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How To Determine Flow
Rates
The flow rate
through a Runtal radiator (or series of radiators) is
dependent on the length of the radiator (or combined length of
the radiator series), and the design Entering Water
Temperature (EWT) and the design Leaving Water Temperature (LWT).
The designer picks the design EWT and LWT. For example, he
might pick 170°F as the EWT and 150°F as the LWT. The median
point between these two temperatures is called the Average
Water Temperature (AWT), and in this example the AWT is 160°F.
The Runtal Heating Capacity charts are based on the heating
capacity per foot of radiator, based on the designer’s chosen
AWT, and this heating capacity per foot is expressed in units
of BTUH/FT @ a given AWT. The required flow rate (GPM) is
figured as follows:
Flow Rate = (Heating Capacity/Foot X Radiator Length)
¸
[(EWT – LWT) X 500]
The (EWT – LWT) is commonly referred to as the “Delta T”, or “DT”.
Therefore, our Flow Rate formula becomes:
GPM = (BTUH/FT X FT of Radiator)
¸
(DT
X 500)
As an example, let’s say our designer needs 445 BTUH/FT
capacity, over a
10’-0” span of wall, and has chosen the design water
temperatures as EWT = 170°F, and LWT = 150°F. This means our
AWT is 160°F. Looking in the Runtal type “R” radiator Heating
Capacity chart, we see that an R-4 radiator gives us the
required 445 BTUH/FT at 160°F AWT. Therefore, the required
flow rate for the 10’-0” long R-4 radiator is:
GPM = (445 BTUH/FT X 10FT)
¸
(20°F
DT
X 500) = 0.445 GPM
Note that there are various combinations of EWT and LWT that
can result in the same AWT. In our example above, for
instance, a 180°F EWT and a 140°F LWT result in the same 160°F
AWT. With Runtal’s unique flattened water tube design, Delta
T’s of up to 60°F are possible without concern that the flow
rate is too low for heat transfer (see the “Design Tips”
section for more information).
As for a maximum flow rate for Runtal radiators, we recommend
no more that 1.5 GPM per water tube. For our R-4 example
above, this would mean a maximum flow rate of 6 GPM for an
opposite end piped radiator, or 3 GPM for a same end piped
radiator (see the “Design Tips” section for more information).
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