Load Calculations Heat Balance Method - Application

Transcription

1 Load Calculations Heat Balance Method - Application Prof. Jeffrey D. Spitler School of Mechanical and Aerospace Engineering, Oklahoma State University

2 Weather Data: Cooling Three types of design days Based on peak dry bulb temperature: room loads. Based on peak wet bulb: cooling towers, evaporative coolers, ventilation Based on peak dew-point: humidity control. See 2001 ASHRAE Handbook of Fundamentals for further information.

3 Peak dry bulb design days See MPS Peak dry bulb temperature > 0.4%, 1%, or 2% Mean wet bulb (MWB) Mean wind speed Daily range. Clearness (See MPS 171)

4 Clearness

5 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.

6 Weather Data

7 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)

8 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.

9 Describing Walls and Roofs Described layer by layer from the outside in. Material properties for each layer (k, ρ, c p ) from MPS Solar 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.

10 External Boundary Condition Important! TOS: normal outside conditions TA: conditioned space on other side TG: ground temperature on other side TSS: outside temperature, but no sun TB: special, user-defined boundary temperature on other side.

11 Stud Walls Replace insulation/stud layer with a composite material. Composite material should have same resistance and same thermal mass. Example to follow later. A 3 x 3 x 1 x 2 x 4 k 3 k 1 k 2 k 4

12 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.

13 Describing Windows

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

15 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)

16 Internal Heat Gains Types: Lights Equipment Occupants Infiltration/Ventilation Radiant/Convective Splits: MPS 262 Schedules

17 Lighting Specify in terms of W/ft 2 Split radiant/ convective fractions. Specify schedule (24 fractions)

18 Equipment Specified similar to lighting. 1 W/ft 2 typical In one study of 44 office buildings, none had more than 1.2 W/ft 2 Data centers, etc. may require more careful calculation.

19 Occupants Specified by number of people, activity levels, sensible/ latent, radiant/ convective, and schedule See MPS 233 for activity levels, sensible/latent split

20 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.)

21 Infiltration Specified in Air Changes / Hour What should that be? See MPS No more than an educated guess in many circumstances.

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

23 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)

24 Questions?

25 Content Notes Weather 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 windows Describing doors Internal heat gains (levels and schedules) Describing thermal mass Infiltration and Ventilation Specifying system parameters Practical things about program why does it do backup instead of save? How to get output. Download latest version from website. Heating Load Calculations Check end of chapter for other items?