Steam Coil

Sound engineering practice!

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What is the principal of operation for a steam powered air dehumidifier?

Are you sure it's a dehumidifier and not a humidifier?

If this component of what may be a system is just a steam condensor or the hot side of a heat exchanger then the outlet needs to be bigger to accommodate (liquid)water at low pressure V high pressure (vapour)steam at inlet....for the reason Joshi said.

Its a steam coil and we use it inside an Air handling Unit to dehumidify the air.

Most of the coil we have for different system is steam inlet pipe dia is larger then Outlet dia, but just in one case it reverse (application is same).

That make me confused!

Just pure speculation.

The steam entering condenser with pressure will rapidly expand & cool faster. The water drain has no pressure & it will flow on more or less gravity. Hence has a larger dia. A layman's explanation. Welcome for a good laugh!

Hi Bashir, I think that in your system there is steam under pressure, but the condensate outlet is to atmosphere. Assuming the basic engineering norm that steam at 100 deg C expands 1600 times to water at the same temperature, you can understand the need to have a bigger diameter pipe at the condensate outlet. Do you have a steam trap at the outlet ?

I hope that I have perceived your system correctly.

Thanks.

We have stem trap in the condensing or outlet line and we use the condensation as well.

just to comment 1.5mm and 2.5 mm sounds a bit too small.The steam is used to raise up the temp isnt it?

Others will be curious about that too. Good point. There's something wrong with the content of the original post.

sorry, its 1.5" & 2.0 ".

Steam System Best PracticesCondensate System Piping

Are you talking about a Re-heat coil as opposed to a "dehumidification" coil? Often the air will be sub-cooled below the dew point to remove moisture and then the air is reheated to bring it up to the desired distribution or comfort temperature.

That's what I was wondering too. The heating coil is a component of a total system not the sole source of dehydration (if that is even possible).

There should be so confusion.

Its a part of the total system and we have pre and post chilled water cooling system with steam heating coil.

There should be so no confusion.

...and yet there was.

.

In your OP you said, We are using a steam coil for air dehumidification. hence the confusion.

You are actually using the steam coil to heat the air prior to dehumidification in a system, as was suspected and queried but only now confirmed by you.

Thanks for clarifying this.

The logic is that the chilled water sub-cools the air and the moisture is condensed out of the air. However the air temperature is too cool to be comfortable so it must be reheated up to 75-77 degrees F for maximum comfort and at the higher temperature the relative humidity is lower and more comfortable. In A/C it is as important to control the RH as the Temperature.

OK so it's reheated after being chilled and dehumidified.

In these parts we actually install rehumidifiers to raise the humidity of chilled air so it is more comfortable and doesn't promote static buildup.

I guess it depends on what you are doing with the air.

What's OP doing with this dry warm air I wonder?

You're right that it often does require a little re-humidification. I didn't bother to go into that since the OP hadn't made it part of his posting. In well engineered systems the dehumidification and re-heating systems should include a final touch up of the humidity if required especially if the outside make-up air is especially dry. This was often done by spray or belt or rotating discs. I wouldn't have thought in your neck of the woods that it would be required.I thought that your area as being relatively humid year round. Of course it also depends on the amount of make-up air being introduced into the system. I was taught and have always advocated that 77 degrees F and 40% relative humidity were optimum A/C settings.

Certainly is humid all year round here. Outside.

Aircon dries the air to below the 40% figure you quoted. 40% is also the target humidity we go for using a misting arrangement in the ducting after the evaporator.

Think telephone exchanges and you'll understand why static buildup is undesirable.

Where non ducted systems are used for smaller equipment rooms low humidity isn't an issue as the door lets moist air in fairly frequently. Being too cold becomes an issue here where it nearly starts raining inside the equipment room when the door is opened!

I only have to think winter in New Jersey when it gets down to 30 to 35% RH inside and walking across a rug and getting zapped touching something. I always run a humidifier to keep it up around 38-40%. Of course sometimes when the outside temp drops real low I do get condensation on the inside of the windows. So your misting arrangement is also needed for heating units to keep the RH up in the winter season around here.

Thanks to all .....

Most of us thinking about OP. So, now I am clarify it further.

The stem heating coil is a part of a HVAC system use for Pharmaceutical Production.

Pre chilled water cooling coil cool and dehumidify the fresh and return air mixture (up to 55 RH & around 24 Deg C) then it goes through a desiccant bed dehumidifier to dehumidify the air below 30 RH. And to dry out the bed media you need dry air and to produce dry air you need a heating coil (steam or electric or any other heating media). And lastly post chilled water cool the air further if required (desired temperature 20-26 Deg C). Beside all of this there are different grade filter and others.

May be we are long away from the main topic! But I have learn a lot how different people think about a problem differently.

Thanks Bashir. That was really interesting. You have quite a responsibility there!!

The original topic...

Have you applied or investigated any of the tips, advice and opinions presented so far?

Most of my confusion clarify with the help of Post: 11.

Thanks to all of you.

Basically user like me learn lot with the communication.

Did you attempt to perform any calculations?

Why do you think the connections appear to be contrary?

Basically in that system, the condensation previously was drained out but from last few years we return the boiler condensation. And as the system is 10 years old, may be at that time URS was prepared in that way ..........

As I conclude ID > OD for condensation return system and OD > ID is for condensation draining system.

It would appear then that you also use a steam coil to heat up the desiccant bed to drive off the moisture. Assume that you have two compartments or more so while one section is in use the other is being dried out.

yes, exactly

Bashir,

My professional focus is bolting but steam is my professional "hobby" (as a result of being well-immersed in the technology in a previous life).

It appears that you've received satisfaction here. However, for my own interest and possibly in order to enable you to optimize your system, I'd like to ask you a few questions:

• Where is the condensate returning to?
  
• Is their a "lift" (ie: elevation) in the condensate return line?
• I assume that there is a control valve on the steam inlet side. Does the system "hunt" or is it relatively steady?
• Along the same lines as above, are your final air conditions steady?
• Does the system experience water hammer?
  

• Where is the condensate returning to? Answer: Its come to the boiler again not directly with the help of feed water tank.
• Is their a "lift" (ie: elevation) in the condensate return line? Answer: yes, due to the gravity and condensate pressure all condensed steam accumulate in a tank (bottom of the building) and automatic float valve with return pump it back in the feed water tank. It's help to maintain a certain temperature in the feed water tank, as we/or anyone do not want to feed cold water into the boiler directly.
  
• I assume that there is a control valve on the steam inlet side. Does the system "hunt" or is it relatively steady? Answer: All individual system have standard inlet and outlet with PRS, strainer, pressure gauge, safety valve, bypass line with manually operated globe valve, float trap module, and automatic electrical control system as required by the machine or system.
  
• Along the same lines as above, are your final air conditions steady? Answer: Yes
  
• Does the system experience water hammer? Answer: At morning time sometime we observe it, but it's only because of the operator when forgot to drain all stem/condensate over night. Its not a 24 Hrs operated system, we only use it 6 am to 8 pm.