[Equest-users] Another process load question
The Watt Doctors - Dave Weigel
Dave.Weigel at thewattdoctors.com
Sat Oct 20 16:32:13 PDT 2012
This is more of a LEED / ASHRAE 90.1 question, but I see a lot of experience in this group. I hope someone has some experience with a LEED review on this issue. It may be a moot point, but I can’t go into the arguments below without a little backup.
A debate has arisen among engineers, architects, and a LEED AP regarding the cooling energy for space conditioning in an area with process loads. This is specifically related to 90.1-2007 exception G3.1.1.b, and to a literal vs. extended interpretation of 90.1-2007 part 3.2, definitions:
Process energy : energy consumed in support of a manufacturing, industrial, or commercial process [implying medical equipment as well] other than conditioning spaces and maintaining comfort and amenities for the occupants of the building.
Process load: the load on a building resulting from the consumption or release of process energy.
This may be a moot point if it has already been the subject of a LEED 2009 CIR, or if there is an official ASHRAE interpretation or addendum. I haven’t been able to find any of those.
The liberal-interpretation side views it this way:
The definition of process load says “consumption or release,” and says nothing about removing the heat from the space.
Process energy is the total input energy to a process. The additional energy to cool the space in which the process resides is not process energy, it is a building space conditioning load.
Example 1: A server room, MRI room, or a room full of autoclaves is allowed to have system 3 (packaged DX VAV) in the baseline model under this exception rather than system 7 (chilled water VAV). While the process load itself must be simulated the same in baseline and proposed design cases, energy savings from the higher efficiency of the space cooling system is included in the bottom line savings. This includes the cooling load imparted to the space by the process load, such as the waste heat from the autoclaves, MRI, or servers.
Example 2: Following the logic in example 1, a process load's waste heat that is removed via a chilled water loop rather than an air system should benefit the bottom line savings through the improved efficiency of the proposed design's cooling plant over the baseline design. A problem is that there isn't a way to substitute a DX VAV air system for a chilled water heat exchanger. Instead, the baseline must include the chilled water cooling loop. This way, you compare a high-efficiency chiller plant to the lower-efficiency baseline chiller plant.
The basic argument is that "process energy" is measured at the input energy source for the process. The total process energy is the sum of the energy actually consumed by the process and the amount released as waste heat. The energy consumed is the process energy. The waste heat is the rest. Therefore, if we add the removal of the waste heat to the process energy, it is a double dip. We have already accounted for the total process energy at its input. If we add the energy to get the waste heat out of the building to the total input energy, we get a number larger than the total energy input to the process. Once its waste heat is released into the building, it becomes an element of the cooling load no different from lights, people, envelope, and others. If not, we are penalized by having to hold constant an amount of energy that is more than the energy delivered into the building to the process. The same should apply to a chilled water loop. If a process piece of equipment is designed for water cooling, it may also have been designed to cool itself with radiators. In that case, it would be just like the MRI or server room. Waste heat would become a space cooling load just like any other.
The liberals also offered up a more complex example, a metal working shop with welders. The energy that goes into the welders is process energy. The waste heat is a part of that process. Cooling it away is a space load, just like the examples above, and is not process energy. But part of the welder’s input energy is expelled in the form of smoke, the by-product of the process. If we must also count the cooling energy from removing the waste heat that is generated by the welding process, should we also have to count the extra exhaust fan energy and the extra load from the increased ventilation air that offsets the exhaust?
The conservative-interpretation side views it from the polar opposite position. They contend that all input process energy plus the energy required to remove its waste heat must be held constant, no matter if the cooling is done by a building air system or by the building chilled water plant.
For the definition of process energy, “energy consumed in support of manufacturing, industrial, or commercial process,” they claim that the energy to remove waste heat is “in support of” the process and must be counted as process load.
In eQuest terms, the conservative side says we must extract the hourly cooling loads that are due to each and every piece of waste heat from process equipment. Then we re-insert that cooling load profile as process energy and hold it constant from the baseline to the proposed design as an external load on the meter. So, the MRI process load turns into the input power plus the energy to cool it.
I'm not saying which side I'm on. I would not be a big fan of extracting and manipulating all those load profiles, especially in the case of that welder.
David R. Weigel, PE
1189 Golden Circle SW, Lilburn GA 30047
678-353-6941 office 901-619-1716 cell
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