CAPE-OPEN

I would like to model a surface condenser for a Rankine Cycle. It appears the approach is to is to assign the condensing vapor to input 1 of the HeatExchanger UnitOp and specify the amount of subcooling as the outlet of temperature stream 1 in the GUI. Condensing pressure is then controlled by setting the outlet pressure of the Expander UnitOp.

q1. Is this the preferred way to achieve this?
q2. Is there a cookbook that shows preferred patterns for solving problems (a la Alexander's 'A Pattern Language')?

TIA

There is no cookbook, but this example may be relevant to your problem: http://cocosimulator.org/down.php?dl=CHP.fsd

Note that on a closed recycle you probably want to include a flow controller unit operation to keep the flow rate fixed. If you have more than one compound, the composition also needs to be fixed, which you could do with a small custom unit that would output a constant composition (e.g. an Excel Unit Operation).

You did not include the flowsheet, so it is hard to interpret what you mean by stream 1.

@Jasper. Sorry about not including a flowsheet. Stream 1 is assigned when connecting to the Heat Exchanger UnitOP.

Also thank for pointing out the CHP example, which uses a Heater/Cooler instead of a Heat Exchanger, and specifies the outlet temperature of the condensing stream. I am also interested in the cascade, i.e. the effect on the cooling stream, so I want to use the Heat Exchanger.

In my case, the cooled stream is superheated as it enters the Heat Exchanger, and as a result encounters a pinch point at the condensing temperature. This prevented the Heat Exchanger from meeting my desired constraint of specified exit temperature for the condensing stream.

Accordingly I used two Heat Exchangers. One to de-superheat, with the temperature of the cooled stream (Stream 1) specified to be at the condensing temperature. The second Heat Exchanger constrains the subcooling of the cooled stream (Stream 1).

What is exactly the reason for the superheated stream not to be able to be cooled in a single heat exchanger? Insufficient heat capacity of the cold stream?

What is exactly the reason for the superheated stream not to be able to be cooled in a single heat exchanger? Insufficient heat capacity of the cold stream?

My inference is that the CounterCurrent mode Heat Exchanger has difficulty handling two pinch points.

Stream 1 (Inlet 1/Outlet 1) has its pressure constrained at the discharge of a turbine. I desire to condense the vapor (which may still be superheated in some cases) and then specify the subcooling using the Stream 1 Outlet Temperature Parameter of the Heat Exchanger GUI. The specified outlet of Stream 1 is much greater than the inlet temperature of Stream 2.

Stream 2 (the heated stream in my case) enters approximately saturated, at a temperature well below Stream 1 and evaporates. I don't care if it fully evaporates at the exit of the Heat Exchanger.

When I run this, I get two Warnings:

message: HeatExchanger warning: heat transfer coefficient cannot be determined; parameter UA is not updated

message: HeatExchange inconsistency: calculated temperature of the cold stream exceed temperature of the hot stream
ATTENTION: calculation continues with modified temperature of the cold stream: T[out,cold] = T[in,hot]

These warning messages are true enough. But the result is that stream 1 is NOT subcooled, but exits at the saturation temperature. I would have expected Stream 1 exit to be the constraint, the heat transfer would be calculated using the enthalpy difference of Stream 1, and then the Stream 2 exit temperature would be calculated using the enthalpy rise for stream 2. Then UA would be calculated and any warnings would be established at this time.

It seems to me that specifying both "countercurrent" and a Stream 1 Exit temperature, the Stream 1 Exit Temperature specification is ignored and the countercurrent mode takes precedence. I had expected instead that Exit Temperature would have precedence, and Countercurrent would be used only to determine the Log Mean Delta T for purposes of UA.

Does this explain the nature of the problem?

If by subcooled you mean a vapor that is not in phase equilibrium (and would form a liquid), this is not possible in COFE; all streams will be in phase equilibrium.

The exit temperature spec should be honoured by the heat exchanger, unless there is not sufficient heat capacity in the other stream to reach the specified temperature. This is what the second message indicates.

If by subcooled you mean a vapor that is not in phase equilibrium (and would form a liquid), this is not possible in COFE; all streams will be in phase equilibrium.

By subcooled, I mean that the liquid phase temperature is less than the saturation temperature.

Likewise, by superheated, I mean that the vapor phase temperature is greater than the saturation temperature.

I would be surprised and perplexed if it was not possible to have 20C water at atmospheric pressure. Are you saying COFE cannot model that?

Here is a more concrete example of what I am trying to model:

Consider a cooled stream of 1bar superheated steam, at say 200C first cooled in a heat exchanger to 100C saturation temperature, followed by condensation at constant temperature, and then subcooling to say 60C in order to provide Net Positive Suction Head for a pump.

The heated stream could be a refrigerant that enters the heat exchanger as subcooled liquid at 40C, evaporates at 80C, and exits at whatever temperature is required by an energy balance. Of course the flow rate must be sufficient so that the pinch point criterion is maintained in order to achieve the specified cooled stream exit temperature.

[It would be a nice feature if one could specify the pinch point, instead of the stream 1 [cooled stream] exit temperature.]

I had expected the heated stream exit temperature to be something less than the cooled stream inlet temperature. But what I see instead is that COFE sets the heated stream exit temperature to 200C, and then applies the energy equation to determine the cooled stream exit temperature. This contradicts the behavior implied by the GUI, which suggests that the cooled stream (stream 1) exit temperature is specified (60C) and then the heated stream exit temperature is a consequence of the heat balance.

Thanks for your thoughts.

My previous post illuminated my problem: The mass flow of the heated stream was insufficient to bring the cooled stream down to the specified temperature. Once I adjusted the flow rate, I could achieve the desired sub-cooling.

I would be surprised and perplexed if it was not possible to have 20C water at atmospheric pressure. Are you saying COFE cannot model that?

Of course it can handle this. Sub-cooled and super-heated are also used for non-phase-equilibrium states. The latter COFE cannot handle.

Just to avoid interpretation errors, can you send me an fsd with your problem setup?

Thanks.

Of course it can handle this. Sub-cooled and super-heated are also used for non-phase-equilibrium states. The latter COFE cannot handle.

Just to avoid interpretation errors, can you send me an fsd with your problem setup?

Yes indeed, COFE can handle this. My subsequent post explained that my flow rates through the heat exchanger were the problem.

So sorry for posting a poor question. I should have edited my post to remove my 'surprised and perplexed' remark. Many apologies.