ENERGY
Comparison of steam and MTHW Systems. Steam
High heat content
Latent heat approximately 2,100 kJ/kg
Inexpensive
Some water treatment costs Good heat transfer coefficients High pressure required for high
temperatures
No circulating pumps required Small pipes
Boiler house gauges.
use of fossil fuels was pinpointed as a key part of the problem evidenced by the increase in carbon dioxide in the atmosphere. The atmospheric CO2 concentration from 800,000 years to 200 years ago varied between 170 to 300 parts per million (ppm). In the last 200 years that has risen to more than 400 ppm. This accelerated rise in CO2
has
been attributed to the burning of fossil fuels and a plan to use sustainable energy sources has been part of the UK government’s strategy on energy use over the past 50 years. In recent years, there has been a trend in healthcare estates to replace steam systems, which are used for both heating and ventilation (HVAC) with Medium Temperature and Low Temperature Hot Water systems known as MTHW and LTHW. Medium Temperature systems have supply temperatures in the range of 120˚C to 175˚C. Low Temperature systems have supply temperatures of 120˚C or lower. These water systems are heated by electricity which is seen as inherently ‘greener’ than fossil fuels. David Livingstone, DRLC managing
director and a former marine engineer trained to maintain steam powered ships, is very clear on this issue. He says: “Steam has many benefits. The advantages of the use of steam as a primary heat source over medium pressure hot water or low pressure hot water are many and various. Steam has a very high heat content capacity. It has a distinct advantage over hot water as it relies primarily on the latent heat of water rather than the sensible heat. The net heat effect for saturated steam at 6.9 bars (100 psig) (170°C) condensed and cooled to 82°C is approximately 2.42 MJ J/kg. “For medium temperature hot water
cooled from 175˚C to 120˚CC, the net heat effect is 0.24 MJ/kg or only about 10 per cent of the energy of the steam. This example indicates a MTHW system must circulate about 10 times more mass than a steam system of similar heat capacity.”
IFHE DIGEST 2024 Easy to control with two way valves
Hot water
Moderate heat content Specific heat 4.19 kJ/kg˚C
Inexpensive
Only occasional dosing Moderate coefficients
High pressure needed for high
temperatures. Limited capacity
Circulating pumps required Large pipes
More complex to control – three way valves or differential pressure valves required
Temperature breakdown is easy
through a reducing valve by reducing the pressure
Steam traps required Condensate to be returned No steam traps required
No condensate Greater volume of water to be returned
Flash steam available Boiler blowdown necessary Water treatment required to prevent
corrosion Reasonable pipework required
No fire risk System very flexible
David, as an advocate of using steam in healthcare premises, continues: “Generating Steam is inexpensive, with marginal costs for water treatment. In addition steam is a very efficient medium as it has excellent heat transfer coefficients. It takes a lot of energy to turn water into steam, energy that is ‘made available’ again when the steam condenses back to water. This makes steam a very effective carrier of heat. A lot of energy available in a small volume means smaller pipes. Smaller pipes equals a less expensive installation which in turn saves on greenhouse gas production.” If more energy is required, steam can be superheated and produced at high pressure for high energy drive – for example, turbines or axial flow turbine pumps. In addition, steam does not require any circulating pumps as steam flows in response to pressure drops along the steam main and thus expensive circulating pumps are not needed, as in
No flash steam
No blowdown necessary Less corrosion
Searching medium, welded or flanged joints usual
No fire risk System less flexible
the case of hot water. As steam condenses, its pressure drops and higher-pressure steam flows into the lower pressure region. No pumps are needed to make the steam flow, a considerable saving in installation and running costs. No pumps also mean no system balancing is required. David has a view on the commonly held position that steam is dangerous and expensive to keep safe: “Steam is inherently a safe medium and reliable. A leak won’t be poisonous. It is also a very forgiving medium. A steam system will seldom simply stop working but with a little ongoing maintenance, operating costs can be kept low. “Steam is easy to control. The control of the amount of steam required for a heating process is easy to control with a two-way valve. Due to steam’s unique property, there is a defined relationship between pressure and temperature. To control the temperature of the steam,
65 Temperature breakdown more difficult
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