Determining steam required for pick heater
Determining steam required for pick heater
(OP)
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!
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!





RE: Determining steam required for pick heater
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.
RE: Determining steam required for pick heater
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
RE: Determining steam required for pick heater
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
RE: Determining steam required for pick heater
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
RE: Determining steam required for pick heater
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.
RE: Determining steam required for pick heater
Reading from tables with my old eyes:
Enthalpy supplied by sat. steam @ 150 psig down to 150oF: 1078 Btu/lb.
Enthalpy gained by water from 70 to 150oF: 75 Btu/lb.
Entalpy of outcoming mixture @ 150oF: 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.
RE: Determining steam required for pick heater
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.