Example 4 Ð Reading and Changing
Chilled Fluid Set Point
KEYPAD | DISPLAY | COMMENTS | ||
ENTRY | RESPONSE | |||
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| SET POINT | System set points |
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| CSP1 44.0 | Present occupied chilled ¯uid | ||
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| set point is 44.0 F |
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| CSP1 42.0 | Press the | . | |
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| Display shows new occupied | ||
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| chilled ¯uid set point is 42.0 F | ||
| CSP2 44.0 | Present unoccupied chilled ¯uid | ||
| set point is 44.0 F |
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| CSP2 50.0 | Press the | . Display | |
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| shows new unoccupied chilled | ||
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| ¯uid set point is 50.0 F | ||
| RESET | Displays the maximum reset | ||
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| set point. The minimum and | ||
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| maximum reference reset | ||
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| set points can also be displayed. | ||
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| These set points are not | ||
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| accessible when reset type has | ||
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| been con®gured for NONE in | ||
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| the service function. |
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Subfunction displays temperature reset set points.
Temperature Reset Based on Return Fluid Temperature Ð The control system is capable of providing leaving ¯uid tem- perature reset based on return ¯uid temperature. Because the temperature difference between leaving and return tempera- ture is a measure of the building load, return ¯uid tempera- ture reset is essentially an average building load reset method.
Under normal operation, the chiller maintains a constant leaving ¯uid temperature approximately equal to chilled fluid set point. As building load drops from 100% down to 0%, entering cooler ¯uid temperature drops in proportion to load. Thus, temperature drop across the cooler drops from a typi- cal 10 F (5.5 C) at full load to a theoretical 0° F (0° C) at no load. See Fig. 5.
At partial load, leaving chilled ¯uid temperature may be lower than required. If this is allowed to increase (reset), the efficiency of the chiller increases. Amount of reset can be de®ned as a function of cooler temperature drop, as shown in Fig. 5. This is a simple linear function that requires 3 pieces of input data for the set function that will vary de- pending on measurement method used as follows (see Table 11):
NOTE: Reset set points are not accessible unless the reset function is enabled ®rst. This is done as a ®eld
con®guration. Select one of the 4 choices for type of reset: Return Fluid Reset, External Temperature Reset,
If dual set point control is enabled (see Field Wiring sec- tion on page 69), the amount of reset is applied to whichever set point is in effect at the time.
Examples
Activating reset based on external temperature or
Example 5A Ð External Reset
In this example, the unit set point is reset from full load at 90 F (32 C) to a maximum reset value of 10 F (5.5 C) at 20 F (±6.7 C) outdoor ambient.
NOTE: All temperatures given in this example are in F.
KEYPAD | DISPLAY | COMMENTS | |
ENTRY | RESPONSE | ||
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| FLD CFG |
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| CRTYP 0 | Scroll past to reset type | |
| CRTYP 2 | External reset selected | |
| ERTYP 0 | Scroll past to space | |
| thermistor sensor selected | ||
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| ERTYP 1 | OAT selected | |
| RESET |
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| CRT2N 0 | Temperature for no | |
| reset is 0 | ||
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| CRT2N 90 | Temperature for no | |
| reset is 90 | ||
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| CRT2F 0 | Temperature for maximum | |
| reset is 0 | ||
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| CRT2F 20 | Temperature for maximum | |
| reset is 20 | ||
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| CRT2D 0 | Maximum reset is 0 | |
| CRT2D 10 | Maximum reset is 10 | |
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Table 11 Ð Reset Amounts
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| MEASUREMENT METHOD |
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INPUT DATA DESCRIPTION | OAT/Occupied Space | Return Water | |||||
or Internal/External | |||||||
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| Variable | Limits (F) | Variable | Limits (F) | Variable | Limits (F) | |
Maximum Reset Amount Ð Allowable range for | CRT1 | ±30 to 30 | CRT2D | ±30 to 30 | CRT3D | ±30 to 30 | |
maximum amount which LWT is to be reset. | |||||||
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Maximum Reset Reference Ð Temperature at | Ð | Ð | CRT2F | ±40 to 240 | CRT3F | 0 to 15 | |
which maximum reset occurs. | |||||||
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Minimum Reset Reference Ð Temperature at | Ð | Ð | CRT2N | ±40 to 240 | CRT3N | 0 to 15 | |
which no reset occurs. | |||||||
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LEGEND |
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OAT Ð |
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LWT Ð Leaving Fluid Temperature |
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39