HVAC tab in model data - Simple HVAC
Check the mechanical ventilation checkbox to indicate that outside air and/or re-circulated air is delivered to the zone. You can use this setting to define mechanical ventilation and air delivered through centrally ducted air conditioning systems or local fresh air systems.
The specification of outside air delivery rates is the same for both options as described in the Outside air section.
The way that mechanical ventilation is modelled in Simple HVAC depends on the Mechanical ventilation method model option:
With the Mechanical ventilation method model option the introduction of outside air through fans is achieved using the EnergyPlus ZoneVentilation:DesignFlowRate data separate from the main HVAC system. Energy consumption and heat pick up from fans is included as described below.
When using Simple HVAC, enter the type of fan. Select from:
Note that in Simple HVAC the Auxiliary energy data accounts for all electric fan and pump distribution energy plus controls and any other electrical energy use associated with HVAC that is not already accounted for elsewhere.
Enter the pressure rise at full flow and standard conditions. Standard conditions are considered 20°C at sea level, 101325 Pa.
You can calculate the approximate fan pressure rise from Specific Fan Power (SFP) data using:
Delta P = 1000 * SFP * Fan total efficiency
Annex E of ISO 5801 shows that by rearranging the formula it can be derived that the SFP is a function of fan pressure divided by the efficiency of the fan system. Therefore the SFP will increase or decrease with a respective increase or decrease in the system pressure.
The SFP is a function of the volume flow of the fan and the electrical power input and is quoted for a particular flow rate;
SFP = Pe/ V
Where:
V is volume flow (l/s)
Peis electrical power input (W) to the fan system or complete air movement installation
[Reference FMA, UK, 2006]
Typical values for various system types are shown in the table below.
|
System Type |
Specific Fan Power (W/l-s) |
|
Central mechanical ventilation including heating, cooling and heat recovery |
2.5 |
|
Central mechanical ventilation including heating and cooling |
2.0 |
|
All other systems |
1.8 |
|
Local ventilation units within the local area, such as window/wall/roof units, serving one room/area |
0.5 |
|
Local ventilation units remote from the local area, such as ceiling void or roof mounted units, serving one room/area |
1.2 |
|
Fan coil units (rating weighted average) |
0.8 |
Source ESTA: http://www.esta.org.uk/
Enter the product of the fan motor and impeller efficiency of the supply fan. This is the ratio of the power delivered to the air to the electrical input power at maximum flow expressed as a percentage. The motor efficiency is the power delivered to the shaft divided by the electrical power input to the motor. The fan efficiency is power delivered to the air divided by the shaft power. The power delivered to the fluid is the mass flow rate of the air multiplied by the pressure rise divided by the air density. Must be greater than 0 and less than or equal to 100.
Enter the percentage of the motor heat that is added to the air stream. A value of 0 means that the motor is completely outside the air stream. A value of 100 means that all of the motor heat will go into the air stream and act to cause a temperature rise. Must be between 0 and 100.
Enter the supply fan placement type. There are two choices:
The default is 1-Draw through.
The calculations for fan power and airflow temperature pick up are detailed in the EnergyPlus Engineering Document. A summary is shown below:
Total Fan Power = Mass flow rate. DeltaP / (Total fan efficiency . Air density)
Shaft Fan Power = Motor efficiency . Total Fan Power
Heat to air = Shaft Fan Power + (Total Fan Power - Shaft Fan Power) . Motor in air fraction
With the Ideal loads option, Mechanical ventilation is modelled with heating and cooling using the EnergyPlus ZoneHVAC:IdealLoadsAirSystem data. In this case there are options to include the effects of heat recovery, economiser, humidification and dehumidification.
Note: With the Ideal loads option, fan energy and heat pickup are not included in the simulation. In this case you can approximate fan energy using auxiliary energy data but if you need more detailed fan modelling you should use either the Room ventilation option or Detailed HVAC model options.
Economisers are used to provide cooling when the outdoor temperature is lower than the indoor temperature. An economiser is a damper opening that draws up to 100% outside air when the outside air is cooler than the temperature inside the building, thereby providing free cooling. An outdoor air economy cycle can reduce cooling energy requirements by some 20% to 30%, or around 5% of the air conditioning energy use and are often required by energy codes for larger air conditioning units.
Choose from 3 options:
The upper limit of outside airflow when the economiser is working in ac/h. The default is 15 ac/h. This value is not used when the HVAC Autosize model option is Adequate.
When heat recovery is active you can choose the type of heat recovery. Select from:
The sensible heat recovery effectiveness, where effectiveness is defined as the change in supply temperature divided by the difference in entering supply and relief air temperatures. The default is 0.70.
The latent heat recovery effectiveness, where effectiveness is defined as the change in supply humidity ratio divided by the difference in entering supply and relief air humidity ratios. The default is 0.65.