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Re: [EnergyPlus_Support] HVAC modelling - Key Temperatures
First, I will offer a general overview of how SIZING inputs relate to the
simulation.
The temperatures in the ZONE SIZING, SYSTEM SIZING, and PLANT SIZING objects
are used only to compute flow rates and capacities to replace "autosize" in the
various component fields. Every replacement of "autosize" will be reported in
the eio output file in the Component Sizing section. In the simplest terms,
ZONE SIZING determines the design airflow rate to each zone. SYSTEM SIZING
determines coil capacities (using the sum of the zone airflow rates served by
that system). PLANT SIZING determines coil design water flow rates and supply
equipment capacities. The sizing algorithms do not recognize any flow rate or
capacity values that may be entered in specific component objects. And the
sizing algorithms have no knowledge of values in setpoint managers or other
controls.
After the sizing is completed, and all occurences of "autosize" have been
replace with a computed size, the simulation runs. The simulatlion has no
knowledge of the data in the SIZING objects. It uses the setpoint managers and
other control inputs to run the simulation.
1. You are correct that there are no warnings if temperatures are
inconsistent. The user must be careful to make sure the various SIZING
temperature are appropriate for the sizing calculations and that the actual
setpoint schedules and other control inputs are consistent with the
temperatures in the SIZING objects.
2. If there is only one boiler, then the boiler output will be controlled to
meet the specified supply temperature.
3. During the simulation, the coil models and controllers will determine the
flow rate through the coil which is required to meet the load. I do not think
that these algorithms would result in a fixed outlet temperature from the
coils. My guess is that this system does not have adequate capacity and the
coils are running at full flow and the boiler is at full load, so the design
delta-T will occur as a result.
4. Yes, unless the loop volume has been specified to be large and the loop
supply temperature is varying.
5. The components created by COMPACT HVAC ignore thermal losses from pipes..
If you want to modelt the thermal losses, you may replace the simple PIPE
objects with PIPE:INTERIOR or PIPE:EXTERIOR.
6. The design temperatures in the SIZING objects are relevant only for the
autosizing calculations. Design temperatures in component objects (such as a
chiller) may also be used in the simulation performance calculations.
7. Is the fan running? The fan operation in the fan coil model is continuos
whenever the availability schedule is >zero.
8. If the outside air flow rate is zero, then the mixed air condition entering
the coil will match the return air temperature which will be the zone setpoint
temperature. If the outside air flow rate is greater than zero, then one would
expect the coil entering condition to be warmer than the zone setpoint.
Mike
On 2 Jan 2008 at 10:22, Ana Oliveira wrote:
> Hi again.
>
> In my (Compact HVAC) Fan Coil model, when I edit the expanded idf files, there
> seem to be several user-defined Design Temperature variables that can be
> independently set, apparently with posible inconsistencies, although no errors
> or warnings are issued regarding that aspect (for simplicity, I did not define
> any internal gains or infiltration, and outside air rate is set to zero):
>
> - I realize many of the following quastions would probably be answered if I
> could decode the relationships between the various nodes in the model, but I
> find it reather obscure to draw that from the expanded file...
>
> 1. I identified 3 sets of design temperatures, under, respectively - Design
> objects (Plant sizing), Boiler (water oulet temperature), Coils - but it seems
> to be that the algorithm ignores all inconsistencies (inlcuding, for instance,
> Boiler outlet Temperature being lower than Coil water temperature) and that the
> Temperatures under Design Objects override all other user-defined design
> temperatures; is that it?
>
> 2. In my model, the water temperature at the oulet of the boiler is always equal
> to the outlet temperature of the hot water plant loop; is that equal by
> definition for all models, or only in this simpler model?
>
> 3. The return Temperature of the hot water plant loop, however, shows a constant
> difference to the inlet temperature to the boiler (0.055ºC). Why? In auto-sizing
> mode, is this temperature driven by the design loop Delta T in Design Objects?
>
> 4. The inlet temperature to the heating coil is, by definition, the same as the
> outlet temperature from the hot water plant loop, right?
>
> 5.. Does it result from the above conditions that the eventual thermal losses
> between the boiler and the zone units are not modelled? can they be implicitly
> modelled by diferent user-defined temperatures (boiler outlet and coil inlet
> temperatures?)
>
> 6. The DESIGN Temperatures are only relevant when auto-sizing? (flow rates "take
> over" when user-defined?)
>
> 7. If I set the outside air rate to ZERO, what might be making the "Zone/Sys
> Sensible Heat Rate" have a non-zero value when the boiler (and chiller) are off
> (this happens in night hours in both winter and summer week, when the thermostat
> schedules are set to such temperatures so as to require no heating or cooling)?
>
> 8. In auto-size mode, it sets the Air inlet Temperature to the Cooling Coil
> equal to the Cooling setpoint temperature of the zone (24ºC).. Is that
> reasonable?
>
> Thanks again for any enlightenment.
>
> Regards,
> Ana Rui.
>
>
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========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support
EnergyPlus-Support@xxxxxxxx
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