ASHRAE Journal - November 2019 - 69

COLUMN ENGINEER'S NOTEBOOK

water supply (CHWS) temperature with 110°F (43°C)
heating hot water supply (HHWS) temperature might
result in a COPcombined of 8.4. The same model selected
for 44°F (7°C) CHWS with 140°F (60°C) HHW might
result in a combined COP of 5.6.
Heat recovery chillers can be provided in many different conﬁgurations. A modular HRC consists of multiple
modules connected to a single header with compressors
in each module. Modules and compressors can be staged
to meet the load while internal automatic isolation
valves can isolate non-operating modules in variable
ﬂow applications. These types of chillers use plate-andframe heat exchangers for the evaporators and condensers and require a good water treatment program with
ﬁne mesh strainers to ensure the reliability of the heat
exchangers. Modular HRCs also have the option of providing an air-source/air-cooled heat exchanger when
heating and cooling loads do not match.
Non-modular HRCs have the beneﬁt of conventional
evaporator and condenser barrels with copper tubes to
allow for tube cleaning but will generally not provide the
low load operating efﬁciency of modular heat recovery
chillers, especially centrifugal chillers where high lift
at low load can lead to surge conditions solved, inefﬁciently, by use of hot-gas-bypass. These can sometimes
be the best option if the project owner cannot maintain
the water treatment program required for the modular
HRCs.
The reasons for using heat recovery chillers can vary.
A project may desire to eliminate combustion sources
for heating and therefore use all-electric, 100% heating
from HRCs, perhaps driven by California's electriﬁcation goals1 where heating generated by electrical sources
is preferred environmentally to fossil fuel sources. New
buildings and major renovations can select the building
hot water heating coils for lower heating hot water supply temperatures to optimize efﬁciency.
Another project may desire to provide most of the
annual heating with HRCs for low-load, low-temperature heating hot water on older buildings that were
designed with higher heating hot water temperatures
to reduce carbon emissions from gas-ﬁred boilers.
Retroﬁt projects in existing buildings may still need the
capability of the gas-ﬁred heating hot water systems to
provide the original design hot water supply temperatures at peak loads with existing one-row heating coils.
In the author's experience, HHWS temperature setpoint

reset controls can substantially reduce annual heating
energy losses. Generally, one-row heating coils selected
at 180°F (82°C) can provide roughly 30% to 40% of the
peak capacity when the supply temperature is reduced
to 110°F (43°C). The existing coils can be modeled at the
lower temperatures to determine their heating capacity
when using lower hot water supply temperatures.
It is critical to understand the cooling and heating
load proﬁles for the project when deciding how to size
and select heat recovery chillers. Standalone building
chilled water systems do not have a means of energy
storage in a campus system and will require an understanding of the simultaneous building cooling and
heating load proﬁles throughout the year to select the
best HRS size to optimize life-cycle cost and to minimize dumping heat when it is not needed. Clearly, if
there are few hours where simultaneous heating and
cooling occur, as might be the case with well-designed
HVAC systems, HRCs will not be cost effective and
should not be considered. However, if the HRC is connected to a continuously operated chilled water plant,
then simultaneous heating and cooling need not occur
within the building itself; heating can be provided to
the building while cooling is provided to other buildings served by the central plant.
Heat recovery chillers that provide only simultaneous heating and cooling are the lowest cost per ton
(kW) while HRCs with auxiliary air-source/air-cooled
heat exchanger can be twice the cost per ton (kW). The
projected electrical and gas rates, including demand
charges (where applicable), for the project location can
have a substantial impact on the life-cycle cost of individual projects.

The Art of Piping Diagrams
The integration of HRCs into the design can add new
complexity into the piping, controls, and operation of
the system. Easy to understand logical piping diagrams
can be a useful tool for evaluating and communicating different piping arrangements and control options.
The goal of the piping diagram is to convey the ﬂow
logic of the connected components. It should be easy for
the operator to determine what equipment is piped in
series and what equipment is piped in parallel. Piping
diagrams with multiple pipes crossing each other can be
difﬁcult for others to understand the ﬂow logic.
The author uses the following simple rules for
N O V E M B E R 2 0 19

ashrae.org

ASHRAE JOURNAL

http://www.ashrae.org

ASHRAE Journal - November 2019

Table of Contents for the Digital Edition of ASHRAE Journal - November 2019