ASHRAE Journal - February 2015 - 41

COLUMN ENGINEER'S NOTEBOOK

ground. Table 1 compares various considerations for both aboveground and belowground TES tanks.2

Hydraulic Integration of TES Tanks

TABLE 1 TES tank location considerations.
CONSIDERATION

BELOWGROUND TES

ABOVEGROUND TES

Site Space Use
Site Aesthetics
Sensitivity to Site Soil Conditions
TES Tank Inspection
TES Tank Costs
TES to CHW System
Hydraulic Differential
TES Tank Pumping Energy

Can Be Preferable
Out of Site Out of Mind
Very sensitive
More difficult
Higher

Can Be Less Preferable
Visible
Less Sensitive
Less Difficult
Lower

Chiller(s)

Atmospheric CHW TES tanks are
typically hydraulically interconnected to
Higher
Lower
chilled water systems by one of the followHigher
Lower
ing common methods:
* Integrating the TES tank within the
CHWR
chiller plant, using the plant's primarysecondary CHW pumping, or
* Siting the TES tank remotely from the
VFD
chiller plant while using dedicated TES
CHW
pumps
TES
When the TES tank can be located next
Tank
to the chiller plant, it can be connected
between the plant's primary and secondary chilled water loops without adding
additional TES pumps as shown in Figure
CHWS
1. The TES tank's upper region of warm
VFD
water connects with the CHWR header
FIGURE 1 Local TES tank.
while the lower region of cold water connects with the CHWS header. Whenever
CV-5
CHWR
the plant's primary CHW flow exceeds
the secondary CHW flow, the TES tank
is charging. When the plant's secondary
CV-2
CV-1
CHW
CHW flow exceeds the primary CHW flow,
TES
the TES tank is discharging. This method
Tank
VFD
of connection is typically the simplest and
CV-2
CV-4
least expensive means of connecting the
CHWS
chilled water TES tank because it allows
CV-6
for charging and discharging, separately
FIGURE 2 Remote TES tank pumping.
or simultaneously, without the use of
automatic control valves to reroute flow.
When the TES tank needs to be remotely located from
automatic valves used to select operating mode as shown
chiller plant(s), a TES pumping station would be required
in Figure 2. This allows the remotely sited TES tank to act
to pump both in/out of the TES tank. The TES tank connects as a load when charging and act like a chilled water source
to the CHWS and CHWR headers similar to above; however, when discharging. Control valves, CV-1 through CV-4 are
the water must always be pumped from the atmospheric
normally two-position valves. CV-5 and CV-6 would be
pressure of the TES tank into higher-pressure chilled water two-position if the TES tank is highest point in the chilled
headers. During discharge, the cold water is pumped from
water system. If the tank is located lower than the chilled
the lower region of the tank into the CHWS header. During
water system piping, CV-5 and CV-6 could modulate to
TES charging, the warm water is pumped from the upper
control back pressure.
region of the tank into the CHWR header. The same set
The objectives of the TES tank pumping system during
of TES pumps are generally used for both charging and
TES tank charging are:
discharging, with interconnecting cross-over piping with
* Pump the proper amount of chilled water to load
FEBRUARY 2015

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