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Objectives. Finish analysis of most common HVAC Systems Learn about the psychrometric related to the cooling towers Cooling Systems Describe vapor compression cycle basics Draw cycle on T-s diagrams Compare real cycles to ideal cycles.

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objectives
Objectives
  • Finish analysis of most common HVAC Systems
  • Learn about the psychrometric related to the cooling towers
  • Cooling Systems
    • Describe vapor compression cycle basics
    • Draw cycle on T-s diagrams
    • Compare real cycles to ideal cycles
vav dedicated outdoor air system doas with occupancy sensors
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

fan terminal units same as fan coil
Fan Terminal Units Same as fan coil

Can be with

or without

recirculation

hvac systems
HVAC Systems

Multi zone

Single zone

All Hydroinic

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 !

summary of hvac systems
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
cooling towes
Cooling towes
  • Similarity and difference between
    • Evaporative coolers and
    • Cooing towers
direct contact processes
Direct Contact Processes
  • Humidification (and dehumidification – see 10.4)
  • Heat rejection
    • Water has better heat transfer properties than air
  • Non dimensional parameter
  • Lewis number, Le = α/D = hc/hD/cP
    • Ratio of heat transfer to mass transfer
    • Assume Le = 1 for evaporative coolers

hc convection heat transfer coefficient

hD mass transfer coefficient

cp specific heat

α thermal diffusivity

D mass diffusivity

air washer
Air Washer
  • Sprays liquid water into air stream
  • Typically, air leaves system at lower temperature and higher humidity than it enters
air washers evaporative coolers
Air Washers/Evaporative Coolers
  • Heat and mass transfer is mutually compensating
  • Can evaluatebased on temperature drop, humidification, or comparison to other energy exchangers
cooling tower
Cooling Tower
  • Similar to an evaporative cooler, but the purpose is often to cool water
    • Widely used for heat rejection in HVAC systems
    • Also used to reject industrial process heat
solution
Solution
  • Can get from Stevens diagram (page 272)
  • Can also be used to determine
    • Minimum water temperature
    • Volume of tower required
  • Can be evaluated as a heat exchanger by conducting NTU analysis
real world concerns
Real World Concerns
  • We need to know mass transfer coefficients
    • They are not typically known for a specific direct-contact device
    • Vary widely depending on packing material, tower design, mass flow rates of water and air, etc.
    • In reality, experiments are typically done for a particular application
    • Some correlations are in Section 10.5 in your book
      • Use with caution
summary
Summary
  • Heat rejection is often accomplished with devices that have direct contact between air and water
    • Evaporative cooling
  • Can construct analysis of these devices
    • Requires parameters which need to be measured for a specific system
efficiency
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
carnot cycle
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)
get real
Get Real
  • Assume no heat transfer or potential or kinetic energy transfer in expansion valve
  • COP = (h3-h2)/(h4-h3)
  • Compressor displacement = mv3
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