1. Objectives More HVAC systems - Sorption (absorption) chillers
- Economizers (Psychrometrics) and control
- Desiccant systems (Psychrometrics)

2. Absorption Cycle
Strong vacuum Refrigerant: water Absorber: lithium bromide
Replace compressor

4. Absorption Cycle Water is typical refrigerant
Strong vacuum - ~0.116 psi or MPa
Boiling point of water?
Differences from vapor compression
No compressor
Chemical (e.g. LiBr - hygroscopic)
Absorbs water vapor turns it into a liquid
Regenerated by removing water
Requires heat

5. Absorption Chillers and Cogeneration
Cooling cycle
Rankine cycle 3 TH 4 2 1 Tc
or boiler

6. Economizer Economizer (fresh air volume flow rate control)
Controlled device is damper
- Damper for the air
- Valve for the CC
fresh
air
damper
mixing
recirc.
air
T & RH sensors

7. Economizer – cooling regime
How to control the fresh air volume flow rate?
If TOA < Tset-point → Supply more fresh air than the minimum required
The question is how much?
Open the damper for the fresh air
and compare the Troom with the Tset-point .
Open till you get the Troom = Tset-point
If you have 100% fresh air and your
still need cooling use cooling coil.
What are the priorities:
- Control the dampers and then the cooling coils or
- Control the valves of cooling coil and then the dampers ?
Defend by SEQUENCE OF OERATION
the set of operation which HVAC designer provides to the automatic control engineer
% fresh air
100%
Minimum for
ventilation

8. Economizer – cooling regime
Example of SEQUENCE OF OERATIONS:
If TOA < Tset-point open the fresh air damper the maximum position
Then, if Tindoor air < Tset-point start closing the cooling coil valve
If cooling coil valve is closed and T indoor air < Tset-point start closing the damper
till you get T indoor air = T set-point
Other variations are possible

9. Sensible and Enthalpy and Desiccant wheels

10. Desiccant wheel
Figure 3 – A desiccant-based cooling system combined with regenerative heat exchanger, vapor compression cooling, and evaporative humidifier (hybrid system).