# Thread: Plate heat exchanger selection help

1. Originally Posted by gas_n_go
Just a question because I find this stuff intriguing. Why cant it be designed using a plate exchanger on the water inlet line so it performs like an insta-hot system. If you have a set volume of water passing over it at a set speed (GPMs), and a set water temp entering it. 20' of 1/2" water line @ 90 degrees @ 5gpm say may exit the plate at 120 degrees. That would make the size of the tank irrelivent
This is called a one pass system, and normally we strive for a constant, such as discharge pressure or outlet water temperature. Both normally change flow rate of water.

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Icemeister and barbar,

I appreciate your time and input. I'm going to study your posts and the pdf files and get back to you tomorrow with answers to the questions you asked and with where I am then.

Thanks again.

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Originally Posted by icemeister
What have we got?......

1.)A tank of water which needs to be heated to 120 Deg F. We don't know how big it is, what the initial temperature is, how quickly it must be heated, etc. so all we really know is that it's a heat sink at 120 Deg F.
We haven't chosen a tank size yet, but the water heating is secondary so we're not designing for water heating at all. The initial water temp will be tap water temp (~60F?). The primary job for the refrigeration system is to cool the ambient air being heated in the duct, so we're treating that as the main design consideration. When the water won't accept the heat rejection any longer (when it gets too warm) a (full capacity) air cooled condenser will come online to help (another part of the design). One of the design decisions will be to decide how long the refrigeration system should run without needing the air cooled condenser, which will decide the volume of the water tank.

Originally Posted by icemeister
Barbar brought up a good point concerning operation with a cold tank of water because that's a real-world problem that us wrench-turners must address after the engineers failed to do so.
Haha. Wait, what? /haha.

Originally Posted by icemeister
120 Deg F water leaving the heat-x will not get you a tank full of 120 Deg F water. To heat the tank, the water temp leaving the heat-x must be higher than 120 Deg F or you'll never reach 120 Deg F....at least theoretically. Perhaps this is why the prof said a tank temp "near 120 Deg F".
I understand, and it is why he said that.
Originally Posted by icemeister
So do we stick with 120 Deg F leaving the Heat-x?
Yes, if 130 condensing temp can achieve 120 water exiting. I must say I don't know (like, know-know) how to reliably predict the water's exit temp yet. But, yes, we're shooting for an eventual (as in no hurry or timetable requirement) 120 temp in the tank.

Originally Posted by icemeister
A smaller [heat exchanger] ... will cause the compressor to run at a higher condensing temperature ...
I wasn't aware of this cause-and-effect. Why does this happen? This seems very important...
Originally Posted by icemeister
As Barbar suggested, you need to have more info on the compressor to know what to expect when conditions change. Study this performance data for you compressor:
Done. I wasn't aware that the more detailed version existed. It is much more informative. Thank you.

Originally Posted by icemeister
I think I would have picked the ZS11KAE-TF5 at about 30*F SST/130*F SCT and selected a condenser based on that.
Thanks for the suggestion. I looked at it and it appears that the _09_'s capacity range at 130F condensing temp fits our anticipated cooling load (~6800 BTUH) a little better than the _11_.

Originally Posted by barbar
where does the 50% come from?
A professor recommended that we oversize the hx by 50% so I added it there. We can leave it off. No big deal.

Originally Posted by barbar
without going the numbers there does seem to be some difference between mass flow of the compressor and your calculation.
Thank you for pointing that out. I have since been told (by my professor) that I should "choose 1 set of design conditions" and, for us, those conditions are described inside one of the squares on the performance table (along with the assumptions stated for that square) which does include a given mass flow rate. Previously I was calculating the mass flow rate. Now I'm taking it from "my" square on the performance table and using it as an input parameter along with the other conditions at Evap Temp=30 and Condensing Temp=130.
Originally Posted by barbar
Selecting a piece of equipment at a said number is easy, understanding why is a little more difficult. and Understanding is the art of refrigeration.

So re-look at your numbers, come up with some new ones and give the reasons why, then we can look into the heat exchanger selection.
Thanks. Here are the design conditions we're using now:

T_evap=30
superheat= +35
P_evap=26 psig
T_cond=130
subcool=0
P_cond=199 psig
capacity=7010 Btu/hr
power=984 watts
mass flow rate=120 lb/hr
amps=3.5
EER=7.1
isentropic effiency=58.4
230 volts
60 Hz
3 phase
95F ambient air over (included this because it's part of specs, even though I am trying to determine a plate hx)

Originally Posted by barbar
Do not rely on software us a PH chart.
Off topic: this was mentioned several times but I'm not sure what it is except (maybe) that I could get an estimate for the amount of compression work into the refrigerant (which contributes to total heat rejection needed). Pressure - Enthalpy chart?

Originally Posted by barbar
This is called a one pass system
Actually, I initially thought about saying that this was my situation, since heating the tank water is only a byproduct and the time it will take to reach near 120 isn't a design concern. But I chose not to go that route because I thought I might say something(s), while trying to "act" like there was no tank, that didn't make sense and you guys would get frustrated and decide not to help.

Originally Posted by barbar
... and normally we strive for a constant, such as discharge pressure or outlet water temperature. Both normally change flow rate of water.
Is this a typo that should say "change with the flow rate"? Isn't the discharge pressure the uniform and constant (if we ignore pressure losses caused by friction) on the entire high side of the compressor?

I hope my replies above show that I've been putting effort into understanding your comments and the ideas. (Because I definitely am. This is very important to me and my team)

So, thanks again for your help!

Am I ready to look at plate heat exchangers? Knowing what I know (if it's enough), how do I decide which ones could work for me?

4. Re: the compressor capacity... I picked the ZS11KAE-TF5 to satisfy the original max cooling load you stated as 8500 Btuh.

The compressor info is available on Copeland's website at www.emersonclimate.com . Register and once logged in, you'll be at the Contractor Portal, then > Tools > Online Product Info and select Compressors. While you're at OPI, go to Software Downloads and get their Product Selection Software. It will be helpful.

The statement I made about the size of the heat exchanger affecting the condensing temperature is all about the system capacity balance of the compressor, the evaporator and the condenser. From the performance data for the compressor you can see how its capacity will vary with changes in evaporating and condensing temperatures. If you look at the capacity data for a given water-cooled condenser, you'll find that its capacity will also vary with water flow, load and the temperature difference between the inlet water temp and the condensing temp...aka, the approach or TD. If you have given condenser with a fixed water flow rate, condensing temp, fixed TD and condensing load you could find the leaving water temperature. Similarly, if all the above were knowns except for the condensing temp, you could predict the condensing temperature.

What if you choose a second condenser with exactly half the Btuh capacity as the first one? If those same values were fixed, what would be the result in your condensing temperature prediction? So what I was saying originally was without regard for the efficiency of the system, I could select a much smaller (and cheaper) condenser at a higher condensing temp and the same with a smaller evaporator with lower evap temp and still satisfy the cooling & heating load requirements. The system COP would be terrible, but I could sell my system and you couldn't. Real-world commercial manufacturers do this all the time. I hope your prof gives credit for practicality.

5. That changes things slightly.

In no order.

Your prof has asked for 50% more because of such things as fouling (when the heat transfer is not as good as when it is clean) and/or in practical systems your evap load is very rarely constant, thus your heat of rejection is not constant (look at comp chart at different SSTs)

In order to have some level of stability, we need to control the variables. "the external forces that act on a refrigeration system"
A refrigeration system will always be at equilibrium with the variables. This does does not mean it is working well, efficiently or within envelope.

When you install your heat exchange, you can not change the surface area/material of the heat exchanger, so we must change the heat transfer co-efficient. So in the case of a water cooled condenser, we change the flow of the water, or the temperature of the water. There are number of methods, either directly of the high side pressure, ( discharge pressure falls, we lower the water flow or increase the water flow temp, visa versa when discharge rises)

A refrig system is not a number of individually designed components, it is a system that uses components and each component effects the system, and thus itself.

The PH chart is the holy document of refrigeration, something you should practice with (not some thing you will use on site), but aids in true understanding of what happens in a refrig system.

So going back to your selection.

So you have your duty (right or wrong does matter at this stage)

We can have very high water flow rates, and limited temp rise of the water, this would normally mean smaller required transfer area, but greater pressure drops through the heat exchanger, care should be taken the the water flow rate ( internal velocity) is not to high that erosion (wearing away) of the heat exchanger does not occur.
Or low flow rates, higher temp rises, means normally larger (more expensive) heat transfer area, care should be taken that velocity is that low where fouling occurs (either increased TDS depositing or just crud in the water sitting in the heat exchanger)

Either way, you should then consider after you have chosen your design what happens when the practical variable change. Then possibly re-select.

6. Your project sounds similar to my research project, except mine is only 5000 BTU/hr, uses R433b, and needs to be able to go up to 140F for dishwashing. I initially looked at plate exchangers but then ended up with a "snake" tube in tube as it's cheaper and less restrictive. You'll definitely need a variable speed pump to regulate the outlet temperature.

BTW, if it's not immediately obvious, you can get an efficiency boost by drawing cold water from the bottom of the tank and putting it in at the top. Extending this idea, the heat pump unit can be quite remote from the tank, as is done in my unit where it simply connects to the hot and cold lines under an existing sink and gets tank temperature readings with a wireless sensor. I'm also using a dsPIC and OpenWRT to do sophisticated controls with web UI, but you can make it work with much simpler stuff.

It's also possible to put copper tubing inside the tank, but that would tie the unit to the tank. That design wouldn't work for my use since that would put the unit uselessly in the attic (or require refrigerant plumbing to be run), but if it can for yours, you can eliminate the cost of the pump. http://ecorenovator.org/forum/geothe...heat-pump.html

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