[Bldg-sim] Equipment Sizing
floodyc at gmail.com
Thu Jun 10 12:49:12 PDT 2010
What's the point in duscussing modular plant arrangements when the
calculations we use are outdated (ASHRAE & CIBSE steady-state) and result in
gross oversizing from the outset?
This is the real problem we face.
On Mon, May 24, 2010 at 09:53, Haynes, Glenn <Glenn.Haynes at kema.com> wrote:
> I fully agree with you, and most of the following diatribe is not meant for
> you, but for the larger design community. None the less, I invite you to
> correct me publicly where you believe I may have missed the boat. I'm still
> When you can specify more than one cooling unit to meet the same load, you
> can afford to oversize a lot before energy efficiency is affected. It's
> mainly the residential and small commercial sectors that concern me
> regarding excessive over sizing vs. efficiency and humidity control.
> But in every case, more capacity costs the client (owner) more money, so
> proper sizing is important, especially during difficult economic times like
> these. If you are like me, you tend to take ownership of the HVAC design,
> and costs may not be as important to you as they are to the client. An
> excellent design may be rejected by the design team based on cost, and the
> easiest way to increase cost, along with the probability of rejection, is to
> oversize the systems.
> With regard to sizing heating systems, I believe there is more leeway in
> terms of equipment (but not floor space or subsystems) costs, and the
> practical need for greater capacity is more common (morning warm-up after
> setback, for example). A good design will consider the real capacity needs
> of the building without exceeding them more than necessary. It's more
> difficult to define over sizing in the large C&I sector because proper
> sizing depends on more variables, so it comes back to the design engineer's
> integrity more often. A highly skilled designer will tend to have more
> confidence in his ability (unless he has been "burned" a few times due to
> design errors) to calculate the real loads more accurately, considering all
> the variables.
> One less confident (possibly including a highly skilled professional with
> burn scars) may tend to assume "conservatively" on all variables
> simultaneously, thus over-calculating the required loads, and then beef
> those up more than necessary. Who, except another highly skilled (and
> brave, and independently wealthy) professional, is capable and willing
> to challenge this approach? And even then it is one's opinion against
> If redundancy is needed (hospitals, for example), then there is still a
> practical limit to the need. It just becomes more indeterminate, or more
> difficult to define. This is where interaction with the design team is more
> critical; this time the HVAC designer needs to ask appropriate questions,
> listen carefully and offer verbal guidance to the team before he
> can properly size the systems. We should not fail to consider that
> installation costs are always greater with larger systems, regardless
> of building sector. All else being the same, this usually applies to O&M
> costs as well. The integrity question is really this; "Do I mitigate my
> personal risk more through willful overdesign, or do I assign more value
> to the owner's financial objectives and/or limitations?"
> To achieve a proper balance, the designer must first determine the true
> capacity requirements with accuracy and confidence, and then add only a
> modest oversize factor to that. It isn't easy, but our choices here
> eventually establish our levels of self-respect and our professional
> reputations within the design community.
> *From:* Varkie C Thomas [mailto:thomasv at iit.edu]
> *Sent:* Saturday, May 22, 2010 11:35 AM
> *To:* Haynes, Glenn
> *Cc:* Jason Humbert; bldg-sim at lists.onebuilding.org
> *Subject:* Equipment Sizing
> I changed the topic name to �Equipment Sizing�. I thought I would add my
> 2 cents worth also. So now Bldg-Sim has 8 cents worth.
> The first building project that I was associated was at the tail end of the
> M-E design at JB&B of the Federal Reserve Bank Building in Minneapolis.
> The building opened in January (I think) in the early 70s. ASHRAE winter
> design at 1% is -16 F. When the building opened it was about -30 F (it
> can go down to -40 F there) with strong winds. Occupants in the US are
> not dressed (with full arm and leg heavy woolen underwear) to tolerate low
> indoor temps even for one day.
> Summer design in Minneapolis at 1% is 92 DB 75 WB but the DB can exceed 100
> F. Occupants can tolerate a little discomfort on the few days that
> extreme summer conditions occur. It�s still better than no AC. Few, if
> any, buildings were air-conditioned, even in the US, before 1940. The
> moral of this story is design for extreme winter conditions in very cold
> climates. Judgment, experience, and common sense have to be applied. It
> depends on the location. In a place like Singapore the temp varies from a
> low of 75 F to 95 F all day and all year. There are no extremes. All
> buildings in tropical countries do not need heating systems.
> Inefficient energy use occurs when there is only one unit of the equipment
> and it is oversized. When there are two or more units, one unit starts
> until it reaches maximum, then second unit comes on and the two shares load.
> The units are rarely operated at minimum load. This is the default in
> DOE2 but LOAD-MANAGEMENT allows you to sequence the use of primary equipment
> in any way you want that is appropriate for the project.
> The lighting and equipment design criteria was 5 watts /sf and 3 watts /sf
> for buildings designed before the energy crises in 1974. No one cared
> about energy before then. Actual lighting density was nowhere close, and
> there was very little equipment in offices. This means for a million sqft
> building you end up with three chillers. One is standby which comes into
> operation when a chiller fails or one chiller has to be shut down for
> maintenance. Specifying standby primary equipment affects first costs and
> does not affect equipment efficiency.
> Energy programs are for comparing alternative energy conservation measures.
> There is no need to size equipment for occasional extreme weather
> conditions. But I think auto-sizing is based on weather data (not design
> data or median weather data) so equipment is sized for the worst condition
> of that year. This means at every other hour of the year the equipment is
> operating inefficiently at part load conditions. Equipment sizes
> estimated by energy programs are used in the design process.
> Equipment schedules in design documents are based on actual equipment
> selected from manufacturers catalogs. The name of the manufacturer and
> the model number are specified and then �or equal� added. Equipment
> schedules are not based on design calculations or generic data because it is
> possible that real equipment cannot meet the performance data. You need
> the dimensions of the equipment (with clearances for maintenance) to design
> them into the space. TRACE energy program is based on actual real
> equipment performance data specified my model numbers. HAP is based on
> real equipment, but they don�t mention model numbers.
> I think the Code of Federal Regulations 10CFR434 (ENERGY CODE FOR NEW
> FEDERAL COMMERCIAL AND MULTI-FAMILY HIGH RISE RESIDENTIAL BUILDINGS) used
> to allow you to size two identical units of primary equipment each at 66% of
> the maximum design. This would be commercial buildings with large heating
> and cooling loads where you would have two units. Perhaps it was a
> special case where failure to perform at all times was not an option. I
> couldn�t find it the latest register.
> *10CFR434 - 403.2.2 Equipment and System Sizing.*
> *Heating and cooling equipment and systems shall be sized to provide no
> more than the loads calculated in accordance with subsection 403.2.1. A
> single piece of equipment providing both heating and cooling must satisfy
> this provision for one function with the other function sized as small as
> possible to meet the load, within available equipment options. Exceptions
> are as follows:*
> *(a) When the equipment selected is the smallest size needed to meet the
> load within available options of the desired equipment line. *
> *(b) Standby equipment provided with controls and devices that allow such
> equipment to operate automatically only when the primary equipment is not
> *(c) Multiple units of the same equipment type with combined capacities
> exceeding the design load and provided with controls that sequence or
> otherwise optimally control the operation of each unit based on load.*
> * *
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Chris Flood BSc (Eng)
Senior Building Analyst & Energy Modeller
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