Cooling Water Heat

[nilanjana2012]

In a 40 minutes batch process water in a closed loop is heated from 45 degC to 65 degC for first 10 minutes and then cools down to again 45 degC. The rate of temperature rise is attached. The flow rate is 200 m3/h. The heated water is cooled by a secondary circuit consisting of heat exchangers, cooling tower. pump etc as shown in the attached scheme.
To save huge amount of cost, my intention is replace the secondary circuit by providing a chilled water tank which will cool down the water to 45 degC. Chilled water is available in the plant at a temperature of 8-10 degC and a DeltaT of 5 deg is permissible. I want to find out what will be the total heat generation and the size of the chilled water tank.

I Think we are overdesigning the plant by taking the heat generation considering enthalpy at 65 degC and 45 degC which is practically not the case.
Any suggestion or help will be appreciated.

Regards

 

[nilanjana2012]

Sorry,The 2nd file is uploaded.

Regards

 

[LittleInch]

Not quite sure what your question or problem is, but as drawn, the diagram implies that the HX / chilled water tank is cooling your hot water from 65 to 45 as it transit through the HX. However your graph and description implies it looses maybe 1-2 deg and circulates gradually cooling. The amount of heat loss over the HX will be dependant on the total mass that 200m//hr represents. If the volume of hot water is less than about 80m3 then the instant heat loss across the HX will be less than 20C to achieve your cool dowm time.

Missing from any of this is any idea of the total mass of hot or cold water and size of the cold water supply.

At a simple level, you have a certain amount of heat energy you want to loose from the water (Mass X 20C x enthalpy of water) in about 25 minutes according to your graph. This gives you J/sec to be used to heat up your chilled water 5C and hence mass flow of chilled water required. Unless I'm missing something here (quite likely) this looks really quite straight forward so I'm not sure what the question is.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[zdas04]

I would say quit the wishful thinking and do a rigorous mass and energy balance of the two systems. Once you know the ergs of energy being rejected you can determine if your heat sink can accept them without causing problems elsewhere. It is my favorite kind of engineering problem because a closed form solution is possible and will come very close to actual field performance.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. ùGalileo Galilei, Italian Physicist

 

[racookpe1978]

You MUST include all of the real world "cost" of the chilled water: Ordinary "cooling water" will vary slightly (greatly ?) in temperature as the winter changes into summer and then back again, but the chilled water will stay at a steady even temperature much colder than the "open" cooling water. But that water itself costs little more than the money to pump the water (unless water rights must be bought.)

BUT. That chilled water costs a lot of money for each liter/gallon per minute because of the costs of pumping and the costs of the chill water refrigerator system and the hourly costs of the chill water refrigerator motor and refrigerant reloads and the chill water overhead heat losses in the lines and tanks. Are ytou really, really sure your economic analysis includes all of the costs of adding to the current chill water heat load?

 

[nilanjana2012]

I am sorry if I am not being able to explain the problem. I will try to explain more about the process - the oxygen is blown into a pool of metal through the lance and due to the exothermic nature of the reaction heat is generated. The rate of increase of temperature with time is shown in the sketch. In this batch process, the rate of O2 blowing starts from 0 minute and peaks at approx. 10-11 minutes and then is reduced to zero at 15-16 minute. The lance is taken out of the system and for the next 24 minutes the water is being cooled to 45 degC as shown in the sketches. The total batch time is 40 minutes and again the next batch starts at 41 minutes and so on.

Now if I simply calculate Q=msDeltaT to find the size of the heat exchanger and the secondary water circuit including cooling tower, pumps, piping instruments etc. the cost of installation and maintenance is huge. I have a chilled water plant in my plant and have spare capacity which I wanted to utilize.
I believe I could be able to bring clarity on the problem.

 

[chicopee]

Other than the economics of your proposal that racookpe1978 mentions, have you given any thoughts about the reason for the spare capacity of chilled water?

 

[nilanjana2012]

I have enough chilled water in the plant and I am producing this from waste heat. As such basic economics of the process has been worked out.