Determining steam required for pick heater

[abamin]

I'm trying to determine the specifications of a pickheater (direct steam injection heater) we are planning on purchasing and am new to the world of pickheaters.

The pickheater is going to be used to heat incoming water from ambient (~70F) to about 150F. The steam available is a 150 psig steam. The water, after leaving the pick heater, should have a flow rate of 75 gpm. I'm trying to detemine the amount of steam required to achieve this...I'm sure it's a basic q=m*cp*dt plus a mass balance type answer but i'm having trouble with actually putting 2 and 2 together. Any help would be appreciated. Thanks!

 

[FMJalink]

Abamin,
The main equations are:

Mass balance in mass flows:
m_water_in + m_steam_in = m_water_out

Energy balance is basically:
Cp_steam x m_steam_in x (T_steam_ in – (T_water_in + T_water_out)/2 ) + dH_condensation x m_steam_in + Cp_water x m_water_in x T_water_in = Cp_water x m_water_out x T_water_out.

Note that the Cp of water and steam vary with the temperature. To calculate more exactly the Cp on the different temperatures should be used and the method would become iterative.

The pressure of the steam should be controlled and set to a specific value. The pressure difference between steam at inlet and the water determines the steam flow. This is apparatus dependant. Vendors of direct steam injection heaters have normally simple charts showing these values.

Normally they also can provide charts showing the steam requirement in order to achieve a specific heating of the water flow.

 

[rmw]

Pay attention to whether or not your 150 PSI steam is superheated or not. That will be necessary to determine your T steam in.

You may be having a hard time trying to solve around your outlet flow. Do you know your inlet flow? It would be simpler to solve for a resulting outlet flow, not that it can't be done your way.

rmw

 

[abamin]

rmw: do you mean it would be easier to solve for an inlet flow or do you mean a resulting outlet flow calculating steam required? I already know the outlet flow to be 75gpm. Also, i know that this is not superheated steam so it carries a temperature of around 360F (guesstimation, not currently at work so don't have steam tables handy).

FMJalink: Thanks, i'll give it a try when I get into work on Monday.

Hope both of you have a happy new year.

cheers,
abamin

 

[rmw]

abamin,

I think it would be easier to start with a reasonably assumed inlet flow (which would seem to me to be a known or something you can control) and calculate the resulting flow after the contribution of the mass of the steam. Then if the resulting flow missed the goal of ~75 gpm range, you could adjust the inlet flow accordingly which is in effect what you are going to have to do.

If your inlet water temperature varies any at all, your outlet flow is going to vary according go the amount of steam needed to control the outlet temperature.

rmw

 

[unclesyd]

Pick did have a very good application group that could give you the information in a few minutes.
I would get all my data together and give them a call. Make sure you decide whether you want a variable or fix outlet flow.

 

[25362]

Reading from tables with my old eyes:

Enthalpy supplied by sat. steam @ 150 psig down to 150[sup]o[/sup]F: 1078 Btu/lb.
Enthalpy gained by water from 70 to 150[sup]o[/sup]F: 75 Btu/lb.
Entalpy of outcoming mixture @ 150[sup]o[/sup]F: 118 Btu/lb

A simple balance shows that the outcoming mixture is composed by 95.7% original water and 4.3% condensed steam.

Please correct me if I'm wrong.

 

[TBP]

From "Hook-Ups" by Spirax Sarco.

GPM x 1.1 x delta-T in F/ 2 = #/hr

75 x 1.1 x 80/ 2 = 3,300 #/hr

This is for a heat exchanger, as opposed to a direct contact heater, and so will likely show a little more steam than the Pick will require. You're likely looking at about 3,000#/hr.

Your safety valve likely lifts at 150 PSIG, so the working pressure of the system is maybe something like 125? In any event, a 2" steam supply line will likely do the job.