Hi all,this is my first post/thread, just starting out as a second year chemical engineer and I seem to have found myself in need of some help.Basically, I'm designing (basic) a heat exchanger where I'm heating a nickel slurry with steam at atmospheric conditions from 25 deg C to 63 deg C. I have the correct heat duty of 12235187 kJ/hr through using Q=m*Cp*?Tm = 4165.19 kmol/hr * 77.302 kJ/K.kmol (avg cp)Now that I have the heat duty that is required to heat up the nickel slurry, I am unable to understand how to get the mass flow rate of the steam required to do this.I was aiming to have the steam enter at 100 deg C and leave at 65 deg C, is this reasonable? How do I work back from the heat duty required using the temperatures mentioned to work out the mass flow rate of the steam?I know that I need to take into consideration condensation and other factors, but I'm just not sure what to do.Any help would be appreciated.
In my experience, the steam conditions are specified, so it is normally just a decision about whether to use high pressure, low pressure, or intermediate. When you know the steam conditions, just look up the latent heat in the steam tables - then divide your duty by the latent heat to get your mass flowrate - but make sure your units are consistent. As some of the other answerers have said, I would not worry about the sensible heat - the latent heat far outweighs this. Its probably okay to assume the steam will enter at 100C and leave at 100C.
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The flow rate of the steam can only be determined by the amount of heat absorbed by the slurry. Under perfect insulating conditions that would be the heat required to maintain the steam volume,the steam temp would remain the same but the amount of condensate return would determine the amount of heat exchanged so at 100 deg C, in other words you would have to maintain 1psi of steam at all times so thats your ideal steam pressure to maintain 100degC. So you use a simple ft type trap and a regulator set at 1 psi. You can also use thermostatic control valves to regulate the steam for a more accurate slurry outlet temp.(the thermostat on the outlet and the control valve on the regulated steam supply.)
Assuming that your steam supply is entering the heat exchanger dry saturated and exhausting to an atmospheric condenser (ie the steam is at least 100 deg C) the heat extracted from the condensing steam will be 2,256 kJ/kg (from steam tables). You can ignore any further heat gain in dropping the condensate temperature to off set against any thermal losses. A heat load of 12,235,187 kJ/hr will require 12,235,187 / 2,235 = 5,474 kJ/hr steam, or 5.5 tonnes/hour. This is quite a lot of steam for a low pressure system and you are probably going to need a 300 mm diameter steam supply to meet this. If higher pressure steam is available it may be better to design around a higher steam temperature/pressure.
