NECB-2017 Article 8.4.4.10
Type of Cooling System:
NECB-2017 Article 8.4.4.10 (1) states that the cooling system serving each thermal block of the reference building shall be determined in accordance with Table 8.4.4.7.A.
A selection of primary cooling equipment based on the details provided in the NECB standard have been provided in the ApacheHVAC NECB reference system library. Where appropriate default information has been assigned to the systems, but these should be reviewed and changed as appropriate to meet the specifics of the building undergoing the analysis.
Energy Type:
NECB-2017 Article 8.4.4.10 (3) states that the energy type of the reference building's cooling system shall be modeled as being identical to that of the proposed building's cooling system.
NECB-2017 Article 8.4.4.10 (4) states that where more than one energy type is used by the proposed building's cooling system
a) the cooling capacities of the reference building's cooling equipment shall match the ratio of the proposed building's cooling equipment capacity allocation, and
b) the operating schedule, priority of use and other operational characteristics of the proposed building's use of energy types shall apply.
The reference systems contain a default energy type of electricity. These should be reviewed by the user to ensure that the fuel type meets the specific configuration of their building.
Part Load Performance:
NECB-2017 Article 8.4.4.10 (5) states that
Cooling equipment performance characteristics as a function of part-load shall be modeled in accordance with the part-load performance curves found in Tables 8.4.4.21.C. to 8.4.4.21.F.
A selection of cooling equipment has been provided in the ApacheHVAC NECB systems library with part load conditions as per Section 8.4.4.21, namely:
· Direct expansion
· Water cooled chillers – scroll, reciprocating, screw & centrifugal
Number and Type of Chillers:
NECB-2017 Article 8.4.4.10 (6) states that
· where the cooling capacity is not greater than 2100 kW, the cooling plant shall be modeled with one water chiller
· where the cooling capacity is greater than 2100 kW, the cooling plant shall be modeled with two water chillers, each having half the capacity of the cooling plant's capacity.
It also states that the types of water chillers shall be modeled as being identical to those of the proposed building.
A chilled water loop has been included by default to match the ‘one water chiller’ requirement. This can be edited to match the requirements specific to the building undergoing the analysis. Note that the default chiller assignment should also be reviewed and updated based on the NECB standard.
It should also be noted that NECB requires a chilled water supply temperature to be set at 7°C with a 6°C temperature rise, both of which are provided by default in the ApacheHVAC NECB chilled water loop.
Chilled Water Pumps:
NECB-2017 Article 8.4.4.10 (6) states that
· the water chiller's pumping system shall be modeled as a primary system with constant speed operation
· the water chiller's pumping flow rate shall be set considering
o the cooling plant's capacity
o use of pure water, and
o a 6°C temperature rise
The default pumping in the ApacheHVAC NECB chilled water loop has been configured as primary only constant speed with the performance curve as per NECB Article 8.4.4.14.
The power demand of the reference pump is dictated by the performance of the pumping in the proposed as per NECB-2017 Article 8.4.4.14:
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Each hydronic pump of the reference building shall have a total static head and efficiency identical to that of the corresponding pump of the proposed building.
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Where the proposed building uses more than one pump in a given hydronic system e.g. a primary secondary arrangement, the peak shaft power demand of the reference building's pump shall be modeled as being identical to the combined peak shaft power demand of the proposed building's pumps.
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Where the total static head or efficiency of the proposed building's hydronic pump is not known, the characteristics of the reference building's hydronic pump shall be based on the peak power demand, in W/(L/s), of the proposed building's pump.
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Pump power, P, versus flow rate, V, shall be calculated using one of the equations shown in NECB-2017 Article 8.4.4.14.(4).
Thus, the specific pump power input required by the VE will have to be entered manually by the user once they know the performance of the proposed cold water pumps or by using the equations in NECB-2017 Article 8.4.4.14.(4).
Direct Expansion:
NECB-2017 Article 8.4.4.10 (8) states that
Where a direct-expansion system is modeled in the reference building, that system shall be modeled as follows:
· the cooling capacity of the system shall be the sum of the cooling loads of the thermal blocks served by the system, multiplied by the applicable oversize factor
· where the cooling capacity of the system is not greater than 66 kW, the system shall be modeled with two stages of equal capacity, and
· where the cooling capacity is greater than 66 kW, the system shall be modeled with a number of stages equal to its capacity divided by 66 kW and rounded up to the nearest integer.
A series of DX systems conforming to the NECB standard have been provided in the ApacheHVAC NECB systems library:
Figure 2: NECB DX units
These can be applied as necessary by the user.
