Load CalculationsHeat Balance Method - Application

Prof. Jeffrey D. SpitlerSchool of Mechanical and Aerospace Engineering, Oklahoma State University

Weather Data: Cooling

Three types of design daysBased on peak dry bulb temperature: room loads.Based on peak wet bulb: cooling towers, evaporative coolers, ventilationBased on peak dew-point: humidity control.

See 2001 ASHRAE Handbook of Fundamentals for further information.

Peak dry bulb design days

See MPS 599-604Peak dry bulb temperature > 0.4%, 1%, or 2% Mean wet bulb (MWB)Mean wind speedDaily range.Clearness (See MPS 171)

Clearness

Weather Data: Cooling

Month: Usually July, but not always.Some rooms can have peak loads in other months, even January.Example: South facing hotel rooms with large windows may have peak loads in January.

Weather Data

Zoning/Rooms

In HVACLoadExplorer terminology:Room: one or more rooms with similar temperatures, loading, and controls.

In large buildings, it may be desirable to combine multiple rooms into a single room description.(Often called a zone in heat balance literature.)

Zone: provided in HVACLoadExplorer for convenience; it is a way of adding up loads for a group of rooms. (e.g. All rooms served by a single air handler)

Dimensions

Outside or Inside?Corner effects are usually small, hence it doesn’t make that much difference.Inside dimensions may slightly underestimate load.Outside dimensions may slightly overestimate load.

Simplify geometry when possible.

Describing Walls and Roofs

Described layer by layer from the outside in.Material properties for each layer (k, ρ, cp) from MPS 127-132Solar absorptivity and thermal emissivity important for inside and outside layers.Do not specify surface conductances in heat balance method.Surface tilt and orientation must be specified for solar radiation purposes.

External Boundary Condition

Important!TOS: normal outside conditionsTA: conditioned space on other sideTG: ground temperature on other sideTSS: outside temperature, but no sunTB: special, user-defined boundary temperature on other side.

Stud Walls

∆x1 ∆x2∆x3

∆x4

k1 k2

k3k4

A3

Replace insulation/stud layer with a composite material.Composite material should have same resistance and same thermal mass.Example to follow later.

Describing attic type spaces.

Program has limited ability to correctly represent attic spaces.An approximation can be made, based on outside surface area and corrected insulation resistance.

Describing Windows

Window properties

Normal SHGC: SHGC=0.87 * SC (SC from MPS 181) or SHGC from ASHRAE HOF 30.25-30.33Interior/exterior emissivity: per manufacturerU-factor: 1/R (surface-to-surface)Transmittance of shade: ideally, from manufacturer; otherwise may be visually estimated.

Thermal mass

Represents interior furnishings, interior partitions not elsewhere described.Necessarily approximate.Has two effects:

Thermal storage (slows room response)Radiative/convective exchange (speeds room response)

“Internal” Heat Gains

Types:LightsEquipmentOccupantsInfiltration/Ventilation

Radiant/Convective Splits: MPS 262Schedules

Lighting

Specify in terms of W/ft2

Split radiant/ convective fractions.Specify schedule (24 fractions)

Equipment

Specified similar to lighting.1 W/ft2 typicalIn one study of 44 office buildings, none had more than 1.2 W/ft2

Data centers, etc. may require more careful calculation.

Occupants

Specified by number of people, activity levels, sensible/ latent, radiant/ convective, and scheduleSee MPS 233 for activity levels, sensible/latent split

Infiltration and Ventilation

Infiltration: outside air that “leaks” into room.Ventilation: outside air introduced intentionally into room by system. (Only affects coil load, but is specified at room level.)

Infiltration

Specified in Air Changes / HourWhat should that be?See MPS 194-208No more than an educated guess in many circumstances.

Ventilation

Required amount set by code or ASHRAE Standard 62Can have a significant effect on coil load; does not affect room load, required air flow

System parameters

Supply air temperature set at zone level. (Affects required air flow rate)Ventilation ACH set at room level. (affects coil load)Room RH defaults to 50% (affects coil latent loads)

Questions?

Content NotesWeather data (explain selection and non-July)Zoning (“rooms”)Geometric description (inside dimensions or outside dimensions? Also simplification.)Material properties (include radiative properties)Describing walls and roofs.Describing stud walls.How to describe attic type spaces.Describing windowsDescribing doorsInternal heat gains (levels and schedules)Describing thermal massInfiltration and VentilationSpecifying system parametersPractical things about program – why does it do backup instead of save? How to get output. Download latest version from website.Heating Load CalculationsCheck end of chapter for other items?