Load Estimating Level 2 Flashcards
Why is it important to understand the sources of the various building load components and how to combine them?
To perform a load estimate for the selection of the heating and cooling equipment that will condition the various building spaces.
What three categories can all load components be classified into?
External, Internal, and System
Explain what infiltration is
The passage of outdoor air into the conditioned space due to positive outside wind pressure.
Explain why it is difficult to accurately estimate the infiltration load component.
Due to the tremendous variability of building geometry, building construction quality, and wind patterns
Under what conditions would ventilation air be considered a room load?
When introduced untreated directly into the space and/or when bypassed through or around and air handling unit.
Name the four supply air losses that exist between the coil of a central air handler and the room supply air terminal.
Supply duct heat gain.
Supply duct leakage loss
Supply air fan heat
Bypassed outdoor air
Would a blow through fan arrangement in a central air handler cause you to calculate the supply air fan heat (SAFH) as a loss between the coil and supply air terminal (Yes or No)? Explain why
No. The load would be calculated on the return side of the coil since this is physically where the fan would be located.
Why is the solar gain through the west glass nearly zero before noon? Why isn’t it zero?
The west is in the shade until noon. Even when in the shade the glass is still subject to diffuse radiation.
Explain why the actual cooling load caused by the sun’s rays penetrating a window is smaller than and occurs at a peak time later than the instantaneous solar heat.
The sun’s rays penetrating the glass in the form of radiant energy must first strike a surface to be converted into heat. Not all of this heat shows up immediately in the space because some of it flows into the mass of the building by conduction. Over time, this stored heat makes its way into the space. Thus the peak load caused by the sun’s rays occurs later in the day and has a lower magnitude than the instantaneous heat gain.
Can the simple steady state equation q = UxAxΔt be used to determine the wall and roof transmission loads?
Even though the storage effect and solar loading of the external wall or roof structure makes the flow of heat time-related and thus not a steady-state heat flow situation, the use of ETD allows the load allocation to be completed using the steady state equation. (Yes)
Explain what Equivalent Temperature Difference (ETD) represents.
Equivalent Temperature Difference is a fictitious temperature difference use for approximation. It allows one to stop the flow of heat at a single point in time and use the steady state heat transfer equation to calculate the load.
It is the approximated equivalent of the complex storage related heat transfer process that is actually taking place throughout eh wall or roof.
Determine the sensible and latent load created by 100 people dancing (moderately) in a room whose temperature is maintained at 75° F
Sensible: 325 Btuh/person
Latent: 525 Btuh/person
*100 each = 32,500 Btuh sensible, 52,500 Btuh latent
Explain why stored energy from lights is generally neglected in calculating the light load component for a building.
The primary target for radiant energy from the light is the floor. Carpet is widely used. Carpet insulated the floor from radiant light rays. The radiant energy strikes the carpet and is converted into heat. The heat is then dissipated almost immediately into the space.
Unless the air conditioning equipment is run for an hour or two before lights are turned on, the mass of the building will already be saturated with stored heat from the night before. As the light heat is absorbed in the mass, the equivalent amount of energy is almost simultaneously released to the space.
Explain why an accurate estimate of supply duct losses is not possible at the load-estimating phase of an air conditioning system design.
At the load estimating phase of the project, the designer has not designed the ductwork or piping systems and in many instances has not yet decided on the type of system to be used.
What is a typical percent of room sensible heat load (RSH) used to estimate supply duct heat gain?
2-3% for mid-level pressure class ductwork (+-2in to + 6in static pressure) Lower pressure class ductwork would likely have 5% leakage.
