Don’t Oversize: The Importance of right sizing HVAC Equipment
Transcript of Don’t Oversize: The Importance of right sizing HVAC Equipment
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Don’t Oversize: The Importance of right sizing HVAC EquipmentBy Mohammed Nader Alamirah
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Presentation TitleThe Importance of right sizing HVAC Equipment
• Can’t be too safe, right?• Wrong! At least when sizing HVAC equipment• Causes adverse effects, even on comfort!
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Topics◦ How and why:- Mistakes at design stage- Added mistakes during construction◦ Importance of maintaining true SHR◦ Effects of Oversizing HVAC equipment:- Effect on the DC plants◦ What do we do? Recommendations
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So, why do we do it and how?1- Mistakes at design stage2- Added mistakes during Construction
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Mistakes at Design Stage◦ Rules of thumb calculations
Committing to crazy deadlines
◦ Communication breakdownOverly Conservative assumptions
◦ High safety marginsDon’t add safety margins, or follow AHJ
◦ Poor SelectionSelection for higher SHR
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Added mistakes during construction◦ Contractor’s fault?
Not paying attention to supplier’s SHR selection
◦ Consultant is also to blame!Worried Inspectors-Not engineers.
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Importance of maintaining Design SHR
𝑆𝑆𝑆𝑆𝑆𝑆 =𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝑆𝑆𝐶𝐶𝐶𝐶𝑇𝑇𝑇𝑇𝐶𝐶𝐶𝐶𝑆𝑆 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝑆𝑆𝐶𝐶𝐶𝐶
◦ It varies based on application (0.5 all the way to 1.0)◦ Very important in part-load (Which is most of the year)◦ Selecting units with higher than design SHR will cause discomfort: Either
overcooling or sweating due to high humidity.
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Importance of maintaining Design SHR
Source: Trane Engineers Newsletter ENEWS 29-04
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Effects of oversizing HVAC equipment◦ Higher capital costs:
Equipment, ductwork, cables, installation, etc.◦ Higher operating costs:
Electrical & Energy bills, maintenance & replacement, shorter DX unit life, DC provider penalty.
◦ Discomfort: Overcooling/poor humidity control, Thermostat wars.
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The bigger picture: DC Plants◦ Oversizing is one of the causes of “Low Delta-T Syndrome”
𝑄𝑄 =𝐿𝐿𝑆𝑆
× 𝐶𝐶𝑝𝑝 × ∆𝑇𝑇
◦ Other reasons include: poor balancing, using 3-way valves, poor control valves, dirty filters, etc.
◦ Remember: 𝑃𝑃𝑤𝑤 ∝ (𝐿𝐿𝑠𝑠)3
◦ Causes pump & chillers to work in full capacities, may starve some terminals too!◦ DC providers enforce penalties on owners because of low Delta-T from their buildings.
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So, what do we do?◦ Design Stage:Communicate with the Architect & the Client (Integrated design):
Reasonable deadlines, design inputsUse proper assumptions: refer to ASHRAE handbooks/standards,
coordinate with lighting engineer, Take credit for treated OA supply.Use reasonable Safety factors: from AHJProper SHR selection
o Construction Stage:Again, proper SHR selection. Educate the supervision team about oversizing risksAccept some compromise on Sensible and Airflow selection
capacities.
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