Flash Tank To Condensate Pump Cooling Leg Analysis

For many installations it is advisable to have a cooling leg between the high-pressure steam system flash tank and its associated condensate pump. Though expensive cooling radiation devices can be utilized or you can utilize special high temperature condensate pump seals, in many cases, the following type of cooling leg will suffice.

To start our analysis we will assume a system with 7000 #/hr of condensate leaving the flash tank at 210^oF, (be aware that in an undersized flash tank the condensate may leave at 215^oF or higher and actually flash in the cooling leg on the way to the condensate pump).  It is advisable to provide a length of the cooling leg to the condensate pump so as to cool the condensate to approximately 180 to 200^oF.  In this case then, the cooling range of the condensate that should take place in the cooling leg will be 30^oF.  The analysis should then go as follows:

Eq. 1
H = (m) (s) (T)
M = mass of condensate
s = specific heat of water

H = 7000 #'s (30T)
H = 210,000 btu/hr

Eq. 2
H = A U T
T = 210 -120^oF = 90^oF

In this case the 120^oF is taken as a worse case for want the maximum room temperature might be in the steam equipment room.

h1 = Conductance of the inside surface film
h2 = Conductance of the outside surface film
Using steel pipe, k = 360, x = .23 = pipe thickness

With new pipe you can figure that h1 and h2 due to scale and corrosion will be equal to infinity, since the films will be very small or nonexistent. This will change with time, but as you can see latter on, the various safety factors that are used will more then take aging into consideration.

With the steel pipe in our example:

U_steel = 360/.23 = 1560
Area = BTUH(U)(T)
Area = 210,000/1560(90)

Area = 1.5 sq. ft., required to dissipate the heat within the condensate to levels that would provide the required cooling effect.

If you select (1 ½") Sch 40 pipe the initial leg length is selected as follows:

• 1.5 sq. ft. Divided by .497 ft²/ft = 3.2 feet, say 4 feet.
• In 1 ½" pipe the velocity caused by the 7000 #/hr condensate flow is approximately 2.2 ft/sec, with about a P = 1.5" per 100 feet.

To allow for proper mixing of the condensate and to apply safety factors to account for future buildup of scale, etc. within the inside of the pipe a factor of about three (3) should be used to come up with the final length.

Of course the actual length need not be straight it could be a simple loop or grid or other form that conveniently fits in the space you have available. In this case, utilizing a factor of three will result in a total recommended installed length of approximately 12 feet.

It would further be advisable to check for a proper mixing Reynolds number as a final determination that you are providing the proper amount of mixing.

The equation will be as follows:

Eq. 5 f = (h_f)(d)(2g)/(l)(v²)

Eq. 6 f = (d)(2g)/V² P

        f = .0255

Referring to a Moody chart you will get an e/d = .00112, with a Re = 60,000. This is an excellent value for this type of usage and indicates very good mixing which will result in a good heat transfer process.

Return to Technical Reports