steam shell pressure 1

[JimCasey]

It seems that customers can never give me this information. I am looking for a way to calculate it with higher confidence than I have.

Consider a tube and shell heat exchanger.
The shell gets steam from a pressurized, saturated steam source. The steam is controlled by a valve before it is released into the shell. Condensate is removed from the shell by a steam trap and the level remains below the tubes.
The tubes are filled with a fluid cooler than the steam. Obviously, I hope, the fluid comes in very cool, and leaves at a temperature somewhat warmer, but not as warm as the steam.
What is the pressure in the shell side of the heat exchanger? Does the condensate subcool to the temperature of the incoming fluid and does the pressure of the steam reflect saturation at the coolest temperature, or at the hottest temperature of the controlled fluid, Or is the shell pressurized to the suppy pressure of the steam and condensed at saturation temperature at that pressure?
This is more information that customers usually can give me, but they want me to calculate valve size for the steam supply valve. I get incomplete process conditions, but if it's wrong it's still "my fault". Energy balance is pretty easy to calculate. Delta-P across the valve is tough. Sometimes I just assume critical pressure drop, and go with that. I will be grateful for any insight.

 

[BigInch]

Jim, I don't know much about steam. Maybe this link will keep you interested until an expert checks in. If I were to guess ... no I'd better not guess. I think I'll just lurk.

http://www.spiraxsarco.com/resource...eat-exchanger-and-steam-load-relationship.asp

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[gerhardl]

This is a standard steam application, and the link from BigInch should help you a lot.

What you need is a steam specialist in the component and layout side to advice you. Serious suppliers will have drawings and advice for free.

A remider: Steam (gas phase of water) is an excellent heat transfer media. What you are after is the energy amount given when steam goes back from gas to condensate water, by giving this energy to the other side of your heat exchanger. This amount is several times higher than what energy you can get out of alredy condensed steam (hot water).

The general idea of a steam heated heat exchanger is to have the exchangeer as small as possible by having maximum of the existing heat surface exposed to steam, and get condensate out, not blocking up the heat transferring surface with condensate at a lower energy level.

Some points, often causing problems when not properly solved for heat exchangers:
a) Steam traps of correct type and size and placement to take air and water out under startup and use.
b) Regulating valve correct size, not too large, to give a normal opening when working. Too large valves will give small opening and tear away seat and disc.
c) The regulating is for normal applications placed on the steam inlet side. If you also try to regulate condensate outlet, this might work against inlet and block up heat surface with condensate. The steamtraps will take care of the outlet side, letting condensate out.

Conclusion: forget the complicated heat transfer and pressure regulating calculations. Install a correct steam pressure reduction station to get correct low pressure steam, if not already available. A direct acting temperature controlled valve will take care of the steam inlet. Steam amount to be calculated as phase transfer energy from steam to water at given pressure. Steam traps and filter before the regulating valve.

(There is although several variations on above general solution, also utilizing rest heat in condensate for larger applications)

 

[TBP]

As BigInch suggested, the Spirax Sarco folks - or pretty much any of their competitors - Spence, Armstrong, Hoffman, etc have lots of resources for you.

See if you can get a copy of "Hook-Ups" by Spirax Sarco. Probably the best practical steam book I've seen. Lots of nice, simple sizing calculation info that works. I've used it for years.

 

[quark]

It depends upon the trap you use. Only thermostatic traps allow subcooling depending upon the thermostatic fluid filled in the capsule.

For all other traps, the condensate pressure is equal to source steam pressure (minus pressure drop across the valve) and temperature is saturated at the condensate pressure.

 

[rmw]

Your condensing pressure/temperature will be a function of some approach temperature to your incoming cooling water depending on certain heat transfer characteristics of your heat exchanger. Changes in your cooling water flow rate can send the condensing pressure all over the map.

Let's say that it will be less than the saturation pressure for the steam entering the shell and more than the saturation pressure for the temperature of the cooling water (or the condensate) leaving the shell.

This if and only if you don't have any air or non condensables present in the heat exchanger.

You might get some sub-cooling as the condensed steam flows across the cooler tubes near the cold water inlet but unless the tubes are flooded, I wouldn't expect much sub-cooling. Heat exchangers that are designed to subcool the condensate have elaborately designed sections dedicated to doing so and in the end, it takes quite a bit of heat transfer surface to get near the CW temperature. Depends on how much sub-cooling you are concerned with. Unless you have a pretty decent sub-cooling section designed into the Hx, consider the condensate saturated for the operating pressure.

You need a HEI or HTRI program to model the heat exchanger based on design flow conditions or your customer needs to get that from their Hx vendor. You can only guess. If I were you, I would design the valve for a reasonable pressure drop somewhere between steam pressure and saturation pressure for the temperature of the incoming cooling water and if that value is critical pressure drop, go with it.

rmw

 

[quark]

If you are aware of the flow rates of both hot and cold fluids then you can cross check the design conditions. Do the energy balance of cold fluid vs steam (considering only phase change, provided your steam is not superheated). If the energy gets balanced then you shouldn't worry about subcooling. Otherwise, check the enthalpy difference corresponding to the heat pick up by cold fluid and match the outlet conditions of hot fluid for degree of subcooling.

Steamtab can be of good help to you.

 

[TBP]

In plants where nobody knows what the flows are - and I've been in LOTS of them, all you can do is pick a reasonable estimate based on line size. If it's a 2" cold water line, go with, say, 75 GPM. Make sure your client has this number in writing, with an explaination that ALL other calculations depend on it.

For a great many applications, the heat exchanger sizing isn't complex. Many industrial plants operate steam systems at 125 PSIG. It's very common for shell & tube HXs to have low pressure steam on the shellside, so we'll say 10 PSIG steam at the HX inlet. Say you're heating 75 GPM water from 40*F to 140*F. Give this info to the HX people, and they'll typically provide a few options with respect to pressure drop on the cold side, availability, pricing, etc. Pick the one that suits the application requirements best.

From "Hook-Ups" - the formula for heating water with steam:

(GPM X 1.1 X delta-T)/2 = #/hr steam

(75 X 1.1 X 100)/2 = 4,125#/hr

Look in the Spirax Sarco (or whichever manufacturer you like) catalogue for a temp control valve for a steam flow of 4,125#/hr with a 125 PSIG steam inlet dropping to 10. I don't have one in front of me, so I'll estimate going with an 1-1/2" valve, supplied by a 2" (assuming you're running maybe 20 feet of pipe) steam line. A good rule of thumb is that the control valve will be one pipe size smaller than the CORRECTLY sized steam line.

A few REALLY important things to consider with respect to steam systems -

1/ DO NOT oversize things like control valves & steam traps. Oversizing causes more problems than everything else combined, and multiplied by about 10.

2/ Pipe the installation CORRECTLY. Hook-Ups shows the correct installation. Do things like install a drip trap upstream of the control valve, and ensure that the condensate from the HX can gravity drain properly.

3/ Install a pressure gauge upstream of the temp control valve.

And - very important for YOU - install pressure gauges and thermometers on the cold side of the HX. Get base line readings when new & clean. Give the pressure drop numbers to the HX supplier, and they'll be able to tell you just how much water is flowing through their unit. Take that GPM number, and the thermometer readings for the delta-T, and you can very accurately calculate just exactly what the steam flow is. This data will give you the ammunition you need to defend your design, if there are problems. And having worked in maintenance for many years - a little bit of basic instrumentation really helps with troubleshooting later on.

 

[Sapphiron]

The answer is: It varies. The pressure can be the inlet steam pressure all the way down to vacuum (if no vacuum breaker is installed)depending on the temperature of the secondary fluid and the flow rate. Just to add to TBP's answer...

If you are calculating the size of the steam control valve you need to be thinking of several things at once.

Flow rate required:

The usual information is needed here GPM flow rate delta T and SpH, SpG. If you can come up with the data sheet on the heat exchanger, it will at least tell you what pressure the unit was sized for. Do not under any circumstances size for the maximum condensing rate of the heat exchanger!

Noise:

You don't want a valve so small that you get noise and high velocities through it creating wear. Any valve sizing program will give you this information. Just because the 1" valve will "do" the flow doesn't make it a better choice over the 1-1/2"

Pressure drop

Different valves work best with different pressure drops. Self-contained valves work best with something around 10: 1 turn down, Control valves can handle more. Valves will work better between the 20% - 80% open range, shoot for that.

Application Details:

If this is for heating hot water in a hotel for example there will be wide variations in demand as people take showers, the kitchen kicks in for example. It may be a better idea to use two valves instead of just one to split the load.

You can do all this exactly right and still get a bad result depending upon how the customer pipes the outlet. Any back pressure in the return line can affect flow and control. It can cause hunting, water hammer and temperature swings. Always use a drip trap prior to the valve, better still install an impingement baffle separator with a trap ahead of it to condition the steam and a wye strainer with a fine (100) mesh screen.