• Q
cmin
= Q
c
x 860 x C
min
• G is the unit’s minimum chilled water flow rate (in m
3
/h)
Heating mode limit on chilled water system water volume
To prevent occupants experiencing cold drafts during defrosting, system design should ensure that the leaving water temperature at the end
of the defrosting operation does not fall below 15⁰C (when the ambient temperature is 1⁰C and the leaving water teperature before starting
defrosting is 30⁰C) or does not fall below 20⁰C (when the ambient -7⁰C and the leaving water temperature before staring defrosting is 35⁰C).
Defin
ing Q
h
as the unit heating capacity (in kW) adjusted for ambient temperature and leaving water temperature, the required minimum water
W
ct
= (Q
d
+ Q
s
) / ΔT
t
Where:
•
W
ct
is the required water volume in liters
•
Q
d
is the heat energy (in kcal) required for defrosting, which may be taken to be 12% of hourly unit output, such that:
• Q
d
= Q
h
x 860 x 0.12
Equation 5
Equation
6
From equation 1:
Water tank design volume
= Chilled water system minimum water volume
- Water pipework volume
- Chiller heat exchanger volume
- FCU heat exchanger volume
Two factors place lower limits on the chilled water system required water volume:
1. Cooling mode limit: To ensure reliable system operation, the chilled water system water volume should be sufficient to
prevent frequent system start/shutdown when operating in cooling mode under relativel
y low ambient temperatures.
Typic
ally systems should be designed such that the system runs for a minimum of 5 minutes following each system start
up in cooling mode.
2.
Heating mode limit: To ensure occupant comfort during the defrosting operation, the water system water volume should be
sufficient to ensure that the leaving water temperature at the end of the defrosting operation has not fallen so low that occupants
could experience cold drafts.
Cooling mode limit on chilled water system water volume
The cooli
ng mode lower limit for the chilled water system water volume is given by:
W
c
= Q
c5
/ (ΔT
s
– (Q
cmin
/ (G x 1000 )))
Where:
• W
c
is the required water volume (in liters)
• Q
c5
is the unit’s cumulative cooling output (in kcal) when running at the unit’s lowest operating speed for 5 minutes less
the cumulative cooling output required to maintain the conditioned spaces at their set temperatures during that time, which
can
be obtained from:
• Q
c5
= Q
c
x 860 x (C
min
– R
min
) x (5/60)
• Where:
• Q
c
is the unit’s cooling capacity (in kW) corrected for ambient temperature (using the lowest
expected cooling mode ambient temperature) and leaving water temperature
• C
min
is the ratio of unit output at its lowest operating speed to its output at maximum operating
speed
• R
min
is the cooling output required to maintain the conditioned spaces at their set temperatures,
as a pro
portion of Q
c
• ΔT
s
is the unit’s deadband temperature range (in ⁰C).
• Q
cmin
is the unit’s output (in kcal/h) when operating at its lowest speed and corrected for ambient temperature
and leaving water temperature, which can be obtained from:
Equation
2
Equation 3
Equation 4
volume is given by:
38
To Next Page