boiler design

[agiraias]

Hi everybody. I´m project engineer working in a new project. Our intention is to recover thermal power from the cooling circuit of an engine. We are going to set up a 3 ways valve in the outlet of the engine (instead of termostates) to maintain 90 ºC in the heat exchangers inlet. The temperature is fixed and the flow through the heat exchanger varies. At full load, we have 300 KWt, but those thermal KWs would be lower when we are working at less powers in the engine. In the other side of the heat exchanger we want to achieve a temperature of 82ºC using a refrigerant fluid as working fluid, my question is , is this possible with a normal heat exchanger?, this means, we know that is possible to achieve that temperature, but we don’t want to install a huge heat exchanger, (with are thinking in a normal size).Another question, if we have a bound of 8ºC (from 90ºC to 82ºC) in the hot side, what is the maximum outlet temperature we can achieve in the fluid refrigerant side? What is the meaning of pitch point? THANKS

 

[athomas236]

agiraias,

I have never heard of pitch point but I have heard of pinch point which is a term used in relation to gas turbine heat recovery boilers.

Pinch point is the difference between saturation temperature of the evaporator bank and the temperature of the gas leaving the evaporator.

athomas236

 

[MJCronin]

Ummmmm...

"Pitch point" is the sound that the "3 ways valve" makes when it actuates.....

 

[rmw]

A properly designed heat exchanger will work with any reasonable pinch point or approach temperature. Pinch point is a term used with any type of heat exchange equipment. An 8C approach or pinch point seems reasonable enough but you have to satisfy the other parameters a heat exchanger needs such as flow (velocity) etc.

For your 3 way valve I recommend Amot valves. your engine manufacturer may want to assure that you don't cool down the return water to the engine too much.

Since you will have variable engine loads, you must not design your heat exchanger for the maximum condition but an intermediate point that you choose. I assume that you will have a normal radiator or cooler for the engine for when the heat exchanger has no load.

rmw

 

[agiraias]

Hi Rmw. Can you tell me the definition of pitch point? why did you recommend amot valves? Please explain me why we must not design the heat exchanger for the maximum condition, can you explain me that? Thank you for all

 

[rmw]

The term is "pinch" point and here is a better explanation than I could give;

http://www.cheresources.com/pinchtech2.shtml

I once used the Amot brand of valve on an application where engine jacket water was used for the driving medium for a salt water desalinator and the engine manufacturer was paranoid about maintaining the return water to the engine at a minimum temperature. The Amot valve did a wonderful job of blending the hot water off the engine with the cold water coming off of the desalinator feed heater and protecting the engine.

If you design your heat exchanger as if the engine will be operating full throttle at all times and then it is not operated at that point, the heat exchanger will under perform because it will be operating off of its design parameters. If you will have a variable engine operation it is better to pick an intermediate point that is a compromise such that it performs well at all operating conditions rather than performing only at full throttle.

rmw

 

[agiraias]

I have been reading literature about boilers and after talking to manufacturers, I assume that that in plate exchangers cross of temperatures are allowed, that means that we are able to reach a higher temperature, in the outlet of the working fluid, than in the outlet of the cooling water.
Exmple;
Cooling water ; from 90ºC to 82ºC
Working fluid; from 33 to 85ºC
That is what it means "cross of temperatures", (negative pintch point). do you agree? And how is this possible?
If we design the boiler for a maximum thermal power available, what happens if the thermal power falls? That is , from 300kwt to 200 kwt. What happens with the water´s outlet temperature?, It drops I know, but how much? Is the pintch point constant?THANKS