tekmar 365 Mixing Control - Manual (Page 4)

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tekmar has developed two significantly different ways of piping variable speed injection pumps for small commercial and residential
hydronic heating systems. Each method has its advantages and disadvantages, and designers should read the tekmar essay E 021
thoroughly in order to correctly choose the best arrangement for their particular application.
The variable speed injection pump should be sized for full load heat transfer at design conditions. Calculations reveal that in most typical
residential and small commercial applications the smallest circulators are of sufficient size and in many cases exceed the maximum
required GPM rating. If an appropriate pump size is not available, a larger pump may be used provided a balancing valve is included to
reduce flow through the transfer loop.
Plumbing Arrangements
Pump Sizing
Reverse Injection
To calculate the required size of the injection pump:
F1 = System Supply flow rate in US GPM
T1 = Hot Loop (Boiler) supply temperature available
T2 = Low Temperature (System) Supply temperature
∆Ts = Low Temperature (System) temperature drop (T2 – Tr)
Note: All values are to be given at design conditions.
Reverse Injection
Reverse injection requires that the water from the boiler loop
is injected into the low temperature loop upstream of the
return to the boiler loop. Mixing occurs directly after the point
of injection. Since some of the mixed water is then returned
back to the boiler loop, higher injection flow rates are required
than in direct injection systems.
F 1 x
∆
Ts
T 1 - T 2
M a x i m u m
Variable Flow
Values at
Design
Conditions
Are:
(Fv) =
This example illustrates an important point to consider when designing variable speed systems. The hotter the maximum boiler supply
temperature is designed for, or the cooler the maximum system supply temperature is designed for, the less injection flow is required. Quite
large systems can be designed with relatively small injection pumps when this is kept in mind.
Variable
Speed
Injection
Pump
Supply To Low
Temperature Loop
Supply From High
Temperature Loop
T 1
T 2
F V
F V
T r
F 1
F 1
∆T ∆T
s "REVERSE" INJECTION
Sample Calculation
T1 = Boiler Supply = 180
°
F
T2 = System Supply = 130
°
F
∆
Ts = System
∆
T = 25
°
F
F1 = System Flow = 10 GPM
Fv = = = = 5 GPM
F 1 x
∆
Ts 10 x 25 250
T 1 - T 2 180 - 130 50
Sample Calculation
T1 = Boiler Supply = 200
°
F
T2 = System Supply = 120
°
F
∆
Ts = System
∆
T = 20
°
F
F1 = System Flow = 60 GPM
Tr = System Return = T2 –
∆
Ts = 100
°
F
Pump Sizing
Direct Injection
To calculate the required size of the injection pump:
F1 = System Supply flow rate in US GPM
T1 = Hot Loop (Boiler) supply temperature available
T2 = Low Temperature (System) Supply temperature
Tr = Low Temperature (System) Return temperature
∆Ts = Low Temperature (System) temperature drop (T2 – Tr)
Note: All values are to be given at design conditions.
Direct Injection
Direct injection requires the water from the hot loop to be
injected into the low temperature loop so that the heat rise and
the mixing occur directly after the point of injection, down-
stream of the return to the hot loop.
F 1 x
∆
Ts
T 1 - Tr
(Fv) =
M a x i m u m
Variable Flow
Values at
Design
Conditions
Are:
Fv = = = = 12 GPM
F 1 x
∆
Ts 60 x 20 1200
T 1 - Tr 200 - 100 100
Variable
Speed
Injection
Pump
Supply To Low
Temperature Loop
Supply From High
Temperature Loop
T 1 T 2
F V
F V
T r
F 1
F 1
∆T ∆T
s "DIRECT" INJECTION
For more details on variable speed pumping, refer to tekmar essay E 021