Technical Notes
 Calculating the Pressure Drop of Runtal Radiators

Calculating the pressure drop for Runtal radiators is a simple process, if you keep the following facts in mind:

1. Do not have a flow of more than 1.5 GPM in any one tube of the radiator.

2. Virtually all of the pressure drop in a Runtal radiator occurs in getting the flow of water in and out of the headers at each end of the radiator. There is almost no pressure drop in the tubes between the headers. Therefore, a long radiator and a short radiator (with the same water flow) would have virtually the same pressure drop.

3. The pressure drops shown in the Runtal Pressure Drop Charts are per radiator, NOT per foot of radiator.

4. The total pressure drop is a combination of two components, the radiator pressure drop plus the pressure drop of the pipe connections.

5. There are 0.0244 gallons/tube per foot of radiator.

6. The pressure drops of type RS2 radiators are difficult to calculate. Contact your Runtal representative and Runtal personnel will calculate the pressure drops of RS2 radiators.

Example #1

R-4 Radiator @ 10’-0” with a 170۫ F average water temperature and a 20۫ F temperature drop.

½” type “A” pipe connections. Capacity = 507 BTUH/FT x 10 FT = 5070 BTUH
 
GPM = Capacity = 5070
    (500) x Temp. Drop   (500) x (20)

GPM = 0.507 gpm

From page # TR-50, using 0.507 gpm, the pressure drop is:

0.01 FOH/radiator x (1) radiator = 0.01 FOH
0.01 FOH/pipe conn. x (2) pipe conn. = 0.02 FOH
Total for the R-4 Radiator = 0.03 FOH

Example #2

R-4 Radiator @ 10’-0” with a 170۫ F average water temperature and a 20۫ F temperature drop.

½” type “C” pipe connections. Capacity = 507 BTUH/FT x 10 FT = 5070 BTUH
 
GPM = Capacity = 5070
    (500) x Temp. Drop   (500) x (20)

GPM = 0.507 gpm

From page # TR-60, using 0.507 gpm, the pressure drop is:

0.05 FOH/radiator x (1) radiator = 0.05 FOH
0.01 FOH/pipe conn. x (2) pipe conn. = 0.02 FOH
Total for the R-4 Radiator = 0.07 FOH

Example #3

(3) R-4 Radiators @ 10’, 8’, and 12’ in an opposite end series of radiators, 170۫ F average water temperature and a 20۫ F temperature drop. ¾” type “B” pipe connections.

Capacity = 844 BTUH/FT x (10 + 8 + 12) = 25320 BTUH

GPM = Capacity = 25320 = 2.53 gpm
    (500) x Temp. Drop   (500) x (20)    

From TRF-50, using 2.53 gpm, the pressure drop for the three radiators is:

0.32 FOH/radiator x (3) radiators = 0.96 FOH
0.08 FOH/pipe conn. x (6) pipe conn. = 0.48 FOH
Total for the 3-Radiator Series = 1.44 FOH

Example #4

(3) R-4 Radiators @ 10’, 8’, and 12’ in a same-end series of radiators, 170۫ F average water temperature and a 20۫ F temperature drop. ¾” type “C” pipe connections.

Capacity = 844 BTUH/FT x (10 + 8 + 12) = 25320 BTUH

GPM = Capacity = 25320 = 2.53 gpm
    (500) x Temp. Drop   (500) x (20)    

From TRF-60, using 2.53 gpm, the pressure drop for the 3-radiator same-end series is:

1.21 FOH/radiator x (3) radiators = 3.63 FOH
0.08 FOH/pipe conn. x (10) pipe conn. = 0.8 FOH
Total for the 3-Radiator Series = 4.43 FOH

Example #1

Model Type R
Pressure Drop: Opposite End

RADIATOR PRESSURE DROP - FOOT OF HEAD PER RADIATOR

 

RADIATOR MODEL
NUMBER OF TUBES FOR FLOW

CONNECTION PD  

PER CONNECTION  

STD
PRESSURE
TUBE

 R-1 R-2 R-3 R-4 R-5 R-6 R-7 R-8 R-9 R-10 1/2"
CONN
PD		 3/4"
CONN
PD
1 2 3 4 5 6 7 8 9 10
                           
→ 0.05 0.21 0.05 0.02 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.01 0.01
  0.75 0.45 0.12 0.05 0.03 0.02 0.01 0.01 0.01 0.01 0.01 0.03 0.01
  1.00 0.79 0.21 0.09 0.05 0.04 0.02 0.02 0.01 0.01 0.01 0.04 0.03
  1.50 1.72 0.45 0.21 0.12 0.08 0.05 0.04 0.03 0.02 0.02 0.09 0.04
  2.00 3.00 0.79 0.36 0.21 0.13 0.09 0.07 0.05 0.04 0.04 0.18 0.06
  2.50 4.61 1.21 0.55 0.32 0.21 0.15 0.11 0.08 0.07 0.05 0.25 0.08
  3.00 6.56 1.72 0.79 0.45 0.29 0.21 0.15 0.12 0.09 0.08 0.35 0.10
  3.50 8.83 2.32 1.06 0.61 0.40 0.28 0.21 0.16 0.13 0.10 0.44 0.15
GPM 4.00 11.42 3.00 1.37 0.79 0.51 0.36 0.27 0.21 0.16 0.13 0.56 0.18
  4.50 14.34 3.76 1.72 0.99 0.64 0.45 0.34 0.26 0.21 0.17 0.63 0.20
  5.00 17.57 4.61 2.11 1.21 0.79 0.55 0.41 0.32 0.25 0.21 0.80 0.25
  5.50 21.12 5.54 2.53 1.45 0.95 0.67 0.49 0.38 0.30 0.25 1.00 0.28
  6.00 24.98 6.56 3.00 1.72 1.12 0.79 0.58 0.45 0.36 0.29 1.20 0.33
  6.50 29.15 7.65 3.50 2.01 1.31 0.92 0.68 0.53 0.42 0.34 1.40 0.37
  7.00 33.64 8.83 4.04 2.32 1.51 1.06 0.79 0.61 0.48 0.40 1.50 0.42
  7.50 38.43 10.08 4.61 2.65 1.72 1.21 0.90 0.69 0.55 0.45 1.80 0.50
  8.00 43.52 11.42 5.22 3.00 1.95 1.37 1.02 0.79 0.63 0.51 1.90 0.56

Example #2

Model Type R
Pressure Drop: Same End

RADIATOR PRESSURE DROP - FOOT OF HEAD PER RADIATOR

 

RADIATOR MODEL
NUMBER OF TUBES FOR FLOW

CONNECTION PD  

PER CONNECTION  

STD
PRESSURE
TUBE

 R-2 R-4 R-6 R-8 R-10 R-12 R-14 R-16 R-18 R-20 1/2"
CONN
PD		 3/4"
CONN
PD
R-3 R-5 R-7 R-9 R-11 R-13 R-15 R-17 R-19 R-21
1 2 3 4 5 6 7 4 4 5
                           
→ 0.50 0.21 0.05 0.02 0.01 0.01 0.01 0.00 0.01 0.01 0.01 0.01 0.01
  0.75 0.45 0.12 0.05 0.03 0.02 0.01 0.01 0.03 0.03 0.02 0.03 0.01
  1.00 0.79 0.21 0.09 0.05 0.04 0.02 0.02 0.05 0.05 0.04 0.04 0.03
  1.50 1.72 0.45 0.21 0.12 0.08 0.05 0.04 0.12 0.12 0.08 0.09 0.04
  2.00 3.00 0.79 0.36 0.21 0.13 0.09 0.07 0.21 0.21 0.13 0.18 0.06
  2.50 4.61 1.21 0.55 0.32 0.21 0.15 0.11 0.32 0.32 0.21 0.25 0.08
  3.00 6.56 1.72 0.79 0.45 0.29 0.21 0.15 0.45 0.45 0.29 0.35 0.10
  3.50 8.83 2.32 1.06 0.61 0.40 0.28 0.21 0.61 0.61 0.40 0.44 0.15
GPM 4.00 11.42 3.00 1.37 0.79 0.51 0.36 0.27 0.79 0.79 0.51 0.56 0.18
  4.50 14.34 3.76 1.72 0.99 0.64 0.45 0.34 0.99 0.99 0.64 0.63 0.20
  5.00 17.57 4.61 2.11 1.21 0.79 0.55 0.41 1.21 1.21 0.79 0.80 0.25
  5.50 21.12 5.54 2.53 1.45 0.95 0.67 0.49 1.45 1.45 0.95 1.00 0.28
  6.00 24.98 6.56 3.00 1.72 1.12 0.79 0.58 1.72 1.72 1.12 1.20 0.33
  6.50 29.15 7.65 3.50 2.01 1.31 0.92 0.68 2.01 2.01 1.31 1.40 0.37
  7.00 33.64 8.83 4.04 2.32 1.51 1.06 0.79 2.32 2.32 1.51 1.60 0.42
  7.50 38.43 10.08 4.61 2.65 1.72 1.21 0.90 2.65 2.65 1.72 1.80 0.50
  8.00 43.52 11.42 5.22 3.00 1.95 1.37 1.02 3.00 3.00 1.95 1.90 0.56

Example #3

Model Type RF
Pressure Drop: Opposite End

RADIATOR PRESSURE DROP - FOOT OF HEAD PER RADIATOR

 

RADIATOR MODEL
NUMBER OF TUBES FOR FLOW

CONNECTION PD  

PER CONNECTION  

STD
PRESSURE
TUBE

 RF-1 RF-2 RF-3 RF-4 RF-5 RF-6 RF-7 RF-8 RF-9 RF-10 1/2"
CONN
PD		 3/4"
CONN
PD
1 2 3 4 5 6 7 8 9 10
                           
  0.05 0.21 0.05 0.02 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.01 0.01
  0.75 0.45 0.12 0.05 0.03 0.02 0.01 0.01 0.01 0.01 0.01 0.03 0.01
  1.00 0.79 0.21 0.09 0.05 0.04 0.02 0.02 0.01 0.01 0.01 0.04 0.03
  1.50 1.72 0.45 0.21 0.12 0.08 0.05 0.04 0.03 0.02 0.02 0.09 0.04
  2.00 3.00 0.79 0.36 0.21 0.13 0.09 0.07 0.05 0.04 0.04 0.18 0.06
→ 2.50 4.61 1.21 0.55 0.32 0.21 0.15 0.11 0.08 0.07 0.05 0.25 0.08
  3.00 6.56 1.72 0.79 0.45 0.29 0.21 0.15 0.12 0.09 0.08 0.35 0.10
  3.50 8.83 2.32 1.06 0.61 0.40 0.28 0.21 0.16 0.13 0.10 0.44 0.15

GPM

 4.00 11.42 3.00 1.37 0.79 0.51 0.36 0.27 0.21 0.16 0.13 0.56 0.18
  4.50 14.34 3.76 1.72 0.99 0.64 0.45 0.34 0.26 0.21 0.17 0.63 0.20
  5.00 17.57 4.61 2.11 1.21 0.79 0.55 0.41 0.32 0.25 0.21 0.80 0.25
  5.50 31.12 5.54 2.53 1.45 0.95 0.67 0.49 0.38 0.30 0.25 1.00 0.28
  6.00 24.98 6.56 3.00 1.72 1.12 0.79 0.58 0.45 0.36 0.29 1.20 0.33
  6.50 29.15 7.65 3.50 2.01 1.31 0.92 0.68 0.53 0.42 0.34 1.40 0.37
  7.00 33.64 8.83 4.04 2.32 1.51 1.06 0.79 0.61 0.48 0.40 1.60 0.42
  7.50 38.43 10.08 4.61 2.65 1.72 1.21 0.90 0.69 0.55 0.45 1.80 0.50
  8.00 43.52 11.42 5.22 3.00 1.95 1.37 1.02 0.79 0.63 0.51 1.90 0.56

Example #4

Model Type RF
Pressure Drop: Same End

RADIATOR PRESSURE DROP - FOOT OF HEAD PER RADIATOR

 

RADIATOR MODEL
NUMBER OF TUBES FOR FLOW

CONNECTION PD  

PER CONNECTION  

STD
PRESSURE
TUBE

 RF-2