ASHRAE Journal - October 2021 - 18

TECHNICAL FEATURE
FIGURE 4 Cool day morning operation of a condenser water TIER system.
42°F
CHWS & R
66°F
60°F
80°F
Cooling
Only vs.
Centrifugal
70°F
HWS & R
100°F
130°F
CHW HW CW
cannot satisfy heating demand, trim ASHPs are used to
charge the storage tank.
Figures 4, 5 and 6, which are simplified and adapted
from a project for which we used a condenser water TIER
design, show an example plant in a few typical modes of
operation to illustrate the design concept.* Flow paths
for chilled water (CHW), condenser water (CW) and hot
water (HW) are traced in each.
Figure 4 illustrates a typical cold morning operation
condition during which the TES tank discharges. Both
red heat recovery chillers are in operation, supplying
hot water to the building at 130°F (54°C) on the condenser
side while extracting heat from the TES tank on
the evaporator side. Any cooling loads that the building
might have-e.g., due to 24/7 IT spaces, data centers, lab
equipment, etc.-are concurrently addressed by a blue
variable speed " cooling-only " machine. The condenser
water rejected from this machine, which is 70°F (21°C)
in this example, is then passed through the trim ASHPs,
which act to boost the condenser water charging the top
of the tank to 80°F (27°C). The amount of heat the blue
cooling-only chiller and the ASHPs are adding to the
tank is less than the amount of heat the red heat recovery
chillers are removing from the tank, so on balance
the tank is discharging (decreasing in temperature).
Later during the same day, when heating loads
decrease and cooling loads increase, the net result is that
the tank charges (increases in average temperature).
During the example condition in Figure 5, only one red
heat recovery chiller is providing heating while drawing
energy from the TES tank. Two-cooling only blue chillers
are cooling the building in a series configuration while
head pressure control on the condenser side is modulating
flow through the cooling-only machines' condenser
barrels to achieve the target condenser water leaving
temperature of 80°F (27°C) needed to charge the tank.
The air-source heat pumps are off because BAS logic
has determined that heat rejection loads alone will be
sufficient to charge the tank by the end of the business
day, i.e., bring the tank up to an average temperature of
about 80°F (27°C).
*Astute readers may notice that the TES tank, which is open to the atmosphere, is not hydraulically isolated from the closed-loop chilled
water and hot water systems. This plant was designed for a low-rise campus of two buildings where the tank was readily located near
the high point of the campus to avoid the energy waste that would have otherwise been incurred by introducing pressure-sustaining
valves. When atmospheric TES tanks are used in high-rise applications, they must be installed low to be structurally feasible; hydraulically
isolating them with heat exchangers avoids pressure-sustaining requirements.
18
ASHRAE JOURNAL ashrae.o rg O CTO B E R 2021
CW
Two-Position
Modulating
Pumps
60°F
Heat
Recovery
Screw
80°F
TES Tank
HX
80°F
Trim Heat ASHPs
HR CW
Pumps
Tower
Pumps
Cooling
Towers
Cond
Evap
Cond
Evap
Cond
Evap
Cond
Evap

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ASHRAE Journal - October 2021

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