Note: Many optimization routines rely on end-use polling of demand or valve/damper positions such as "most open valve" routines. This can be done cost effectively without actual measurement of position - by polling the individual "percent commanded output". This is referred to as "implied position" and is acceptable n most cases in lieu of actual position.

Note also that routines that use polling have the potential to be inefficiently operated if one errant measurement exists. For polling space temperatures, for example, limiting the user adjustment is strongly recommended in conjunction with demand polling of space controls. Additionally, it may make sense to 'discard' the high and low values from such polling to prevent errant operation

VAVBox Optimal Minimum Setting

The usual method of calculating VAV box minimums is to assume full load conditions and the outside air percent of the total supply air at those conditions. With constant outside air control, the mixture is richer in outside air at supply air flows that are below maximum. Therefore, the VAV minimum stops can be reset automatically based on the actual OA mix as loads change. Example

A 20% VAV minimum stop is required based on 25% OA at design conditions. This same VAV box could have a minimum position of 10% when the supply fan is at 50%, since there is now twice as much OA in that air stream The effect of this is to reduce over-cooling and reheating when the VAV box is at minimum while the zone thermostat continues to call for less air.

Optimal Supply Air Static Pressure Setting - Variable Air Volume (VAV) Systems

This requires polling of individual VAV boxes for air valve position and reduces system pressure until at least one box is 90% open, thereby providing the optimal system duct pressure (just enough pressure). This reduces fan horsepower.

Optimal Supply Water Pressure Setting - Variable Pumping Systems

This requires polling individual air handier for control valve position and reduces system pressure until at least one control valve is 90% open, thereby providing the optimal system water pressure (just enough pressure). This reduces pump horsepower.

Optimal Evaporative Cooling Setting

This uses outdoor air wet bulb temperature, which can be measured directly, but is usually calculated from temperature and humidity. The evaporative process can get close to, but never reach or exceed, the wet bulb temperature. By knowing the wet bulb temperature, the control system will know its boundaries and won't try to achieve something it cannot For water-cooled refrigeration equipment, the low limit for the reset is normally around 55 or 60 degrees F and the chiller mfg needs to be consulted to confirm. Resetting the cooling water temperature down in this way, in lieu of a constant temperature setting, will reduce kW/ton energy use and demand. NOTE: achieving colder condenser water is a trade off between improved chiller kW/ton and increased cooling tower fan kW/ton, so evaluation for diminishing returns is required.

Figure 22.18 Advanced HVAC Control Applications

Figure 22.18 Advanced HVAC Control Applications


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