The benefits of a larger ∆T
Operating a heating system with a greater difference between flow and return temperatures than the traditional 20 K can bring numerous operating and capital cost benefits — as Trevor Struck of Hamworthy Heating explains.
At traditional operating temperatures of 80/60°C flow/return a condensing boiler will only achieve marginally higher efficiencies than a high-efficiency non-condensing boiler. If the operating temperatures are reduced, typically to 50/30°C, or the temperature differential can be widened to offer a lower return temperature (80/50°C) then there are significant efficiency gains to be had.
A lower return temperature provides greater opportunities for condensing boilers to actually condense. Plus, being able to maintain the higher flow temperature is useful for hot water heating.
For example, with a flow temperature of 80°C and return temperature of 50°C using natural gas, the boilers have the opportunity to condense whilst being able to provide the required higher-temperature water for driving a calorifier or plate heat exchanger.
Using a boiler with maximum 20 K differential would require the flow temperature to be reduced to 70°C to allow 50°C return (enabling condensing operation). The heat-up times for hot water would be longer, or the heat transfer area would have to be greater to deliver the same performance.
The pursuit of maximum efficiency from condensing-boiler systems is one driver behind the demand for wider temperature differentials.
The larger flow/return temperature differential was traditionally the reserve of large steel boilers with high water content but no condensing capability. Now specifiers and consultants want this capability as well as all the benefits associated with smaller condensing boilers — fully modulating with better turndown, higher operating efficiencies, low NOx emissions, rapid response, and compliance with Government legislation.
Designing a system that operates with lower flow rates made possible by a greater difference between flow and return temperatures can provide a number of benefits to both the installer and end user/owner.
Lower flow rates lend themselves to smaller pipe diameters. Smaller pipes are cheaper to purchase, easier to install and will waste less energy as they have lower heat-emission rates due to a smaller surface area.
The water content of the heating system will also be lower, having a direct saving on chemical dosing and inhibitors. With the dosing rate being typically 1% it will cost less to dose the system and be quicker to administer at both the installation stage and with ongoing maintenance and re-dosing.
Some modern condensing boilers can have quite significant pressure losses when operating with narrow differential temperatures. By operating with a wider differential temperature and lower flow rates the pressure loss can be reduced, which will have a substantial effect on pump size.
Consider, for example, a 250 kW condensing boiler operating with a ∆T of 11 K, a flow rate of 5.4 l/s and a pressure loss of 1300 mbar.
By comparison the same boiler operating at 30 K ∆T with a flow rate of 2 l/s will only have a pressure loss of only 180 mbar. This is a reduction of more than 1100 mbar and will present an opportunity for savings in circulating-pump sizing. With smaller pumps being much cheaper to purchase, this could be a saving of over £2000 (based on list price of circulating pumps for this example).
The purchase cost of pumps has become of particular importance since the introduction of the Energy Related Products Directive (ErP). From January 2013 the impact of ErP has resulted in circulating pumps becoming variable speed, meaning that they will only run as fast as they are required to, rather than the fixed-speed pumps that work at maximum performance all of the time. Pumps that are compliant with ErP are more energy efficient, but also more complex and, therefore, more expensive.
Not only are larger pumps more expensive, but they may also require a 3-phase electrical supply — which in turn is more complex and time consuming to install, along with higher associated running costs.
For companies providing the total-energy solution, often the case in district-heating schemes, allowing condensing boilers to condense more frequently through the use of wider differential temperatures helps maintain a high level of efficiency for energy production.
With the continued drive for carbon reduction and related incentives for adopting renewable energy, more projects are now specifying dual heating sources — with condensing boilers being used to support the renewable-energy source.
We are increasingly seeing a requirement for a 30 K temperature differential in new-build projects, particularly for district-heating schemes where boilers are supporting combined heat and power (CHP) plant for when the base load is exceeded.
CIBSE guidance ‘AM12 – Combined heat and power for buildings’ makes specific reference to designing district-heating schemes with a minimum of 30 K differential temperature. This is to keep flow rates and pipe sizes small, and to maintain a low return temperature, even at low-load conditions, for more efficient plant operation.
Using modern condensing boilers capable of operating with design differential temperatures that complement the requirements of district-heating systems, helps simplify pipe and control systems, as well as providing rapid response to frequent and often sizeable changes in heat demand.
When designing a system for operating at 30 K temperature differential, careful consideration must be given to the heat emitters. If the flow out of the boiler is 70°C and return is 40°C then the average system temperature is likely to be 55°C, lower than if the system was operating at say 80/60°C. This means that the heat emitter needs to be larger to deliver sufficient output for the area to reach design temperature within the stipulated heat-up time.
Hamworthy’s Wessex ModuMax Mk2 condensing boilers, for example, can now operate at 30 K temperature differential.
Trevor Struck is product manager at Hamworthy Heating
