PART 1 OF 2 HVAC&R MODULE 85 HVAC&R Skills OPTIMISATION Workshop – VENTILATION AND AIR FLOW

Over the next few months, we will be giving readers an insight into an ever-evolving topic of HVAC&R Optimisation. First up, we will discuss energy consumption reduction approaches involving air flows and ventilation systems – more specifically, economy cycle strategy.

months, O/A can be used to provide cooling ECONOMY CYCLE for spaces, with or without the assistance of AC Economy cycle is a strategy that uses outside air , thereby reducing the system energy DID YOU KNOW? directly for space cooling, with the aim to reduce consumption. Economy cycle allows for a the energy consumption by reducing operation of When outdoor conditions are not suitable, typically 15–20 per cent reduction of energy AC equipment. This strategy provides the added due to high temperature and/or , the consumption of AC compressors benefit of improved IAQ by significantly increasing economy cycle must be disabled and the AHU must and improved IAQ. outside air ventilation rates during periods of mild revert to minimum O/A. The economy cycle must weather. also be disabled during Fire Mode Operation, which is an important consideration when retrofitting. to inadequate maintenance. When they fail, PRINCIPLES The economy cycle is one of the energy saving the HVAC system consumes more energy than AND EQUIPMENT: control strategies normally included in HVAC necessary. Common reasons for failures of existing PULLOUT DDC system controllers or BMSs. economy cycles include: For most parts of Australia, the ambient outside air often has a lower (total energy) than the Often, existing economy cycles have been • Malfunctioning sensors air within conditioned spaces in buildings (except set up incorrectly and/or they have failed due (especially humidity sensors) for the hot and humid northern regions). In such conditions, air returned from the occupied space can be exhausted (rather than being returned) 100% and the lower enthalpy O/A can be used as open Spill air Return air S/A, reducing the operation of AC systems and reducing energy consumption, see Figure 15. When an AHU operates in cooling mode, it M typically cools S/A to 12–13°C (for CHW systems) Return air or 8–9°C (for DX systems). During mild weather open M damper conditions, cooling requirements can be satisfied closed with a higher temperature of S/A, sometimes even as high as 20°C. It should be noted that in such situations, the ability of AC systems to dehumidify is significantly diminished and this optimisation strategy will not be viable where humidity control 100% Supply air is paramount. This is not the case for typical office open M Supply air type buildings where the space relative humidity activated can typically drift between 40–60 per cent (or Damper even to 35–65 per cent for brief periods) without open Energy Saving Tip significant discomfort. Don’t use relative humidity of outside air Taking into account the typical S/A temperatures alone as a justification noted above, as well as typical climate conditions Essential criteria for activation of economy cycle: for disabling economy for the southern part of Australia during the cooler • Call for cooling activated cycle – also consider • Outside air has less total energy (enthalpy) than return air absolute humidity • Outside and internal air temperature and relative of outside air PROUDLY SPONSORED BY humidity are both within operational limits (dew point criteria)

Figure 15: Activated economy cycle.

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• The economy cycle is only used when O/A temperature is close to Often, existing economy cycles S/A temperature (12- 13°C). Such a strategy severely restricts the use of economy cycle (reduces its operating hours) as it only operates when have been set up incorrectly and/or O/A temperature is close to the off-coil S/A temperature. • O/A temperature is lower than R/A temperature. This approach overlooks they have failed due to inadequate the fact that sometimes even warmer O/A, rather than R/A (but much drier, lower humidity) can be utilised for the economy cycle. maintenance. When they fail, the A contributing factor to the under-utilisation of the economy cycle is HVAC system consumes more energy inappropriate space temperature set points and ranges. Optimised control strategies and control parameters should take into account both the than necessary. minimisation of the use of AC compressors and the comfort of occupants. OPPORTUNITY • Failed dampers or damper actuators resulting in discomfort FOR OPTIMISATION • Building pressurisation issues Typical control strategies (as described above) in place for economy cycles are not optimised. As such, they either operate within overly-restrictive conditions • Deactivated due to high energy consumption. or have excessive operation, both of which leads to energy wastage. Significant energy savings can be realised by optimising existing Economy cycles are typically beneficial in the following ambient conditions: settings, or retrofitting HVAC systems to incorporate economy cycles. • Temperature: 10–20°C Figure 15 shows the use of the O/A economy cycle as a first stage of cooling, • Enthalpy: < 52 kJ/kg when O/A conditions are favourable. • Dew point: < 12°C. Minimum required information In order to maximise the operating hours of the economy cycle, The minimum information required for an economy cycle program includes: it should be enabled when the O/A temperature: • Return Air (R/A) temperature and enthalpy (based on temperature • is lower than the zone cooling set point temperature and humidity) • is below the R/A temperature • A temperature and enthalpy (based on temperature and humidity) • Control strategy eg lock-out conditions • has a dew point below 12°C or an enthalpy which is at least 10 kJ/kg below the R/A enthalpy. • Control parameters eg O/A conditions This is the most efficient control strategy. • Conditions for activation • Economy cycle enabling time. The source of O/A should be located away from polluted air (loading docks, traffic, etc) and preferably on the southern or eastern side of the building Minimum required equipment to maximise economy cycle operation potential. The minimum equipment required for the economy cycle program includes: • Field sensors (outdoor and R/A temperature and humidity) ENERGY SAVING POTENTIAL, • DDC controllers COSTS, BENEFITS AND RISKS • Economy cycle software The economy cycle is typically a very cost-effective HVAC energy efficiency • O/A and relief air dampers (of adequate size) and motorised improvement; however, it does require some investment in sensors, motorised dampers, filters and possibly ductwork modifications. The economy cycle modulating actuators (see Figure 15) immediately reduces the energy consumption of the HVAC system by reducing • O/A inlet, exhaust air outlet and ducts sized to accommodate its load, as well as improving IAQ. This strategy can save up to 20 per cent 100 per cent O/A. of energy consumed by AC compressors. A mixed air temperature sensor is very useful for this strategy. Modern BMSs If a building has smoke hazard management, most of the infrastructure can now also use weather data feeds from the internet for more accurate is probably already in place and only new controls would be needed. and reliable data. CURRENT PRACTICE TIPS Many HVAC systems that are controlled by a DDC or BMS either do not have In cooling mode, a higher space temperature an economy cycle installed/ enabled or it is underutilised due to an inefficient 1 control strategy and/or inappropriate control parameters/limits. set point increases the use of economy cycle. Typical control strategies that lead to reduced opportunities for saving energy Economy cycle often runs together with regular include: 2 mechanical cooling, reducing the heat load • The economy cycle is used when O/A temperature is lower than the on the mechanical HVAC equipment. air temperature set point for the economy cycle (16-19°C), which is cooler air than the space temperatures. As such, the AC load is reduced. Don’t use outdoor Relative Humidity (RH) The disadvantage of this strategy is that it restricts the operation of 3 as a control criterion for an economy cycle – the economy cycle at higher ambient temperatures. This represents a diminished opportunity for energy conservation, especially in systems it’s irrelevant. Instead, use enthalpy, absolute where the enthalpy of the O/A is monitored and the O/A is only used humidity or dew point instead. when it has lower enthalpy than R/A.

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Figure 16: Air plenum of AHU showing motorised O/A and R/A volume control dampers.

The economy cycle, or the use of O/A for AC, can also form part of a mixed- mode HVAC design (eg automated opening/closing of windows, dampers, DID YOU KNOW? or any other O/A provision). When implementing economy cycle or other For new installations, the use of economy cycles is mandated in Section J HVAC optimisation strategies to existing systems, of the NCC. it is important to ensure that the original design intent The economy cycle is not typically suitable for applications where a narrow for Fire Mode Operation is not altered in any manner control band for both space temperature and RH is required (eg museums, without consultation with the relevant authorities. art galleries, laboratories, clean rooms, etc). GETTING STARTED The total benefits arising from the use of economy cycle can include: The economy cycle is one of the energy saving control strategies • Reduced energy cost normally included in HVAC DDC system controllers or BMS packages. • Reduced CO₂ emissions The control strategy has often been set up incorrectly and/or has failed due to inadequate maintenance. To implement a working strategy • Improved IAQ (at times of increased O/A flows) for an existing economy cycle the points to check include: • Reduced HVAC maintenance costs (less run time) • Malfunctioning sensors (especially humidity sensors) PULLOUT • Facilitation of other energy saving initiatives • Failed dampers or damper actuators resulting in incorrect airflows (early morning warm-up/cool-down, DCV, night purge). • Building pressurisation issues The potential risks arising from the use of economy cycle that should be • Failure of the cycle to generate energy savings due to high fan energy considered include: consumption. n • Increasing energy consumption by bringing in O/A that is too hot or too cold, due to faulty sensors or control settings. This risk is mitigated by implementing regular energy efficiency maintenance inspections and using high-quality sensors • IAQ impacts due to O/A pollution events MORE INFORMATION • HVAC hygiene impacts that may come with increased O/A (can be mitigated by filtration; however, filters can increase fan energy use) This month’s • Issues with O/A /moisture content if enthalpy sensors are not used. Skills Workshop was adapted from APPLICATION NOTES the NSW Office Economy cycle can be applied to centralised HVAC systems with CHW, of Environment DX AHUs and ducted packaged DX systems with motorised O/A dampers connected to a central (BMS) or local HVAC control system. AHUs facilitate and Heritage’s HVAC this energy-conservation optimisation strategy as they are the source of O/A ventilation for buildings. When economy mode is enabled, the damper Optimisation Guide. actuation should operate in sequence with the HHW and CHW valve modulation. It is important to use dampers with low leakage rates and to ensure that modulating dampers close when the AC or ventilation system is off to eliminate ; this is also a requirement of Section J For more information, visit www.environment.nsw.gov.au of the National Construction Code (NCC).

Next month: HVAC&R Optimisation – Ventilation and air flow. Part 2

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