FMI Energy Conference 2006
HVAC
Load Calculations
Perry Morrow & John Parker
Brandt Engineering, Inc.
Richmond, Virginia
Key Concepts to Consider
as we look at Load Calculations
•
•
•
•
Interior Design Criteria
Exterior Design Criteria
Kitchen Hoods and Make Up air
Walls that aren’t as well insulated as they
appear.
Why are Supermarket loads Unique?
Why is moisture control important?
Illustration provided by Todd Smith of Munters Corporation.
Where does the moisture come from?
• Infiltration
• Ventilation
• Makeup Air
• Miscellaneous Loads
Makeup Air
• There have been
many attempts at
using tempered
makeup air. This
one appears the
most successful;
however, some of
the humid air
introduced here
doesn’t go out the
hood.
Image Courtesy of Food Service Technology Center publication
Design Guide 2 - Optimizing Makeup Air
Summer Weather Data – Richmond, VA & Phoenix, AZ
Data From 2005 Ashrae Fundamentals CD
HVAC Load Basic Terms
•
Zone: A single area with
similar thermal and use
characteristics.
•
•
Envelope Loads or
Skin Loads: Loads
associated with the Roof,
Walls, Windows, Doors,
Floor and Partitions.
•
Infiltration: Air that
leaks into a building
through the building
envelope.
•
Humidity: The quantity
of water vapor present in
air. It can be expressed
as an absolute, specific
or a relative value.
Humidity Measurement Terms
•
Absolute humidity is the actual mass of water vapor present in the air water
vapor mixture. The absolute humidity may be expressed in pounds of water
vapor (lb).
•
Specific Humidity Ratio or Humidity Ratio is the ratio between the actual
mass of water vapor present in moist air - to the mass of the dry air.
Humidity Ratio is normally expressed in pounds of water vapor per pound of
dry air or in Grains of moisture per pound of dry air. There are
approximately 7000 grains in a pound.
•
Relative Humidity: The ratio of the amount of water vapor in the air at a
specific temperature to the maximum amount that the air could hold at that
temperature, expressed as a percentage.
•
Dew Point temperature: The temperature below which moisture will
condense out of air. - Air at a given Humidity Ratio has a constant Dew
Point. If air is cooled below this point, moisture condenses out thus
changing its humidity ratio.
Definitions courtesy of wikipedia.org and .engineeringtoolbox.com
Relative Humidity vs. Humidity Ratio
As a given mass of moisture laden air is heated or cooled the air
volume changes but the moisture does not. Thus there is a change
in Relative Humidity, without a change in actual water content.
The mass of both water and air remain unchanged, so the humidity
ratio (Grains/Lb ) does not change. If the air continues to cool, the
relative humidity will reach 100% and water will begin to condense
on surfaces at 63oF so this is the Dew Point of the air.
Load Calculation Methods
• Manual Calculations
• Spreadsheet
• Specialized Software:
Elite Software - Chvac - Commercial HVAC
Loads
Carrier - Hourly Analysis Program
Trane - Trace family of programs
Department of Energy – ENERGYPlus, BLAST
and DOE-2. Text based software with graphic
interfaces sold by 3rd party software vendors.
Loads the New Way
Screen Capture from Carrier Hourly Analysis Program © Carrier Corporation
HVAC Load Basics
Although HVAC load calculation includes a
significant amount of number crunching, it
begins with digging out the answers to a
number of questions.
A lack of good information up front can mean
bad answers at the end of the process.
Garbage In = Garbage Out
HVAC Load Questions
1.
2.
3.
4.
5.
6.
1
Establish allowable Indoor Temperature and Humidity
Range.
Determine Outdoor Weather Conditions to use.
Choices are 0.4%, 1% and 2% Ashrae data or Other.
Determine mass and insulation value of all building
envelope components (Walls, Roof, Glass, Doors…).
Determine all internal load components (Anything that
consumes electricity, gas, solid fuel or food. Also
anything that has energy piped to it or away from it.).
Calculate heat flow into and out of each zone, at summer
and winter outdoor weather conditions, varying
temperature and sun location for each hour of the day and
each day or month of the year.
Repeat steps 4 & 5 for every zone (thermally similar
space of interest) in the building.
ASHRAE Indoor Air Temperature and
Humidity Recommendations
• … The industry chose 75°F db and 64°F wb
(55% rh, 57.5°F dew point{71.8 grains}) as
summer design conditions. This is the
ambient condition at which refrigeration
load for food store display refrigerators is
normally rated. Store humidity is one of the
most critical variables that can affect
performance of display refrigerators and
refrigeration systems.
Excerpt from 2006 ASHRAE Refrigeration Handbook, Chapter 46
Refrigerated Case Loads
110%
Percent Load
100%
90%
80%
70%
60%
30
40
50
60
70
36.7
42.3
48.5
53
57.2
80
Humidity Ratio (gr./LB)
60.8 Dew Point (oF)
Data From 2003 ASHRAE Applications Handbook 2.3
Building HVAC Cost vs. Store Dewpoint
$34,000
In Miami
$3,000 A/C
energy COST
if “setpoint” is
50 versus 58
dewpoint
$30,000
$28,000
(excludes auxilary heat cost)
Energy Cost to operate A/C for 1 year
$32,000
$26,000
$24,000
$22,000
$20,000
In Baltimore
$400 A/C
energy COST
if “setpoint” is
50 versus 58
dewpoint
$18,000
$16,000
$14,000
$12,000
$10,000
$8,000
In Miami: 90% of a year the
store needs humidity control
40 42 44 46 48 50 52 54 56 58 60
Store Dewpoint Setpoint
BALTIMORE
MIAMI
Building HVAC Cost vs. Store Dewpoint
In Baltimore: 45% of a year
the store needs humidity
control
Excerpt from 2002 FMI Presentation by Lee Churchill of Seasons-4 Inc.
Rack Energy Cost vs. Store Dewpoint
$110,000
$108,000
$106,000
$104,000
$102,000
Cost to Operate 3 Racks (annual)
In Miami
$18,000 rack
energy savings
if “setpoint” is
50 versus 58
dewpoint
$100,000
$98,000
$96,000
$94,000
$92,000
Baltimore, MD
Miami, FL
$90,000
Salt Lake City
$88,000
Pittsburg, PA
$86,000
Houston, TX
$84,000
$82,000
$80,000
In Baltimore
$8,000 rack
energy savings
if “setpoint” is
50 versus 58
dewpoint
$78,000
$76,000
$74,000
$72,000
$70,000
40
42
44
46
48
50
52
54
56
58
60
Store Dewpoint SETPOINT
Excerpt from 2002 FMI Presentation by Lee Churchill of Seasons-4 Inc.
HVAC + Rack Energy
Cost vs. Store Dewpoint
$130,000
Cost to Operate 3 Racks & A/C (annual)
$125,000
$120,000
$115,000
$110,000
Baltimore, MD
$105,000
Miami, FL
$100,000
Conclusion to study:
50 F dewpoint is an
optimum setpoint
$95,000
$90,000
$85,000
$80,000
40
42
44
46
48
50
52
54
56
58
60
Store Dewpoint SETPOINT
Excerpt from 2002 FMI Presentation by Lee Churchill of Seasons-4 Inc.
HVAC Load Questions
1.
2.
3.
4.
5.
6.
2
Establish allowable Indoor Temperature and Humidity
Range.
Determine Outdoor Weather Conditions to use.
Choices are 0.4%, 1% and 2% Ashrae data or Other.
Determine mass and insulation value of all building
envelope components (Walls, Roof, Glass, Doors…).
Determine all internal load components (Anything that
consumes electricity, gas, solid fuel or food. Also
anything that has energy piped to it or away from it.).
Calculate heat flow into and out of each zone, at summer
and winter outdoor weather conditions, varying
temperature and sun location for each hour of the day and
each day or month of the year.
Repeat steps 4 & 5 for every zone (thermally similar
space of interest) in the building.
ASHRAE Weather Data
• 2 Types of Summer Data
• Cooling
Dehumidification
DB / MCWB
&
DP / MCDB
# of Hours Exceeding Summer Data
– 0.4%
35 hr/Yr
– 1%
88 hr/Yr
– 2%
175 hr/Yr
• Types of Winter Data
# of Hours Exceeding Winter Data
– 99%
88 hr/Yr
– 99.6%
35 hr/Yr
2005 Ashrae Example Weather Data
Dehumidification Hours
by City
• Based on 74oF/55% RH
• ( 67.5 Gr/lb ) Internal Set point
Hours by City
7,000 to 9,000
6,000 to 6,999
5,000 to 5,999
4,000 to 4,999
3,000 to 3,999
2,000 to 2,999
1,000 to 1,999
Illustration provided by Munters Corporation.
0 to 999
2005 Richmond, VA
Summer Weather
0.4%
1.0%
Carrier Hourly Analysis Program – Opening Screen
HAP – Weather Input
HVAC Load Questions
1.
2.
3.
4.
5.
6.
3
Establish allowable Indoor Temperature and Humidity
Range.
Determine Outdoor Weather Conditions to use.
Choices are 0.4%, 1% and 2% Ashrae data or Other.
Determine mass and insulation value of all building
envelope components (Walls, Roof, Glass, Doors…).
Determine all internal load components (Anything that
consumes electricity, gas, solid fuel or food. Also
anything that has energy piped to it or away from it.).
Calculate heat flow into and out of each zone, at summer
and winter outdoor weather conditions, varying
temperature and sun location for each hour of the day and
each day or month of the year.
Repeat steps 4 & 5 for every zone (thermally similar
space of interest) in the building.
Typical Wall Constructions
Illustration from 2001 Ashrae Fundamentals Handbook, Chapter 25
Illustration from 2005 Ashrae Fundamentals Handbook, Chapter 25
HAP – Wall Construction
Similar Calculation for 6” Metal Studs with
R-19 insulation, No Insulated Sheathing
Simplified parallel Path calculation for Example purposes.
See 2005 Ashrae Fundamentals, Chapter 25 Modified Zone Method for less conservative method.
Insulated CMU Walls
R-12 or R-5 ?
•
•
•
•
Illustration from 2005 Ashrae Fundamentals Handbook, Chapter 25
CMU walls exhibit similar thermal
bridging to the Stud Wall example
with the addition of heat flow
parallel to the wall surface around
the insulated cores.
One insulating product’s published
data indicates a 12” CMU wall
with their product including air
film coefficients provides R-12.5.
The fine print indicates that more
detailed calculations are available.
We requested the calculations, they
indicate R=9.6.
Often for structural reasons, one in
four of the CMU cores are grouted
solid and have rebar added. This
added thermal bridging brings the
wall assembly down to
approximately R=5.1
Note that the problem is not with
the insulating material, but with the
fact that there are numerous paths
around the insulation.
Carrier Hourly Analysis Program – Opening Screen
•
•
Information describing the Roof,
Windows, Doors and any overhangs
(Canopies) are entered in the library so
they will be available as we define spaces.
When equipment will be operated, when
the lights are programmed to be on, and
estimated hourly occupancy rates are
entered under schedules.
HAP – Space General
Ventilation Air
The minimum quantity of ventilation air required is set by
the building code in most areas. In most spaces, this is
defined as X cfm / person. In some spaces, usually
ones that are sparsely populated, ventilation rates may
be set on a cfm / sq. ft. (floor area) basis.
In general, people exhale a predictable quantity of CO2 as
they breathe. The CO2 level can be monitored and used
to control outdoor air dampers to reduce the amount of
Ventilation air during lightly occupied hours.
HVAC Load Questions
1.
2.
3.
4.
5.
6.
4
Establish allowable Indoor Temperature and Humidity
Range.
Determine Outdoor Weather Conditions to use.
Choices are 0.4%, 1% and 2% Ashrae data or Other.
Determine mass and insulation value of all building
envelope components (Walls, Roof, Glass, Doors…).
Determine all internal load components (Anything that
consumes electricity, gas, solid fuel or food. Also
anything that has energy piped to it or away from it.).
Calculate heat flow into and out of each zone, at summer
and winter outdoor weather conditions, varying
temperature and sun location for each hour of the day and
each day or month of the year.
Repeat steps 4 & 5 for every zone (thermally similar
space of interest) in the building.
Internal Loads
Dry (Sensible Heat)
Mixed
• Lights
• Equipment:
(Contain Sensible & Latent Components)
– Self Contained Display
Cases
– Slicers, Registers,
Computers
• Hooded Equipment
– Broilers
– Ranges
– Ovens
• People
• Steam table
• Holding Cabinets
• Coffee Brewer
• Case Credits
HAP - Internal
Hooded Loads – Where Applicable
• Spreadsheets or pads of paper are good ways to total
the various components that make up the
Miscellaneous portion of the internal load.
Case Credit - Summary
• Case Credits can be calculated a number of different ways
that will be discussed in up coming slides.
Non-Hooded Loads and Totals
Case Credit Calculation
Simplified Method
• We have seen this fairly simple method recommended by
at least one refrigerated case manufacturer and more than
one client. Although relatively simple, the results appear
to be fairly accurate.
Case Credit Calculation
Ashrae Method
Ashrae publishes a table listing Sensible and Latent Case
Credit values on a BTU/Foot basis for various case types.
Some case manufacturers have this type of data available
for their specific cases.
Case Credit Calculation
BTU / Ft & Return Air
One client has taken the BTU/LF or Ashrae method a step
further. This calculation bases the case credits on a btu/lf
method and then assigns only a portion of this credit to the
space. Where under case returns are utilized, the remainder of
the credit is assigned to the unit cooling coil.
HAP - Wall
HAP – Roof
HAP - Infiltration
Infiltration
• From Carrier HAP 4.20a - Help File
– Items in the CFM/sqft column define infiltration in terms of airflow per
unit of exterior wall area. Because infiltration occurs through exterior
walls - especially through windows and doors in these walls - rule of
thumb infiltration rates are sometimes tabulated in this CFM/sqft format.
• From: Ashrae 2005 Fundamentals – Chapter 27, Page 23
– NONRESIDENTIAL AIR LEAKAGE
– …Typical air leakage values per unit wall area at 0.30 in. of water are
0.10, 0.30, and 0.60 cfm/ft2 for tight, average, and leaky walls,
respectively.
Wall Construction
Tight
Average
Leaky
Infiltration rate (cfm/ft2)
0.10
0.30
0.60
HAP - Floor
HAP – Partition Input
• Partitions are internal walls, ceilings or floors that
separate conditioned spaces from un-conditioned or
partially conditioned spaces. Machine rooms and
Receiving Areas can be partition loads to the
adjacent spaces. These walls should be insulated.
HAP – Partition U Value Calculation
•
Use the built in Wall and Roof property calculation portions of the
program to calculate the U-Value for any partitions. Manually
transfer this data to the partition tab of the space input form.
HVAC Load Questions
1.
2.
3.
4.
5.
6.
5
Establish allowable Indoor Temperature and Humidity
Range.
Determine Outdoor Weather Conditions to use.
Choices are 0.4%, 1% and 2% Ashrae data or Other.
Determine mass and insulation value of all building
envelope components (Walls, Roof, Glass, Doors…).
Determine all internal load components (Anything that
consumes electricity, gas, solid fuel or food. Also
anything that has energy piped to it or away from it.).
Calculate heat flow into and out of each zone, at summer
and winter outdoor weather conditions, varying
temperature and sun location for each hour of the day and
each day or month of the year.
Repeat steps 4 & 5 for every zone (thermally similar
space of interest) in the building.
HVAC Load Questions
1.
2.
3.
4.
5.
6.
6
Establish allowable Indoor Temperature and Humidity
Range.
Determine Outdoor Weather Conditions to use.
Choices are 0.4%, 1% and 2% Ashrae data or Other.
Determine mass and insulation value of all building
envelope components (Walls, Roof, Glass, Doors…).
Determine all internal load components (Anything that
consumes electricity, gas, solid fuel or food. Also
anything that has energy piped to it or away from it.).
Calculate heat flow into and out of each zone, at summer
and winter outdoor weather conditions, varying
temperature and sun location for each hour of the day and
each day or month of the year.
Repeat steps 4 & 5 for every zone (thermally similar
space of interest) in the building.
Systems
1
• After all spaces to be conditioned have been entered, we
can begin grouping them together into zones and systems.
• Only spaces with similar thermal and occupancy
characteristics should be grouped together as a zone.
There will be one thermostat per zone generally
• The systems usually correspond to air handling units.
These can be split systems, central station air handling
units or rooftop units..
• A typical rooftop unit or split system can only serve one
zone, and is controlled by one thermostat. So in this case
each system consists of only one zone.
• There are other system types that allow one unit to serve
multiple zones, but in our experience, they are not often
applied to supermarkets.
HAP – System Input
General
HAP – System Components
Systems
2
•
When we were describing spaces, we entered information that will give us
the total outside air required for ventilation.
•
In stores with cooking, this is only part of the outside air story. You need
make-up air for hoods and other exhaust sources. The software is aware
of this, so all you need to do is enter this value as an exhaust quantity as
you describe the system.
•
If the zones where the exhaust loads occur are open to the main sales area
and each other you have a decision to make. Where to allocate the make
up air.
•
You can either design for one large semi-custom unit capable of handling
most of the outside air or multiple semi-custom units.
•
If the zones are not open to one another, you have no choice. Make Up
air must be provided in the system where it is actually exhausted.
Building Air Balance
HAP – Zone Components
Load Software
Air System Sizing Summary Output
Notice Warning
Load Software
Air System Design Load Summary
Heating Load Calculation – “Case Credits”
• Heating loads are calculated for the maximum required
heating from the system without taking credit for any
internal lights, equipment and people. External heating
credits such as solar effects are also omitted. Because the
software ignores miscellaneous loads while calculating the
heating load, the “case credits”, which in heating mode are
an additional heating load not a credit, they must be added
to the required unit capacity manually.
• If sufficient heat from the refrigeration rack is reclaimed
by the mechanical systems, it may not be necessary to
increase the unit heating capacity to offset this added
heating load. But be careful, often the reclaim heat is not
available.
Summary
•
Humid Climate
129.4 Total Tons
0.67 Coil SHR
•
- Pie
Dry Climate
119.7 Total Tons
0.97 Coil SHR
Summary
129.4 Total Tons
0.67 Coil SHR
– Stacked Bar
119.7 Total Tons
0.97 Coil SHR
Take Home Points
• Infiltration / Transmission Load thru Walls.
• Interior & Exterior Design Conditions
• Costs of Controlling vs. Not controlling
space humidity.
Credits
Presentation includes excerpts from:
• ASHRAE 2003, 2005 and 2006 Handbooks
• Screen Outputs from Carrier Hourly Analysis Program
HAP version 4.20a
• Humidity illustrations provided by Todd Smith of Munters
Corporation.
• Rack Energy Cost vs. Store Dewpoint data provided by
Lee Churchill of Seasons-4 Inc.
Software Resources
• Load & Energy Simulation Software
–
–
–
–
Carrier Software - Hourly Analysis Program
US Dept of Energy - EnergyPlus and DOE-2
Elite Software - Chvac - Commercial HVAC
Trane - Trace Family of software
• Psychometric Calculation Tools
– PsyCalc98 - Linric Company
– PsyFunc is a collection of psychometric
functions for Excel sold by Linric Company.