Objectives

More HVAC systems

- Sorption (absorption) chillers

- Economizers (Psychrometrics) and control

- Desiccant systems (Psychrometrics)

Absorption CycleStrong vacuum

Refrigerant: water

Absorber: lithium bromide

Replace compressor

Absorption Cycle

• Water is typical refrigerant• Strong vacuum - ~0.116 psi or 0.0008 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

Absorption Chillers and CogenerationRankine cycle Cooling cycle

Tc

TH3

4

1

2

or boiler

EconomizerEconomizer (fresh air volume flow rate control)

mixing

damper

fresh

air

T & RH sensors

recirc.

air

Controlled device is damper

- Damper for the air

- Valve for the CC

Economizer – cooling regime

How to control the fresh air volume flow rate?

% fresh air

Minimum for

ventilation

100%

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

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

Sensible and Enthalpy and Desiccant wheels

Desiccant wheel

»Figure 3 – A desiccant-based cooling system combined with regenerative heat exchanger, vapor compression cooling, and evaporative

humidifier (hybrid system).