Prior to 1935, boilers and water heaters were separate products. Boilers were
of the large volume and mass type as were the water heaters. Around 1935, General
Fittings Corp., Warwick, R.I., developed a heat exchanger that fit inside the
boiler. This eliminated the need for a stand-alone water heater and was the
beginning of the tankless heater design. Since oil costs were in the $.12 per
gallon range, efficiency was not an issue.
Although advertisements promised free hot water during the winter, there was
a cost penalty for keeping the boiler at temperature during the summer. During
the 1950s test laboratories determined that the tankless heaters with the large
volume boilers were about 18% efficient during the summer or non-heating season.
In 1981 Amtrol developed an indirect fired water heater storage tank that answered
the problem of low efficiency and the increasing problem of insufficient hot
water. Boilers that used the tankless heater had to have the nozzle sized for
the DHW rather than for the space heating demand.
On a retrofit, when the indirect fired water heater was installed, the nozzle
size could be reduced, resulting in the heat having more resident time in the
combustion chamber. This increased the boiler efficiency by up to 15%. The low
limit on the boiler was also reduced, again, resulting in lower stand-by losses
and an increase in efficiency with less cycling of the boiler.
Design
The design concepts of the indirect fired water heater consist of improved efficiency
of boiler system, extended boiler control life and provisions of sufficient
hot water.
1. Improved Boiler Efficiency. Boilers can now be designed for low volume
with a firing rate to meet the space heating demand. Due to the small amount
of energy required, added boiler output is not required to handle the domestic
hot water demand in most cases. When they do, the system can be set for priority.
During periods of no space-heating demand, the use of indirect fired water
heaters that add to the boiler water system reduces seasonal efficiencies. The
intent of the indirect fired water heater design is to reduce the system volume,
not to add to it.
2. Extended Boiler Control Life. The high output, low volume boilers
need a flywheel effect to reduce the number of boiler cycles. This is accomplished
by adding an indirect fired water heater storage tank to the system. With a
properly designed heat exchanger in the indirect fired water heater, during
recovery, the boiler should not cycle.
Having to keep the boiler at high temperatures when a tankless or sidearm heater
are used is not only inefficient, but results in the boiler cycling between
a narrow temperature control setting. The heat exchanger for the indirect fired
water heater storage tank can be either in or outside the tank.
In both cases the domestic hot water is stored in the highly insulated tank
that further reduces the number of boiler cycles. Special purging of the boiler
to remove the remaining BTUs is an added feature that increases efficiency and,
at the same time, reduces boiler cycling.
3. Provide Sufficient Hot Water. The sizing of an indirect fired water
heater requires the understanding that there is an initial quantity of stored
hot water that once used will depend on the output of the boiler to meet the
additional demand. The amount of useable stored hot water that can be taken
from the indirect fired water heater is directly related to the tank volume,
demand flow and the cold-water inlet design.
The cold-water inlet is critical in that if not properly designed, mixing will
occur and only a small percentage of the stored hot water will be delivered
at the desired temperature. Amtrol's BoilerMate™ utilizes a specially
designed cold-water inlet that radially dispenses the water into the tank regardless
of flow that results in the minimum mixing of the stored hot water with the
incoming cold water. Up to 87% percent of the tank hot water storage volume
can be utilized at temperature.
The thermostat is positioned in the BoilerMate™ to activate and turn on
the boiler and circulator after 20 gallons have been withdrawn. The total demand
that the BoilerMate™ can satisfy is now dependent on the output of the boiler, the surface
area of the heat exchanger and the heat exchanger design.
The BoilerMate™ heat exchanger has been designed using finned copper material, which
has the greatest heat transfer capabilities due to its thermal conductivity.
The integral finned tubing heat exchanger has been designed to provide counter
flow, which is the most effective heat transfer condition. The thermal circulation
causes the water being heated to travel directly to the top of the tank, resulting
in immediate use of all BTUs being provided by the boiler.
When analyzing the demand required for the system, up to 87% of the tank volume
is immediately available. For example, if an application requires 70 gallons
and the output of one boiler is 100,000 Btuh, a BoilerMate™ storage tank
that has 80 gallons in volume with no recovery will handle the demand. If the
demand is repeatable over a period of time, the time to recover becomes important.
Continuing with the above example, the following equations can be used to determine
the time to recover.
(t) Time (minutes) = BTUs to reheat x 60
Boiler output (BTU/HR)
Using the basic definition of a BTU as the raising of one pound of water 1 degree
F, a change in temperature of 90 degrees F and if there is 8.25 pounds per gallon,
the time (t) can be determined.
t = 70 x 8.25 x 90 x 60 = 31 minutes
100,000
If the next equal (70-gallon) will take place in less than this time, added
storage is required.
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Joseph Lane is Technical Marketing Manager, Amtrol Inc., Warwick,
R.I. He can be reached at (800) 736-1149, ext.250
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