# Oil Bath heat exchanger

1. Jul 12, 2010

### curio

I have an oil bath that has a jacketed heat exchanger I wish to use to heat another water bath.

The temperature of the oil bath is 100 C and the water bath temperature I require is 54 C. My question is will this be either too much heat or not enough.

My basic design is to use a thermistor to monitor water bath temp and adjust the pump speed (12V) accordingly. I think I have the circuit downpat but want some help with the theoretical heat capacity calculations.

Without wanting to go and renivent the wheel I was hoping someone could run through some quick back of napkin calculations with me.

Cheers.

2. Jul 13, 2010

### Lok

What oil is it? (heat capacity varies a little between oil types)
How much oil and water is there? As water has a very high heat capacity, there is a lot of oil needed to heat a bit of water. Also what is the surface of the heat exchanger?
What is the oil's heat source? Assuming it has one that keeps it at 100 'C.

3. Jul 13, 2010

### curio

The heat exchange surface area is 0.00054 square meters submersed in a silicon oil bath which holds 0.00016 cubic meters, the water bath size is 0.000036 cubic meters, 0.225 times the size of the oil bath which has a thermostat controlled heating element holding it at 100 C.

I'm just looking for rough calculations here, no second order differentials or partial derivatives lol.

4. Jul 13, 2010

### Lok

Silicone oil has a spec heat cap. of 1370 (J/Kg*C) while water has 4187. 3 times less so the oil has to cool down 3 deg for water to heat 1deg. (Completely ignoring oil density 0.92 kg/l). You have 4 times more silicone then water, it sounds ok (or more then needed).

So if you heat the silicone to 100'C and then transfer the heat to the water it will be enough to heat the water to the desired temp without the need to reheat the silicone. For long time use that will be necessary. Use the thermocouple ( thermistor ) on the water bath and if you can you can set the oil bath to less temp for economic and safety purposes. 80'C sounds good.

The time it takes is a "second order differentials or partial derivatives" thing. So just do it. :P

5. Jul 13, 2010

### curio

Thanks Lok.

The volume fo the oil bath is 0.000028 cubic meters not 0.00016, i forgot to divide the diameter of the oil bath by 2 :0 So its roughly the same size as the water bath.

The bath needs to stay at 100 C as it used to measure oil viscosities. It is safely plumbed, there should be no issues with this.

Being that the volumes of the oil and water bath are approx equal, will this overtly effect the temp of the oil bath seeing that a three degree reduction in oil temp is required to increase the water temp by one degree. If I'm heating water from 24 to 54 C are you saying the oil will need to decrease by 90 C?

My plan was to set up a PWM routine in a PIC, add a pot to one of the a2d pins and use this to vary the PWM whilst monitoring the temp of the water bath with a thermometer and thermistor/op-amp (adding a resistir to the thermistor to improve linearity) with a DMM.

Any further thoughts?