Plate/Frame Heat Exchanger Questions
Is there a way to determine temperature inputs/outputs of a plate/frame heat exchanger if you know a few variables?
I have a heat exchanger with the following characteristics. The hot side of the heat exchanger goes to a condenser water loop for water source heat pumps. The cold side of the heat exchanger goes to a cooling tower. What information do I need to ascertain what the temperatures will be on all four pipes, if every component is clean and operating as designed? I do know the following:
- Cold side (Tower side): 5,325,000 btu/hr capacity, 900 total gpm, 2.5 gpm per ton, 12 degree delta T
- Hot side (Condenser water): 5,124,000 btu/hr load, 854 total gpm, 2.5 pgm per ton, 12 degree delta T
- Desired hot side outlet (Condenser water supply) is 80 degrees
I'm assuming I will need to know the surface area of the heat exchanger, I do not have that information on hand as I write this. Is there anything else I need to know to figure out the temperatues on all four pipes?
Comments
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Ah… you have the delta T values… what more do you need?
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
The critical missing temperature for this problem is:
You need the cold-water temperature entering the heat exchanger from the cooling tower.
That temperature is determined by:
- Outdoor design wet-bulb temperature
- Cooling-tower design approach
- Actual tower selection and fan performance
- Water flow through the tower
Your required exchanger duty is:
- 5,124,000 BTU/hr
- Approximately 1,500 kW
- Approximately 427 refrigeration tons of heat transfer
That is a very large water-to-water duty. I would expect this to be handled by:
- A large gasketed plate-and-frame exchanger, or
- Multiple brazed-plate exchangers piped in parallel
A single ordinary brazed-plate exchanger may not be the most practical choice at 854–900 GPM.
What are you working on, may I ask?
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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you can measure the temperatures under operating conditions
If you are troubleshooting you need to know the load it is trying to carry and some gpm info.
Most all the fphx manufacturers have free sizing software. I use the Swep program, you need to just register to get in. I did an inline tutorial to learn some of the finer points.
In most cases a fphx will outperform any other type of HX. You have two moving flows, lots of surface area and turbulent conditions.
The plate and frame versions come in very large sized.
Wessels , is but one brand, they can handle up to 50,000,000 btu/hr!
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
If I was doing this I would start with the cooling capacity of all the heat pumps in tons. The GPM on the condenser side of the HP system is typically 3 gpm/12,000 btu of cooling (maybe more with glycol). But I am assuming no glycol with the plate and frame.
The tower (or closed circuit cooler) will have to reject (1.3x x tons of cooling x 12000btu + any HX inefficiency). The tower will have to reject this BTU load at the outdoor design conditions where the tower is installed knowing the db & wb at that location.
Normally towers are sized at a 10 degree tD but 12 may work depending on the location.
The heat pumps but the BTUs into the water loop.
Water gpm and tower capacity will determine the expected water temps.
Is this a large chiller or multiple heat pumps? If multiple heat pumps are they all in cooling at the same time? or will some be heating like a tower/boiler system?
Your going to need information from the tower MFG and HX mfg to work your way through this.
Alfa Laval is popular in these larger units.
We replace one in a high rise in Hartford. It had to go up an elevator that couldn't handle all the weight.
We had Alfa ship it to us broken down and it went up the elevator in pieces (the two end plates are still a huge chunk of metal). 13th floor I think it was. loaded it on and the guys upstairs called the elevator no one road up with it……..just in case…………LOL
We had Alfa send a guy up to supervise the reassembly
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Do you usually rely on the manufacturer's selection software from the start, or do you rough-size the exchanger first and then verify it with the software?
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it depends on how critical of an application.
For a residential solar or wood boiler, you might ball park or error on the large size. These hx are not too expensive. The wood boiler industry seems to ball park them also.
For a specific commercial application I would call a rep and have them size and spec it.
I just recently learned to use the software after we put together Idronics.29
I have been on the hunt for a Swep B3, to mess with, it is about the size of a small I-phone.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
The reps for the plate and frame HXs do this all day every day. They are not going to want a problem so if they have the right information, they will nail the design.
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