HVAC Control Systems

Announcements End of the class course evaluation Final project presentation Monday (5/8/17) at 10 a.m. Send me the ppt file before 9:45 a.m. Objectives Learn about automatic control PID and Control terminology Sequence of operation Control optimization Course summary

The Real World 50% of US buildings have control problems 90% tuning and optimization 10% faults 25% energy savings from correcting control problems Commissioning is critically important Practical Details

Measure what you want to control Verify that sensors are working Integrate control system components Tune systems Measure performance Commission control systems Modulating Control Systems Example: Heat exchanger control Modulating (Analog) control Cooling coil

air x water (set point temperature) Modulating Control Systems Used in larger systems Output can be anywhere in operating range Three main types Proportional PI

PID Electric (pneumatic) motor Position (x) fluid Volume flow rate Vfluid = f(x) - linear or exponential function Proportional Controllers x A K (Tset point Tmeasured ) x is controller output A is controller output with no error (often A=0)

Kis proportional gain constant e = Tset point Tmeasured is error (offset) Unstable system Stable system Issues with P Controllers Always have an offset But, require less tuning than other controllers Very appropriate for things that change slowly i.e. building internal temperature

Proportional + Integral (PI) K x A K (Tset point Tmeasured ) (Tset point Tmeasured )d Ti K/Ti is integral gain If controller is tuned properly, offset is reduced to zero Figure 2-18a

Issues with PI Controllers Require more tuning than for P But, no offset Proportional + Integral + Derivative (PID) Improvement over PI because of faster response and less deviation from offset Increases rate of error correction as errors get larger But HVAC controlled devices are too slow responding Requires setting three different gains

Ref: Kreider and Rabl.Figure 12.5 The control in HVAC system only PI x K (Tset point Tmeasured ) K (Tset point Tmeasured )d Ti Proportional Integral

value Set point Set point Proportional affect the slope Integral affect the shape after the first bump HVAC Control

Example 1: Economizer (fresh air volume flow rate control) Controlled device is damper damper fresh air - Damper for the air - Valve for the liquids mixing recirc.

air T & RH sensors Economizer % fresh air Fresh air volume flow rate control enthalpy damper Fresh (outdoor)

air TOA (hOA) mixing Recirc. air T & RH sensors 100% Minimum for

ventilation 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? % fresh air 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.

100% Minimum for ventilation 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 HVAC Control Example 2: Dew point control (Relative Humidity control) damper fan fresh filter cooling heating

air coil coil filter mixing T & RH sensors Heat gains Humidity generation We should supply air with lower humidity ratio (w) and lower temperature We either measure Dew Point directly or T & RH sensors substitute dew point sensor

Relative humidity control by cooling coil Cooling Coil Mixture Supply TDP Room Heating coil Relative humidity control by cooling coil (CC)

Cooling coil is controlled by TDP set-point if TDP measured > TDP set-point send the signal to open more the CC valve if TDP measured < TDP set-point send the signal to close more the CC valve Heating coil is controlled by Tair set-point if Tair < Tair set-point send the signal to open more the heating coil valve if Tair > Tair set-point send the signal to close more the heating coil valve Control valves Fresh air mixing cooling coil

heating coil Tair & TDP sensors Sequence of operation (ECJ research facility) Mixture 3 Set Point (SP) Mixture 1

DPTSP Mixture 2 Control logic: DBTSP Mixture in zone 1: IF (( TMTSA) increase heating or IF (TSPDPTSA) increase humidifying or IF (DPTSPTSP) & (DPTMTSA) decrease cooling

Humidifier: IF (DPTSP>DPTSA) increase humidifying or IF (DPTSPDPTSP) ) cooling/dehumidifying and reheatin Cool. coil cont.: IF (DPTSP>DPTSA) increase cooling or IF (DPTSPTSA) increase heating or IF (TSP

UTs CHP Course Summary Course Objectives: 1)Learn about advanced building HVAC systems 2)Obtain knowledge about district cooling and heating systems. 3)Gain the skills and tools necessary to evaluate integration of sustainable energy production systems to a given building site. 4)Study application of combined heat and power systems in a specific building or group of buildings. 5)Conduct thermal, hydraulic and economic modeling of integrated building energy systems for planning and design. Course Topics:

Class intro and HVAC systems 1.Building ventilation heat recovery systems 2.Thermal (solar and waste heat) powered desiccant systems 4. Centralized (compressor and sorption based) cooling systems 5. Centralized heating systems 6. District heating and cooling distribution systems 7. Combined heat and power systems 8. Systems integration and control

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