HVAC Load Calculations Guide

(1)

FMI Energy Conference 2006

HVAC

Load Calculations

Perry Morrow & John Parker Brandt Engineering, Inc.

(2)

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.

(3)

(4)

Why is moisture control important?

(5)

Where does the moisture come from?

• Infiltration

• Ventilation

• Makeup Air

(6)

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

(7)

Summer Weather Data – Richmond, VA & Phoenix, AZ

(8)

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.

(9)

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.

(10)

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.

(11)

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 3

rd

party software vendors.

(12)

Loads the New Way

(13)

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.

(14)

HVAC Load Questions

1

1. Establish allowable Indoor Temperature and Humidity Range.

2. Determine Outdoor Weather Conditions to use.

Choices are 0.4%, 1% and 2% Ashrae data or Other.

3. Determine mass and insulation value of all building

envelope components (Walls, Roof, Glass, Doors…).

4. 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.).

5. 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.

6. Repeat steps 4 & 5 for every zone (thermally similar

(15)

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.

(16)

60% 70% 80% 90% 100% 110% 30 40 50 60 70 80

Refrigerated Case Loads

Data From 2003 ASHRAE Applications Handbook 2.3

Percent L

o

ad

36.7 42.3 48.5 53 57.2 60.8 Dew Point (oF)

(17)

$8,000 $10,000 $12,000 $14,000 $16,000 $18,000 $20,000 $22,000 $24,000 $26,000 $28,000 $30,000 $32,000 $34,000 40 42 44 46 48 50 52 54 56 58 60

Store Dewpoint Setpoint

E n er g y C o st t o o p e ra te A /C f o r 1 year (e xc lu d e s au xi la ry h e at c o s t) BALTIMORE MIAMI In Miami $3,000 A/C energy COST if “setpoint” is 50 versus 58 dewpoint In Baltimore $400 A/C energy COST if “setpoint” is 50 versus 58 dewpoint

In Miami: 90% of a year the store needs humidity control

In Baltimore: 45% of a year the store needs humidity

control

Building HVAC Cost vs. Store Dewpoint

Building HVAC Cost vs. Store Dewpoint

(18)

$70,000 $72,000 $74,000 $76,000 $78,000 $80,000 $82,000 $84,000 $86,000 $88,000 $90,000 $92,000 $94,000 $96,000 $98,000 $100,000 $102,000 $104,000 $106,000 $108,000 $110,000 40 42 44 46 48 50 52 54 56 58 60

Store Dewpoint SETPOINT

Cost to Operate 3 Racks

(annual)

Baltimore, MD Miami, FL Salt Lake City Pittsburg, PA Houston, TX In Baltimore $8,000 rack energy savings if “setpoint” is 50 versus 58 dewpoint In Miami $18,000 rack energy savings if “setpoint” is 50 versus 58 dewpoint

Excerpt from 2002 FMI Presentation by Lee Churchill of Seasons-4 Inc.

(19)

HVAC + Rack Energy

Cost vs. Store Dewpoint

$80,000 $85,000 $90,000 $95,000 $100,000 $105,000 $110,000 $115,000 $120,000 $125,000 $130,000 40 42 44 46 48 50 52 54 56 58 60

Store Dewpoint SETPOINT

C o st to O p er ate 3 R acks & A /C (an n u a l) Baltimore, MD Miami, FL

Excerpt from 2002 FMI Presentation by Lee Churchill of Seasons-4 Inc.

Conclusion to study: 50 F dewpoint is an

(20)

HVAC Load Questions

2

1. Establish allowable Indoor Temperature and Humidity

Range.

(21)

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

(22)

(23)

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 0 to 999

Dehumidification Hours

by City

• Based on 74oF/55% RH

• ( 67.5 Gr/lb ) Internal Set point

(24)

2005 Richmond, VA

Summer Weather

0.4% 1.0%

(25)

(26)

(27)

HVAC Load Questions

3

Range.

3. Determine mass and insulation value of all building envelope components (Walls, Roof, Glass, Doors…).

(28)

Typical Wall Constructions

(29)

(30)

Similar Calculation for 6” Metal Studs with

R-19 insulation, No Insulated Sheathing

Simplified parallel Path calculation for Example purposes.

(31)

Insulated CMU Walls

R-12 or R-5 ?

• 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.

(32)

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.

(33)

(34)

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.

(35)

HVAC Load Questions

4

Range.

4. 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.).

(36)

Internal Loads

Mixed

(Contain Sensible & Latent Components)

• People

• Steam table

• Holding Cabinets • Coffee Brewer • Case Credits

Dry (Sensible Heat)

• Lights

• Equipment:

– Self Contained Display Cases – Slicers, Registers, Computers • Hooded Equipment – Broilers – Ranges – Ovens

(37)

(38)

Hooded Loads – Where Applicable

• Spreadsheets or pads of paper are good ways to total

the various components that make up the

(39)

Case Credit - Summary

• Case Credits can be calculated a number of different ways that will be discussed in up coming slides.

(40)

(41)

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.

(42)

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.

(43)

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.

(44)

(45)

(46)

(47)

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 Infiltration rate (cfm/ft2) Tight 0.10

Average 0.30

(48)

(49)

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

(50)

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.

(51)

HVAC Load Questions

5

Range.

5. Calculate heat flow into and out of each zone, at summer and winter outdoor weather conditions, varying

(52)

HVAC Load Questions

6

Range.

6. Repeat steps 4 & 5 for every zone (thermally similar space of interest) in the building.

(53)

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.

(54)

(55)

(56)

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.

(57)

(58)

(59)

Load Software

Air System Sizing Summary Output

(60)

Load Software

(61)

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.

(62)

Summary

- Pie • Dry Climate • Humid Climate 119.7 Total Tons 0.97 Coil SHR 129.4 Total Tons 0.67 Coil SHR

(63)

Summary

– Stacked Bar

119.7 Total Tons 0.97 Coil SHR 129.4 Total Tons

(64)

Take Home Points

• Infiltration / Transmission Load thru Walls.

• Interior & Exterior Design Conditions

• Costs of Controlling vs. Not controlling

space humidity.

(65)

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