Presentation George Berbari

2011

HVAC Design for High Ambient Conditions

Sustainability Beyond CertificationSept 19th 2011 – Beach Rotana Hotel, Abu Dhabi- UAE

Presented by George Berbari

What Are the Key Elements of HVAC Efficient Design for High Ambient Conditions

• Design Conditions• Indoor Design Conditions• Outdoor Design Condition

• Envelop Optimization beyond code compliance• Roof & Wall Insulation and U factor• Fenestration Light transmittance and U factor• Glass to Wall Ratio• Glass Shading• Tightness of Envelop

• Ventilation (Fresh & Exhaust Air)• Why some standards encourage to exceed Ashrae 62.1 by 30% or more?• Heat Recovery between fresh and exhaust air.• Variable speed fans operation.• Double Skin Commercial Hood??

Proper Design Conditions

• Design Conditions• Indoor Design Conditions Ashrae Standard 55 -2010 Allows to raise the typically

used DB of 23.3 to 24.4 OC in hot ambient to a range of 24.4 to 25.5 O C while Int’l Energy Conservation code (ICCC 2009) specify minimum indoor design conditions of 24 O C

• Outdoor Design Condition DB / WB = 46.1 / 29.44 OC typically used Vs. Ashrae Design Conditions 44.9 / 23.2 OC and 35.3 / 30.6 OC lead to :

• Over estimation of sensible load.• Under estimation of latent load.• Over estimation of Total load where dry bulb and wet bulb temperatures

peak at different time.• Wrong selection of Fresh Air Handling unit Coil which should be selected at

35.3 / 30.6 OC.• Current design conditions is leading to HVAC system and plant oversizing by

more then 10% while other safety factors compile the problem to typically of 30% oversizing leading to larger HVAC system and inefficient system operation.

Proper Design Conditions (Extracted from previous Ashrae Presentation Given in Dubai on 22-3-2005)

115 / 85 °F (46.1 / 29.4 °C) Variance 94 / 86 °F (34.4 / 30 °C) Variance 107 / 75 °F (41.7 / 23.9 °C) Variance

74°F (23.3°C) / 50% rh 492.1 7.3% 423.1 -7.8% 462.1 0.7%

76°F (24.4°C) / 50% rh 482.3 5.1% 414.5 -9.7% 458.8 BASE

115 / 85 °F (46.1 / 29.4 °C) Variance 94 / 86 °F (34.4 / 30 °C) Variance 107 / 75 °F (41.7 / 23.9 °C) Variance

74°F (23.3°C) / 50% rh 76.8 -12.9% 81.0 -8.2% 46.5 -47.3%

76°F (24.4°C) / 50% rh 84 -4.8% 88.2 BASE 53.7 -39%

115 / 85 °F (46.1 / 29.4 °C) 94 / 86 °F (34.4 / 30 °C) 107 / 75 °F (41.7 / 23.9 °C)

74°F (23.3°C) / 50% rh 568.9 11.0% 504.1 -1.6% 508.6 -0.8%

76°F (24.4°C) / 50% rh 566.3 10.5% 502.7 -1.9% 512.5 BASE

Internal Load (TR)

Fresh Air Cooling Coil Load (TR) for ~ 30,000 CFM (14,200 l/s)1,2

Outdoor Conditions

TOTAL Building Load (TR)Outdoor Conditions

Indoor Conditions

Indoor Conditions

Indoor Conditions

Outdoor Conditions

Envelop Optimization beyond code compliance

• Roof & Wall Insulation and U factor

• Meeting the local and international code is an adequate strategy as decreasing the Wall U factor below 0.57 Watt/m2.OK (0.1 BTU/Hr/OF.ft2) is often not economical unless codes dictates that as in the case of Abu Dhabi. Ashrae Std. 90.1-2010 specifies for residential Buildings in hot and humid climate (Zone 1A) wall U Factor = 0.857 Watt/m2.OK (0.151BTU/Hr/OF.ft2) and roof U Factor = 0.273 Watt/m2.OK (0.048 BTU/Hr/OF.ft2)

• Department of Municipal Affairs user Guide for Int’l Building codes in the Emirates of Abu Dhabi (IBC for AUH) interpret Int’l Energy Conservation Code 2009 specifies Wall U Factor of 0.324 Watt/m2.OK (0.057 BTU/Hr/OF.ft2) and Roof U Factor of 0.148. Watt/m2.OK (0.026 BTU/Hr/OF.ft2)

Envelop Optimization beyond code compliance

• Fenestration Light Transmission and U factor• Meeting the local and international code is an adequate strategy in low cost electric

power where 2.1 Watt/m2.OK (0.37 BTU/Hr/OF.ft2) and Glass Shading coefficient (SHGC) of 0.25 to 0.4 depending on Projection Factor. Triple glazing and double glazing with argon gas filler are feasible options where electric power are priced above US $ 0.15 /Kwh and where architects and owners prefers large glass facades. Select Glass with Light transmission preferably higher than 40% that meat the SHGC requirement.

• Thermally brake Glass have little impact on the thermal performance especially with 4 sided silicon structure glass where there is no exposed aluminum structure and in general was designed as a solution for winter condensation in colder countries. Use whenever code requires that as in the case of Sharjah.

Glass To wall Ratio

• Codes are calling for smaller Glass to Code Ratios Ashrae std. 90.1 recommends 40% maximum glass to wall ratio while IBC for AUH limit it to 30% . However Architects and Buildings owners and developers still prefer higher ratios such as:

70% G to W Ratio 50% G to W Ratio <50% G to W Ratio

Notice Cladding & Glass Colors

Glass To wall Ratio : Why is it important?

• Glass facades cost between US $400 to $500 per m2 compared to aluminum cladding of US $150 to $ 200 m2 and wall finishes US $100 to $400 per m2.

• Glass Passes 6 to 9.5 times more heat than walls!!


Heat Transmission Through Walls & Glass (Abu Dhabi)

Design Heat Transmission - Watt per m2

U value Glass Shading Exposure

Watt/m2.OK BTU/Hr/Ft2.OF Coeficient North East South WestAverage -

W/m2

Average - BTU/Hr /ft2

Wall 0.568 0.1 12.1 11.3 14.6 17.0 13.7 4.350.318 0.056 5.6 7.6 12.9 8.8 8.7 2.76

Glass 2.129 0.375 0.287Solar Conductance 32.5 32.5 32.5 32.5 32.5 10.3Solar Transmission 13.5 39.6 80.0 68.3 50.3 16.0

Subtotal Glass 46.0 72.1 112.5 100.8 82.9 26.3

Glass / Wall Heat Transmission (U=0.1) 3.8 6.4 7.7 5.9 6.0

Glass / Wall Heat Transmission (U=0.056) 8.3 9.5 8.7 11.5 9.5

Glass To wall Ratio : Why Mechanical Engineer are Passive?

• Mechanical Engineers are still on the receiving end with the architect and most often don’t comment at all on the envelop or offers any advice to the architect and developer.

• Architects tend to finalize the building architectural impression and offer it to the owner at early stage of the project prior to the involvement of the mechanical engineer.

• Architect are reluctant to change the ratio as often that involves unpaid work variation that means time and cost.

• Shading to the southern façade is quite effective but can be rarely incorporated once design is finalized.

• Building tightness test are now required in Abu Dhabi Emirate.

Fresh Air: The magic of Heat recovery Preferred Solution


Minimum Wheel Efficiency 75%

Minimum HP Effectiveness 45 to 50%

Fresh Air: The magic of Heat recoveryAlternative Solution


Fresh Air: The magic of Heat recoveryNon Recommended Solution


1) FRESH AIR AHU WITH ENERGY WHEEL ONLY

FRESH AIR

BAG FILTER2" PRE FILTER

RECOVERY WHEELTOTAL ENERGY COOLING COIL

EXHAUST AIR

SUPPLY AIR

RETURN AIR

PURGE AIR

CHILLED WATER44°F/54°F

2" FILTER

OPTIONAL VARIABLESPEED CONTROL

Minimum Wheel Efficiency < 65% Fresh Air > 30% of Ashrae

Std 62.1 Supplied at around 13OC

Why are This type of AHU’s still being specified and used?!!!

Fresh Air: Key Recommendations


• Fresh Air should not exceed the Amount specified in ASHRAE Standard 62.1 2010 in hot and humid countries and ignore the additional points achieved in few Green buildings codes as you will be penalized in the energy consumption part. EPA Indoor Air Quality Guide Quote: “source control is also a more cost-efficient approach to protecting indoor air quality than increasing ventilation because increasing ventilation can increase energy costs”. i.e. eliminating frying at your homes will reduce the smell, fire hazard and your fat intake and improve your Health and safety.

• Fresh air should not exceed that value of exhaust air by more than 10%.

• Residential Kitchen hood should be a circulating hood while permanent kitchen exhaust of 50 CFM should be installed at the kitchen ceiling plenum level.

• Residential Kitchens and Toilet exhaust can utilize the same duct and both should be used for heat recovery.

• Fresh air of 2 ACH should be supplied to lift lobbies to stop any cooking smell mitigation.

Fresh Air: Key Recommendations (cont’d)


• Double Skin Commercial kitchen hoods are controversial where the rule of thumb state that 80% make up air portion can be used in commercial kitchen is a recipe of trouble

• Peripheral perforated plenum with low velocity is recommended with the possibility to cool the air down to 30 deg. C.

• Preference to use UL listed hood with minimum exhaust flow and make use of transfer air from dinning areas.

• VFD on both exhaust and make up air fans.

X

Low air velocity < 0.4 m/s (75 FPM)

Recommended

Thank youThank you

email: [email protected]

15

Documents

Presentation George Berbari