermediate pressure steam is normally 60 or 80 psig. It is used for sterilizers, autoclaves, wash mixers, and pumping stations. This steam is usually generated from distribution pressure by a pressure reducing station.
Low pressure steam is normally 10 to 15 psig. It is used for heating air and water. The steam water heater system are either “service” or “domestic”. Domestic water is for the hot side of sinks and showers. Service water is first heated by the steam, then it circulates through coils and baseboard to heat air. It’s usually mixed with glycol to avoid freezing.
Between service and domestic, most facilities use at least 50% of their steam for heating water. Condensate system pressure is normally 0 psig. Industrial process dryers sometimes use intermediate 30 to 60 psig return systems, which then “cascade” down into a 0 psig system.
Principle of using a vertical flooded steam heat exchanger
The Maxi-Therm steam water heater system uses a control valve on the condensate side for temperature control on a vertical heat exchanger. This difference allows two advantages. First, high pressure steam can be used. A pressure reducing station is not needed, and steam piping is smaller and lighter. Second, the heater utilizes the latent and sensible heat of the steam. It subcools the condensate to 200F or below so flash steam losses are eliminated.
When flash steam is formed with a conventionnal system, it flows up through the vent on the condensate return station. It’s lost in the atmosphere through the vent pipe. In the deaerator discussion above, remember that makeup must be added, treated, and heated when flash steam is lost.
The condensate pump station behaves better with the cooler condensate. Hot condensate cavitates as the pump pulls it into the volute. The pump will sound like it’s full of gravel, and the internals will wear out quickly.
In addition, the unbalanced impeller wobbles and causes the shaft seals to leak.
The Maxi-Therm eliminates the need for a dedicated condensate pump. The 200F condensate has high pressure behind it, so it does not need a pump. Because there is no flash lost, we call our vertical flooded heat exchanger a condensing heat exchanger for steam applications.
This piping is sized like a water line, so smaller and lighter pipe can again be used.
In order to deliver these benefits, the control valve on the condensate side is a vital component. Every precaution is taken to be sure it does not leak. The system is normally furnished with a temperature controlled steam regulator to prevent high water temperatures in the rare event that the control valve leaks. A steam trap is also provided to prevent a leaking control valve from discharging live steam.
In summary, the Maxi-Therm condensing heat exchanger for steam applications has many advantages:
- Lower installed cost than other fluid heaters
- Utilize steam that is wasted by other heaters
(Save 20% of energy usage at 125 psig steam, save 5.4% of energy usage at 15 psig steam.)
- Smaller footprint
- No dedicated condensate pump required
- No dedicated PRV station required
- No vacuum breaker required during normal operation
- Control liquid leaving temperature at ± 2F on building heat, at ± 4F on domestic
- High quality heat exchanger exchanger with ASME Stamp for high pressure steam operation of 150 psig and more
You can use the Maxi-Therm for:
- Domestic Hot Water
- Heating Water / Glycol for Building Heat
- Hot Oil or other Heat Transfer Fluids
- Wash Stations
- Emergency Showers
- Reactors, Pasteurizers, Jacketing