Heat Loss Thorugh SS Tank & Hot Water System 2

[dungarah]

Hi,

I am trying to find justification to why electrical heating would be better to use on our new SS tank rather then the currently existing hot water system. to compare energy usage of the hot water pump and that of the energy required to maintain the temperature in the tank if using a new electrical system.

Hot water system : Uses a Pump 27m3/hr at 27m head . 4kw & 2900rpm

New electrical system: tape trace heating

The HCHO Tank is a 3.5m diameter, 6.2m high Stainless Steel Tank with a flat base and domed room. It will be sat on a plinth ~ 0.7m high. It is required to maintain the contents between 30 & 40°C.

• Specific Heat of Liquid cal/g/°C at 25-40°C : 0.78
• Boiling Point °C : 95°C
• Freezing Point: ~15°C
• Contents in T114 to be kept at ~38°C
• Max temperature no more than 60°C

the quote recieved says For temperature maintenance, the system offered will maintain 38C against a minimum ambient of -20C.
Our system will also heat raise from 0C to 38C. With a full tank we calculate a heat raise time of 30 to 32 hours. With the tank half full, this would be achieved in around 17 to 19 hours.
Heat load: 15kw.

currently the pump is running contantly 12 hours a day.
with the lectrical heating, once the temperature drops to 30degrees it will turn on to heat it back to 38~40 degrees.

Please advise how much heat loss is obtained in the tank, the power required to put it back to 40degrees and whether is it better then the hot water system.

thanks

 

[BigInch]

You're never going to be able to make a comparison like that.

You must find out how much money its costing you to heat the water in your current water system. All you have given so far will ONLY allow comparison of pumping cost of the hot water system to the total cost of heating the tank with electric tracing.

You must know the cost of heating your water. Cost of pumping it is minor in relation to that. If you now, for example, heat your water with gas, perhaps you are paying $5.00 to $10.00, or something, for 1000 ft3 of gas, say equal to 1 million BTUs.

I calculate that you need around 2 million BTUs to heat your tank from 30 to 40 deg C. If you pay $5.00 for each million, that's about $10.

The energy needed to heat that tank from 30 to 40 degrees is roughly the same, if it is electric, or if it is hot water. So you need 2 Million BTU for electrtic heat and 2 Million BTU for gas too.

2 Million BTUs gas heat is equal to about 600 kWh of electricity. If your electricity cost is $0.12 /kWh then for 2 million BTUs you will pay $72.00 for that electricity.

If you now heat your water with electricity then the cost of electric energy will be the same for both systems, except for the small pumping cost of the water system. PLUS to use electric tracing you must buy and install it. Probably a relatively big cost. How are you going to recover that cost based on equal energy usage, if they were both electric, or a greatly increased cost, if your present system is gas heat.????

So, hang up the phone and stick with your present system, or tell us something very, very interesting about how you plan on saving so much energy with the new expensive elect tracing system that it will make it so much cheaper that you can pay for it.





If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[abeltio]

Using insulation is critical in this kind of application. The heat loss of an uninsulated tank is the greatest cost.

saludos.
a.

 

[trashcanman]

Using a tank temp of 104 and ambient air at 0, insulation thickness=.05" (my program doesn't like 0 thickness), I come up with a heat loss through top and side = 24716 BTUH.
It will take 7.23 KW of electrical heat. What did the quote say is the wattage required?

The tank should be insulated.
With just 1" Fiberglass, heat loss = 8536 BTUH = 2.5 kw.

 

[BigInch]

Granted insulation would be a great idea, but it still does not influence the selection between gas/electric. With insulation you would pay the same relative difference in cost of energy, just lower and slower.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[willard3]

In that gas cost is approx $6/million btu and elec cost is approx $20/million btu, the answer is obvious. Don't make a mountain out of a molehill.

 

[willard3]

I will correct my previous post's gas cost. Gas costs $3-4.5/million btu making the electric option even less desirable.