Chilled Drinking Water Systems
by Mike Frankel, CPD
April 16, 2007
Selection
and sizing of fixtures, piping and insulation are the key design factors to
focus on.
Drinking
water systems provide the fixtures and low temperature water used only for
drinking purposes. This article describes the fixtures, design criteria and the
piping distribution system used to supply and distribute chilled water to all
fixtures of the system.
Applicable standards and codes are the Air-conditioning and Refrigeration
Institute (ARI) Standard 1010 (standard rating conditions) and ARI Standard
1020 (standard rating criteria), as well as the local, ADA and other codes for
the number and type of fixtures.
Types of Fixtures, Accessories and Chiller Assembly
Bottled
Water Unit. Bottled water units are not considered plumbing fixtures
because they have no connection to any plumbing system. These units are
completely self-contained and do not require any electrical connections, a
piped water supply or a drain. Water is supplied from bottles, generally
containing 5 gallons of water, that must be replaced when empty. Water is
discharged from faucets and dispensed into cups. Spills are contained in a receptacle
on the unit under the faucet and must be emptied when full.
Drinking Fountains (DF). Drinking fountains are considered
plumbing fixtures and are directly connected to plumbing potable water and
drainage systems. Water to the unit is supplied either from a remote, central
chilled water unit or ambient temperature water from the potable cold water
system. Water is dispensed under pressure through a bubbler or spout and does
not require a cup. Excess water that is not consumed is piped direct to the
sanitary drainage system.
Electric Water Cooler (EWC). An electric water cooler is a
completely self-contained plumbing fixture consisting of an integral, directly
connected drinking fountain and water chilling unit serving only this unit.
These units are supplied with pressurized ambient temperature water, which is
chilled by the self-contained refrigeration unit. Water is dispensed under
pressure through a bubbler or spout and does not require a cup. Excess water
not consumed is discharged directly into the sanitary drainage system,
generally through a coil that additionally cools the incoming water. If a
refrigeration unit is intended to serve more than one drinking fountain, a
maximum of 10' distance from the unit is recommended.
There are many different accessories and components that can be provided for
many models of drinking fountains, such as glass fillers, hot water dispensers
and foot pedals.
The central chilled water assembly is a mechanical refrigeration unit
specifically designed to chill incoming water and distribute the chilled water
to multiple drinking fountains. It consists of a compressor and condenser,
evaporator, storage tank, recirculation pump and filters.
System Design Concerns
System
design involves determining the number and type of fixtures needed, system
space requirements and sizing the storage tank.
Regarding fixtures, the first step is for the architect to select the location
of the drinking fountains. The applicable plumbing code generally has
requirements for the minimum number of drinking fountains based on the amount
of people served and the type of building. Additional drinking fountains may be
provided for the convenience of the
building occupants.
Free standing units typically require a
minimum of 18" (460 mm) square, with a recess of 30" (760 mm) wide by
24" (600 mm) deep. The chiller pack for an electric water cooler requires a recess under the unit of approximately 24" (600 mm) square and
9" (230 mm) deep. The recess for the drinking fountain varies with the
model selected.
The storage tank is normally sized for one half of the calculated system hourly
demand. For example, if a 100 gph (380 L/hr) demand is calculated, a 50-gallon
(190 L) tank would be selected. The designer should select a standard size tank from the
manufacturer.
Pipe and Insulation Sizing and Selection
The
most common piping specified is copper tube, conforming to ASTM B88, with soldered joints. The pipe should be
insulated with a minimum of 1-inch-thick insulation with a vapor barrier to keep the heat gain to a minimum and to
prevent condensation.
The maximum recommended velocity for circulating water is 3 feet per second
(FPS) or 1 meter per second (1m/sec). The friction loss through the piping
should be limited to approximately 10 ft. (3 m) of head for each 100 ft. (33 m)
of pipe. Dead-end piping should be limited to approximately 10 ft. (3 m) in
length.
System and Component Sizing
General
Criteria. Tests have shown that water at less than 50°F (10°C) is
considered too cold for most people;
furthermore, the flavor buds of the tongue are numbed by the cold. Based
on this study, a slightly higher temperature of discharged water is considered to be appropriate.
The maximum recommended pressure for any system is 125 psig (870 kPa). If the
pressure is higher than 125 psig, a
second zone should be established. There has been a general trend away from
central systems serving many units except for buildings with cores. It is
usually more economical and allows improved maintenance to use groups of four
to six units served by a small chiller.
Sizing Criteria. The standard used by manufacturers for
the capacity of chilled water produced from a single water cooler is based on
ARI standard 1010. This standard uses an inlet water temperature of 80°F
(27°C), a discharge water temperature of 50°F (10°C) with the drinking fountain installed in an area with an ambient
temperature of 90°F (35°C). Table 1 lists standard rating conditions for a
variety of installations. If any conditions are different from the standard,
use Table 2 to determine a factor that will calculate the flow of chilled water
at those non-standard conditions.
The
primary criteria in the sizing process, whether individual electric water coolers or a central system,
is the number of persons served either at a specific location or by the entire
system. Table 3 is the recommended number of persons served per gallon (liter)
of water from a single drinking
fountain. This table is used by determining the approximate number of people
served by the unit and dividing the figure in the table by the number of people
served. The answer will provide the
capacity for the unit.
Table 4 has been developed by ARI standard 1010 and is used to find the number
of people served per hour. A similar table, often found in manufacturers'
literature, may also be used. There is usually a wide discrepancy between
information for offices, schools and hospitals found between Tables 3 and 4.
The use of Table 3 will generally result in one half the size unit than the use
of Table 4. This should be reviewed and evaluated with the selected
manufacturer to arrive at an appropriate figure. Experience has shown
that a value of between 5 and 8 gallons (19 and 30 L) per hour for average
conditions is sufficient for drinking fountains found in these areas.
Individual fluid requirements for various heavy work loads are given in Table
5. This figure should be used if necessary in lieu of any previous figures
calculated. Sufficient unit capacity should be allowed for the required amount
of water replenishment provided at the unit based on the number of people the
unit is serving.
Individual Unit Selection
The
selection process involves the following five steps:
1. Determine the number of units required from where shown on the plans,
required from the code, or as established by the architect.
2. Determine the ambient environment where the units will be installed.
3. Find the specific area where the installation will be made to see if there
is water, drain piping and space necessary for the style of unit to be
installed.
4. Determine children or ADA requirements (for mounting
height).
5. Size the refrigeration requirements for individual units based on Tables 3,
4 and 5, using judgment as to the various conditions encountered.
Central Water Chiller Sizing
To
properly size the central water chiller, follow these
steps:
1. Calculate or obtain the total number of drinking
fountains.
2. Determine a single fountain usage by using Tables 3, 4 and 5, as applicable,
to find the number of gallons per hour required for one
unit.
3. Calculate the total water usage (makeup) for all units served by the central
chiller by multiplying the figure found in step 1 by the figure calculated in
step 2.
4.
Calculate a portion of the chiller cooling load, in Btus, necessary to cool the
makeup water for drinking purposes found in step 3. First, select the inlet
water temperature. Next, use Table 6 by entering the water temperature and read
the makeup heat gain in Btu/hr/per gallon (W/hr/L) for each 100 ft. (30 m) of
pipe.
5. Select a preliminary size for the distribution loop header. The size will be
selected to reduce the friction loss in order to keep the circulation pump
horsepower as low as possible. A general starting point is 1-in. (DN 100) size
line. This is an iterative procedure to find the final pipe size.
6. Calculate the capacity of the chilled water circulation pump. First, select
the gpm of the pump. This is accomplished using Table 7. Using the table with
the pipe size (found in step 5) and ambient room temperature, find the gpm
required to limit the heat loss from the pipe to 5°F (2.8°C) at the
intersection of the two figures. The actual pump selection must overcome the friction
loss of the flow in the distribution loop, which can be found using a standard
engineering text. Selecting the size is an iterative procedure using
manufacturers’ pump curves to select a cost-efficient pump.
7.
Calculate the heat gain from the piping distribution system by using Table 8.
Knowing the pipe size and ambient room temperature, read the Btu per hour (W)
heat gain. To calculate the total heat loss from the pipe distribution system,
use the total measured feet (m) run of piping, in increments of 100 ft. (30 m),
by the heat loss value found in Table 8.
8. Calculate the heat generated by the circulating pump that must be made up.
Use Table 9, entering the circulating pump horsepower from the size selected in
step 6.
9. Calculate the heat gain from the chilled water storage tank. Usually,
1-1/2" fiberglass insulation is selected. This thickness has a
conductivity of 0.13 Btu/ft (0.4W/M). Obtain the area of the tank from the
manufacturer.
10. Calculate the actual central chiller size in Btu/hr (W) by adding the
results of steps 4, 7, 8 and 9. It is suggested that an additional 10% be added
as a safety factor.
References
ASPE Data Book, Chapter 27, Water Coolers, 1994.
Acknowledgements
Elkay/Halsey Taylor Mfg. Co.
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