Figure 4 4 15 Control devices and IO signals Stage 3 We are now in a position

Figure 4 4 15 control devices and io signals stage 3

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Figure 4-4-15 Control devices and I/O signals Stage 3 We are now in a position to draw up the points schedule. This is given in Table El.l.1. The points schedule is now used to select the number and disposition of outstations or UCs. We have a need for nine inputs and 10 outputs. For example, suppose we chose to adopt the Unitron system based on UC16 controllers. The UC16 has eight universal inputs and eight analogue outputs. For the plant considered, we would need two UC16s leaving us with seven spare input points and six spare outputs (the switched voltage binary outputs can be done from the analogue board). Stage 4 The following control logic and functionality can now be identified.
111 a Control — switching x IF (preset run times prevail) THEN (switch fans to run (BO120 and BO130)) THERWISE (off) x IF (frost coil thermostat opens due to low limit (BIl10)) THEN (open heating valve (BOll0)). b Control — positioning x Maintain space temperature (AI130) by controlling the heating coil (valve; AO160), heat recovery device (dampers; AO140, AO150), mixing dampers (AO110, AO120, AO130) and cooling coil (valve; AO170) in sequence. x IF (supply air temperature (AI120) falls outside preset maximum and minimum limits) THEN (this signal shall take priority over space temperature (AI130)). x IF (fresh air enthalpy signal (AIl10) is greater than return air enthalpy signal (AI140)) THEN (position heat recovery device dampers to closed (AO140) and open (AO150)) AND (fresh/return air dampers to minimum (AO130, AOl10) and recirculating damper to open (AO120)). c Status — monitoring x Monitor and store key plant performance parameters likely to be of interest for long-term energy management. For example, fresh air temperature (AIll0), supply air temperature (AI120), space air temperature (AI130) and return air humidity (AI140). d Status — alarm x IF (filter differential pressure switch (BI120) closes) THEN (report alarm condition ‘FILTER EXHAUSTED’). x IF (plant frost protection thermostat (B1l30) opens) THEN (shut down both fans (BOl20, BO130)) AND (close heat recovery device dampers (AO140, AO150)) AND (report alarm condition ‘PLANT SHUT DOWN ON FROST PROTECTION’). x IF (supply fan differential switch (BI140) open) AND (fan is signaled ‘on’ (BOl20)) THEN (report alarm condition ‘SUPPLY FAN FAILURE’). x IF (return fan differential pressure switch (BI150) open) AND (fan is signaled ‘on’ (BO130)) THEN (report alarm condition ‘RETURN FAN FAILURE’). In this chapter we have taken a fairly broad look at many of the practical HVAC control systems; how they are configured and what, in principle, they seek to achieve. These are actually the routine considerations of HVAC control in practice. Having selected a system and identified what we want it to do the fundamental question remains: Will it work? In the next chapter we will move one step closer to equipping ourselves to answer this question by taking a look at the characteristics and selection procedures for the various components that make up a control system.
112 4.7. System Modeling and Multistage Control 4.7.1. Simple room modeling for control analysis (Also for Fan-Coil Unit) T µ U1 T r n r

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