Pressurisation system for district heating system

[Yobbo]

Hi,

I have been looking for literature about the different possibilities of pressurisation of a district heat system, but alas I couldn't find anything proper. We are currently working with a buffer vessel on height, that takes care of the required static pressure in the system. This static pressure is needed to prevent the water from cooking. Temperatures are 105 until 135 degrees Celcius.

This system seems rather basic and requires a construction for the buffer vessel.

I would rather think of a pressurized vessel on ground level with a gas cushion inside, that is being kept under pressure by the gas, the supply of water by a centrifugal pump and a by-pass line in order to bypass excess water ( due to excess pressure in the system to the storage vessel.

If such a system would be feasible or is already applied I would like to know how to size the pump, the pressurized vessel, the gas cushion and the by pass valve. Especially I am interested how solution of the gas in the cushion into the proces water can be prevented.

Any help is welcome !

Regards,

Yobbo

 

[quark]

This type of system has been used for quite some time, but for thermic fluid applications. For water I didn't see so far but I strongly feel you can give it a try. I used such a system to increase boiing point of DowthermA to 380⁰C.

Basically, it is better to keep the tank at the top most point. Nitrogen blanketing is used for pressurisation. For you case (135⁰C) you may require 2 Bar (g) pressure. You need not put extra head to the pump. Mind that pressure difference across pump is same whether or not you go for pressurisation. But just take care that mechanical design of the pump is suitable for heigher pressures.

We used CP model pump of KSB. If possible just have a look at Hot Oil Systems.

 

[khal200ca]

I think you can use a common expansion tank (diaphragm type, rated for the application temprature) which will eliminate the problem of solution of gases in process water and further will result in a much smaller tank without the need to install the tank at a high elevation.

You will need a compressor to maintain the gas cushion.

I am not sure I usnderstand what you really mean by that by-pass thing....I mean, you are using a buffer (or expansion) tank to maintain the working pressure in the system and if anything - I believe - you need an automatic relief valve with let's say 1.5-2 bar above your working pressure.

Good luck.

 

[TBP]

This isn't a particularly high pressure/temperature system. In fact, it's on the low end. The only system I've seen with a nitrogen blanketing system used big N2 storage tanks like those at hospitals. But this was on a huge hot water heating system that operated at 400*F. Depending upon how much water is in your system, you should be able to find a commercially available expansion tank. These will typically have a bladder in them. The system water is on one side, and compressed air on the other. The tank has a valve stem on it just like a car tire.

I'm not sure what you mean by pump and bypass. As the system water heats up and expands, the tank gives the increased volume a place to go. As long as the valve is open between the system and the expansion tank, the water will just enter the tank, and compress the air on the other side of the diaphragm. Plain steel tanks without bladders work just fine too, as long as you install an Airtrol (or other make) fitting. This is a special fitting that goes into the bottom of the tank, with a copper tube that extends above the tank water level. It prevents the air in the tank from being absorbed into the system water. A plain steel tank will work without one of these, but it will water log every year or two, and need to be drained.

You could run the system without an expansion tank, but the relief would lift every time the system heated up, and the make-up valve would add water to relace it when the system cycles off, the water cools, and the pressure falls. (This is very bad from a corrosion point of view.)

 

[chicopee]

For district heating, your are dealing with thousands of pound mass of hot water, therefore the buffer tank which would be sizable would be the approach that I would use--not diagphram tanks. The size of the buffer tank(s) would be based on the amount of water being expanded when heated and equations for volume expansion are readily available in any mechanical engineering hdbk.

 

[Whoops]

Wotcha, Yobbo.

If you’re in the UK (I've assumed you are, from the handle) I’d suggest that you get BS 7074: part 2 1989 - Application, selection and installation of expansion vessels and ancillary equipment for sealed water systems
Part 2: Code of practice for low and medium temperature hot water heating systems

This document only covers MTHW systems up to 120 degC. However, the method of calculation of the water expansion volume and the initial and final pressures of the gas cushion are, I think, given in detail and this would be equally applicable to your HTHW system.

If you are not familiar with the equipment selection methods, then I’d suggest that you should obtain a quotation from a specialist contractor, who would provide a system designed and tested to comply with the relevant standards. This isn’t something you should be learning ‘on-the-job’. Try Aquatech or Pressmain, who both suggest nitrogen cushion systems for HTHW.

http://www.aquatech-ltd.co.uk/Products/Pressurisation Units.htm

http://www.pressmain.com/index2.htm

I’ve had experience with a MTHW (120 degC) district heating system with a high level expansion tank. This system was in operation 24/7, so the system only got cold if something had failed. The expansion volume was therefore relatively small and was only required to accommodate small daily variations in the system temperature. The only problem with this system was that, since the water treatment chemicals were injected into the circulation system at ground level, the RO make-up water in the tank and the cold feed pipe was static and did not receive any benefit from the injected chemicals. Additional chemicals had to be shot dosed into the F&E tank. The tank & cold feed pipe is corroding, and is likely to fail, but they’ve lasted 30 years.

If your system is only required for space heating & HWS, then you should consider amending the system to LTHW, and/or replace HTHW/LTHW heat exchangers with small de-centralised gas boilers. The energy savings from the reduced standing losses will be dramatic. There would also be a major saving in the insurance inspection regime for a LTHW system, as against a MTHW system, to comply with the UK Pressure System Regulations.