Page 37 - Climate Control Magazine September 2018
P. 37

Chiller Feature
Currently, most HVAC systems have a fixed temperature set point for HHW, CHW and CW, or b.manually regulated floating temperature set points for the heating/cooling fluids which are not optimised.
HVAC equipment is designed to consider the order of highest heat load that can be expected, in terms of both external and internal heat loads.
HVAC equipment sized using these design conditions actually operates at part-load condi- tions for the majority of the time. This means the heat load of HVAC systems can be satisfied, for most of the time, with control parameters that are not as stringent as those required under the design for operation at full-load conditions.
Temperature reset is a control optimisation approach in which the temperatures of the cool- ing or heating media are altered in order to mini- mise the overall energy consumption of the HVAC system.
Temperature reset can allow for minimised energy consumption of HVAC equipment when operating in the part-load conditions that occur throughout most of the year.
The minimum required information for the control of temperature reset strategies includes:
• current space temperature set point
• operational characteristics and limits of the
HVAC equipment that is subject to tempera- ture reset temperature reset limits an indi- cation of actual heating or cooling load based on thermal energy metering, prevail- ing ambient conditions, the percentage opening of field modulating valves or flow/ return temperatures
• supply and return temperature of CHW or CW
MINIMUM REQUIRED EQUIPMENT
The minimum required equipment for tempera- ture reset includes:
• field temperature sensors
• controllers and data processors
• temperature reset software
• control interface between HVAC equipment
and HVAC controls.
OPPORTUNITIES FOR OPTIMISATION
OPTIMISATION OF
HEATING HOT WATER
Reducing HHW temperatures will reduce distri- bution losses (the heat lost or gained through the pipes and pipeline components) and slightly im- prove the thermal efficiency of conventional boilers while significantly improving the effi- ciency of non-condensing boilers.
If the return temperature of a conventional (non-condensing type) boiler is reduced below 55°C, any small gains in efficiency will be negat- ed due to boiler ‘back end’ corrosion, also known as fireside corrosion, which will cause prema- ture failure of the boiler. This must be where con- densing boilers are installed, they are more effi- cient when the return temperature is maintained below 55°C – which is the typical dew point of flue gases for natural gas-fired boilers.
As such, it is important to maintain the boilers at as low a HHW flow temperature as possible, except for the relatively short periods of the year when full heating system capacity is required;
typically during extremely cold weather and/ or during early morning warm-up periods. Dur-
ing periods of high heating demand, the HHW flow temperature should be reset to 80–85°C when condensing boilers have reached their maximum output.
The HHW flow temperature should be reset to 80–85°C and the condensing boiler used as the lead boiler before additional (non-condensing type) boilers are brought online.
OPTIMISATION OF
CHILLED WATER
For every 1°C increase in chilled water tempera- ture, chiller compressors consume 2–3 per cent less energy for fixed-speed compressors and 4–5 per cent for variable speed compressors.
Typically, CHW is supplied in most systems at temperatures between 6–7°C for most sys- tems under design conditions; however, this temperature can be reset upwards to around 10–12°C during milder weather, providing there are no adverse effects such as the loss of de-hu- midification (humidity control). Increased CHW temperature will reduce chiller energy consumption; however, it could require addi- tional pumping energy and/or increased S/A flow. These increases should be considered when calculating the overall benefits from this optimisation strategy.
For CHW systems that have long CHW cir- cuits, the pumping power is significant. In these circumstances, variable CHW flow, on secondary circuits, may be a more appropriate strategy for energy saving than CHW temper- ature reset and should be assessed on a case- by-case basis.
Generally, for short CHW circuits, CHW tem- perature reset is more energy efficient as sav- ings in pumping power are smaller, relative to potential gains in chiller efficiency through CHW reset.
Variable speed chillers typically have a better response to increased CHW temperature than fixed-speed chillers. Similar behaviour occurs when CW temperature is reduced.
Figure 10 shows that, during mild weather conditions, warmer than standard CHW tem- peratures can satisfy the cooling load. Raising the CHW temperature will reduce the electricity consumption of chillers.
Figure 10: Benefits from higher chilled water temperature (chilled water temperature reset)
CLIMATE CONTROL NEWS SEPTEMBER 2018
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