First Law for the Open System (Control Volume Formulation)
Heat Transfer ( Q ) — Net Rate of Work ( W ) = Net Rate of Energy Flow (across the system boundary)
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D_E^ Dt |
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+ — |
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F 2 A V gz m i +————- + — |
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Gz |
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-Z |
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I + — |
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— + |
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M |
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2gc g |
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2Sc Scy |
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Special case of the 1st law is for steady-state flow that has constant flow across the boundary and no mass or energy change in CV.
TOC o "1-5" h z For a typical HVAC system two assumptions are valid: d-E dm
1) Steady-state (—— =0, —— =0) flow across the system boundary
Dt dt
2) KE and PE terms usually small Therefore, for the typical HVAC system:
Out in
A house/building is a thermal system and its envelope is the boundary. Let us consider some energy transfer in a single family house.
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Q infiltration 150 Btu/h Q conduction 200 Btu/h
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The thermal mass of the house is assumed to be 700 Btu/°F.
(a) Is the system in equilibrium?
Since Qi„ = Qout, the system is in equilibrium. The heat flows are steady state. The temperature will not change.
(b) What will happen if the A/C is shut off?
O
Posted in Fundamentals of Heating. Ventilating, and Air-Conditioning