Post on 13-Mar-2018
transcript
Supply Air Temperature and Pressure Setpoint Reset in VAV Systems Based on Zone Demand
Steven T. Taylor, PEReinhard Seidl, PE Taylor Engineering Alameda, CA
Outline
Supply Air Temperature and Static Pressure Setpoint Reset in VAV Systems •Control Strategies•Energy impacts•Control SequencesCommissioning•Tuning• “Rogue” Zones
Static Pressure Setpoint Reset
Fan Energy at Varying SP Setpoints
0%
20%
40%
60%
80%
100%
120%
0 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Percent Fan CFM
Perc
ent F
an k
W SP setpoint = TSPSP setpoint = TSP*.75SP setpoint = TSP/2SP setpoint = TSP/3SP setpoint = 0
Surge Region
Real Fan Systems Many Hours At Low Loads
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0.5
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1.5
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0 20,000 40,000 60,000 80,000 100,000 120,000 140,000Total CFM (SF-1 + SF-2)
Pres
sure
Dro
p A
cros
s Fa
n ("
H2O
)
Actual DataPerfect System Curve (i.e. SP reset)Design Point (145,000 CFM at 4.0")Likely Actual System Curve (No SP Reset)
Static Pressure Setpoint Reset Control Logic Options
Option 1: Reset static pressure setpoint based on PID Loop on VAV damper positionOption 2: Reset static pressure setpoint using “Trim & Respond”based VAV damper position or airflow demand
Best Choice: Trim and Respond
Advantages•Easier to tune•Can “respond” more quickly than
“trim”•Does not require knowledge of
damper position•Easier to ignore “rogue” zones
Trim & Respond SP Setpoint Reset Logic
Static pressure setpoint shall be reset using trim and respond logic within the range 0.15 inches to 1.5 inches. When fan is off, freeze setpoint at the minimum value (0.15 inches). While fan is proven on, every 2 minutes, decrease the setpoint by 0.04 inches if there two (adjustable) or fewer pressure requests. If there are more than two (adjustable) pressure requests, increase the setpoint by 0.04. Where VAV zone damper position is known, a pressure request is generated when any VAV or underfloor damper served by the system is wide open. Where VAV zone damper position is unknown, a pressure request is made when the ratio of the zone’s actual supply airflow to supply airflow setpoint is less than 90%. Adjustable to limit
dominance of rogue zonesSet by TAB Contractor
Trim & Respond SP Setpoint Reset Logic
Example of heating fan on dual duct system, without calls for heating.
System runs close to minimum: 75°F for temperature SP and 0.24”wg for pressure SP.
Note that actual temperature is much higher than SP, this is because of the mechanics of the burner staging, not because of the reset logic.
Trim & Respond SP Setpoint Reset Logic
Example of heating fan on dual duct system, with calls for heating. Note that calls for heat (3) and calls for pressure (3) coincide here, but don’t necessarily do this.
System runs at maximum: 95°F for temperature SP and 1.5”wg for pressure SP.
Note that, again, actual temperature is much higher than SP, this is because of the mechanics of the burner staging, not because of the reset logic.
Variations on this Sequence
A damper position (valve position, loop output, etc.) of 80% counts as one request, while a position of 95% counts as two requests.The response rate is a function of the number of requests, e.g. response = 0.04” times the number of requests.
Tuning – Unstable Control
y = 0.6709x + 0.2656R2 = 0.5971
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0 0.2 0.4 0.6 0.8 1 1.2 1.4
Actual Static Pressure
Stat
ic P
ress
ure
Setp
oint
Tuning – Stable Control
Supply Air Temperature Reset
Supply Air Temperature Control Optimization
Analyzed for:• Mild California climates• Chilled water plant at building (not central)• Variable speed fans
Probably applicable to more extreme climates as well• Humid climates should use low end of
outdoor air temperature reset ranges
Impact of SAT on Energy Usage Sacramento Office Building
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Sou
rce
Ene
rgy,
B
tu/s
f
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Out
door
Air
Tem
pera
ture
, F
50 55 60 OAT
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Sou
rce
Ene
rgy,
B
tu/s
f
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Out
door
Air
Tem
pera
ture
, F
In very hot weather, increased fan energy more than eliminates savings in compressor energy (green line above blue line)
In cool weather, supply temperature reset saves significant energy (longer use of economizer and no excessive fan energy as result of low SAT)
Recommended SAT Reset Logic
Reheat Economizer SAT Logic
None None Constant SAT at design (~55ºF)
Low(e.g. DFDD,
FPU)Yes
Constant SAT at design. Allow to float up by locking out cooling until zone request indicates need for CHW
Medium to High
(e.g. VAVRH)
Yes
Reset by zone cooling demand during cool weather (<55ºF to 60ºF), then ramp down to design SAT in warm weather (>60ºF to 70ºF)
Reheat System SAT Reset Logic
55 65
55
70
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75
65
Outdoor Air Temperature
Supp
ly Ai
r Tem
pera
ture
Set
poin
t
Tmin = 53F
SAT reset by demandwithin this range
VAV Reheat System SAT Reset Logic
During occupied mode, the setpoint is reset from T-min (53°F) when the outdoor air temperature is 70°F and above, proportionally up to T-max when the outdoor air temperature is 60°F and below. T-max shall be reset using trim and respond logic within the range 55°F to 65°F. When fan is off, freeze T-max at the maximum value (65ºF). While fan is proven on, every 2 minutes, increase the setpoint by 0.2ºF if there are two (adjustable) or fewer zone cooling requests. If there are more than two (adjustable) cooling requests, decrease the setpoint by 0.3ºF. A cooling request is generated when the cooling loop of any zone served by the system is >99%. Lower if reheat reduced
using “dual maximum” logic and central plant
Adjustable to limit dominance of rogue zones
Performance – Oversized Zones
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Outdoor Air Temperature (oF)
Sup
ply
Air
Tem
pera
ture
Set
poin
t (oF
)
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7/280:00
7/2812:00
7/290:00
7/2912:00
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7/3112:00
8/1 0:00 8/112:00
Tem
pera
ture
(oF)
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1
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ber
of C
oolin
g R
eque
sts
SAT SetpointOutdoor Air TemperatureCooling Requests
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Outdoor Air Temperature (oF)
Sup
ply
Air
Tem
pera
ture
Set
poin
t (oF
)
Performance – Rogue Zones
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52 54 56 58 60 62 64 66 68
Outdoor Air Temperature (oF)
Supp
ly A
ir T
empe
ratu
re S
etpo
int (
oF)
Rogue Zone
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6/27/06 3:00 6/27/06 6:00 6/27/06 9:00 6/27/06 12:00 6/27/06 15:00 6/27/06 18:00 6/27/06 21:00
Cool
ing
Loop
Out
put (
%)
VAV2-20 VAV2-21 VAV2-11 VAV2-22 VAV2-23 VAV2-24 VAV2-25VAV2-27 VAV2-30 VAV2-31 VAV2-6 VAV2-7 VAV2-28 VAV2-9
Rogue Zones
Causes• Undersized zone/unexpected loads• Thermostat setting too low• Duct design problem – high pressure drop
fitting or duct section
Handling Rogue Zones
Exclude them from reset sequence• May result in temperature problems in
these zones• Not advised for boss’ office
Limit thermostat setpoint adjustmentsFix duct restrictions/sizing issuesAdd auxiliary cooling to augment VAV
Supply Air SP vs Duct Static SPHighest temp. reset occurs at warmest point of the day. Note that pressure remains high almost all day, possibly rogue zone
Pressure setpoint (blue)
Temp. setpoint (red)
Outside Air Temp. (green)
Supply Air SP vs Duct Static SP
Same building, next day – completely different pictureboth resets now working – might look like they are running in “counter-phase”, but incidental
Pressure setpoint (blue)
Temp. setpoint (red)
Outside Air Temp. (green)
Supply Air SP vs Duct Static SP
Same building, next day – now both resets almost run “in phase”
Pressure setpoint (blue)
Temp. setpoint (red)
Outside Air Temp. (green)
Poor Supply Air Temp controlActual temperature does not adhere to setpoint
Actual temperature (green)
Temperature setpoint (orange)
Poor Supply Air Temp controlTemp. setpoint does not appear to follow prescribed logic, and actual temperature does not adhere to setpoint)
Actual temperature (green)
Temperature setpoint (orange)
Temperature setpoint per sequence of operations (red)
Poor Supply Air Temp controlPressure setpoint (blue)
Temp. setpoint (red)
Actual Temp. (orange)
Outside Air Temp. (green)
Pressure control
Number of pressure requests – when these drop below 5 requests, dP setpoint is lowered
Pressure setpoint (signal stays high even when requests drop, because more than 5 requests)
Pressure control
VFD speed drops to maintain pressure after initial system start, then builds again as VAV’s open after 9AM, then drops as pressure setpoint is reduced.
Pressure setpoint signal stays high (>5 requests)
After initial morning start, VFD speed and pressure setpoint go hand in hand
Summary
Static pressure setpoint reset• Use Trim & Respond SP setpoint reset logic based on
damper position or airflow demandSupply air temperature setpoint reset• Not effective for all systems and climates• Use combination of Trim and Respond SAT reset and
outdoor air temperature reset to minimize fan/cooling/reheat energy use
Commissioning• Tuning is essential for stable operation• Review trend data of normal operation to identify (and fix)
rogue zones