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Lecture Objectives:

Lecture Objectives:. Summarize AHUs Learn more about cooling cycles. VAV Dedicated Outdoor Air System (DOAS) with occupancy sensors. Exhaust. 100%OA. VAV box. VAV box. CO 2. CO 2. For ventilation and humidity control. Fan coil units for heating and cooling.

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Lecture Objectives:

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  1. Lecture Objectives: • Summarize AHUs • Learn more about cooling cycles

  2. VAV Dedicated Outdoor Air System (DOAS) with occupancy sensors • Exhaust 100%OA VAV box VAV box CO2 CO2 For ventilation and humidity control Fan coil units for heating and cooling

  3. Fan Terminal Units Same as fan coil Can be with or without recirculation

  4. More about AHUs and Heat and Air Distribution System 1) Chapter 3 in your Building Environmental System Textbook A copy is provided in the handouts section of the course website 2) ASHRAE Handbook HVAC System and Equipment 3) ASHRAE Handbook HVAC Application

  5. HVAC Systems Multi zone Single zone All Hydronic or DX that relay on infiltration VAV CAV CAV VAV With and without humidity control With and without reheaters Dual duct Dual duct With reheaters DOAS with fan coils DOAS with fan coils This is not the complete list !

  6. Summary of HVAC Systems • Show HVAC processes on a Psychrometric chart • Define SA point • Think about processes and different ways to get to SA point • Analyze HVAC processes for real buildings • Single zone • Multiple zone

  7. Vapor Compression Cycle Expansion Valve

  8. Efficiency • First Law • Coefficient of performance, COP • COP = useful refrigerating effect/net energy supplied • COP = qr/wnet • Second law • Refrigerating efficiency, ηR • ηR = COP/COPrev • Comparison to ideal reversible cycle

  9. Carnot Cycle No cycle can have a higher COP • All reversible cycles operating at the same temperatures (T0, TR) will have the same COP • For constant temp processes • dq = Tds • COP = TR/(T0 – TR)

  10. Get Real • Assume no heat transfer or potential or kinetic energy transfer in expansion valve • COP = (h3-h2)/(h4-h3) • Compressor displacement = mv3

  11. Example • R-22 condensing temp of 30 °C (86F) and evaporating temp of 0°C (32 F) • Determine • qcarnot wcarnot • Diminished qR and excess w for real cycle caused by throttling and superheat horn • ηR

  12. Comparison Between Single-Stage and Carnot Cycles • Figure 3.6

  13. Subcooling and Superheating • Refrigerant may be subcooled in condenser or in liquid line • Temperature goes below saturation temperature • Refrigerant may be superheated in evaporator or in vapor (suction) line • Temperature goes above saturation temperature

  14. Two stage systems

  15. Multistage Compression Cycles • Combine multiple cycles to improve efficiency • Prevents excessive compressor discharge temperature • Allows low evaporating temperatures (cryogenics)

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