controlling for saturation temp instead of superheat

archibald tuttle

with TXVs aimed at relative superheat and the system charged on subcooling the general approach allows for a varying of saturation temp. I am hooking up a condensor to a chiller application using a flat plate heat exchanger and in order to use a small enough flat plate unit to keep refrigerant flow up and not get it gummed up with oil which i was warned to do by the flat plate supplier I have to run at least a 7 and preferable 8 or 9 degree TD between saturation temp and desired water outlet.

i could let the outlet temp rise a little when it is cooler outside but our target water temps are generally 42 to 45 which means i need 34 or 35 evaporating temp when I need the coldest water which is getting towards freezing so i'm trying to contemplate whether there is an expansion valve that works more like a regulator targeting a pressure downstream of the metering device such that i can target the evaporating temp without moving into freezing territory. then i would charge off superheat as with fixed metering device . . . and watch that over a range of conditions?

I guess maybe another indirect way to accomplish this is with hot gas bypass targeted at keeping the apparent load the same. or wouldn't hurt to have variable condensor fan that ramps up with outdoor temp so that i get similar target temp of liquid out of evaporator regardless of outdoor conditions.

then finally maybe either a variable refrigerant flow compressor or electric TXV that could have both absolute and relative evaporator temps figured into its algorithm. I can see some ways to do this. I'm just wondering given the world of whats available off the shelf what might be the best (or best comination) of approaches.

thanks,

brian

pecmsg

Adjust your GPM higher or lower for a higher or lower approach temperature.

Never heard of the oil issue with Plate Frame and we have a lot of them!

archibald tuttle

maybe they are being belts and suspender but as soon as i thought about going to a bigger plate size or more plates to handle the high load closer approach to outlet temp the kelvion engineer immediately got concerned about oil build up on refrigerant side.

i assume if you are talking GPM you are talking tailoring waterflow. I would need lower waterflow with closer approach temp to obtain desired outlet temp and I that would reduce the BTU effectiveness of the exchanger and then i'm down to flow reduction of refrigerant again i think.

what i really need to do is to control that approach temperature. the more i think about it the more i like the idea of a variable speed condensor fan motor. I'm wondering if there are any retrofits and i can key the fan speed to an outdoor thermister. i think that could be my easiest accomodation. i'm staging units. the old unit had two stages internal and it had 3 condensor fans with analog fan staging with 1 fan until 80 degrees 2 fans until 90 degrees and then the 3rd fan kicked in. so i'm really looking at doing about the same thing with a variable speed fan.

pecmsg

AXV's are exactly that, a Pressure Regulating Valve.

Add a head master to the condensing unit for a minimum head pressure.

pecmsg

I assume your looking a refrigeration condensing units!

archibald tuttle

chiller, refrigeration, its more or less the same only gotta not freeze it unless i want to glycol which i don't. and i don't want to freeze the fan coil units anyway.

I'll look at AXVs but the more i think about the unit i am replacing i realize about the only difference between a this built chiller and a standard condensor hooked up to hyrdonice instead of air handling heat exchanger is they varied the condensor effectiveness by turning off fans and in a very rudimentary way. i bet i could do better on matching approach temps, or even inverting them, i.e. higher so higher approach temp when its colder outside, with a variable speed fan. The unit i took out used an OEM but very typical sporlan style TXV.

Of course i like the operational safety of having a control that watches superheat at evaporator exit. But maybe there are AXV hybrids that respond to both.

archibald tuttle

and i can keep looking but its pretty clear sporlan wasn't thinking of AXV operation for my circumstance as their offerings are 0-90 PSI adjustable and the R410A PT chart says i need between 105 and 110!

i suppose one other way to deal with this is to use a different refrigerant although once upon a time when the government liked it, before they didn't like it, I thought I should try to standardize there . . . .

EBEBRATT-Ed

If you don't want glycol and your going to operate that close to freezing I thing you will be required to use HGB. HGB will nail the desired water temp if sized correctly and yes you need something for head pressure control. I doubt any chiller mfg would allow that water temp without it (unless using glycol) I know Carrier used to be 38 deg minimum with water

archibald tuttle

Ebe? Eddy?

so walk me through sizing/controlling the bypass. i'm liking my variable condensor fan idea although the HGB is desirable from the point of view of keeping refrigerant velocity up in the plate exchanger. but thinking about it in theory, if i slow the condensor fan down when its cooler out then the condensed refrigerant will be hotter than it would have been which is kind of what the hot gas bypass would do. if i had a truly variable fan, carefully matched the blades and could vary it as a function of approach temperature i bet I could peg it the same way, but i might have to settle for a little more stilted control. maybe a 3 speed . . .

and maybe i'm not off from that carrier spec. my water exit is actually warmer at 42, but i'm going to have that 8 degree split against the evaporating temp so I will have close to freezing refrigerant in there. as long as i have a good flow switch to check for circulation i think i'll be ok.

thanks,

brian

EBEBRATT-Ed

HGB does a few thing but the most important is to prevent freeze up of the chiller barrel (plate and frame..whatever). You need 4 pc of information:

Lowest suction pressure...chiller barrel temp
Lowest head pressure to be maintained
type of refrigerant
# of tons to be bypassed when the system is at minimum load.
select the HGB to bypass the required tonnage at the selected pressure differential

Because HGB is a necessary energy pig (the compressor runs but is doing little work) what used to happen was that they would unload the compressor with electric unloaders and as the load drops off and then use HGB to maintain the minimum suction pressure to maintain freeze up

aircooled81

I think controling the evaps saturtion temp is the most solid,
program an EXV to maintain the saturation pressure you want to keep and cut the ac off when you hit 33*F, like a fail safe. if this doesnt have a feature to maintain a min stable sh, then you might need an accumulator.... hmm maybe EXV isnt way to go?

then you can adjust the saturation temp in the coil, just make sure you have a flow switch cause your riding the ragged edge,

archibald tuttle

ebe and aircooled,
i haven't used an EXV. sounds like i could program it to maintain the evaporating pressure i desire and still watch the superheat. While i don't think load is going to be a problem because of my staging design does the EXV have an end switch that could cut out the demand call if the load is low enough that it can't maintain proper superheat in the suction line with that evaporating pressure . . .

i tried looking at EXVs just because I thought they would have an addressable control regime that might allow AXV and TXV properties in a single unit, but I was not easily able to understand the package required between the valve itself and the controller and they did seem to run up the cost. But if they nail all the problems I'm first in line for one. I had looked at some ALCOs like EX-4 but what brands have you used and have best balance of programming features.

I wonder if the EXV is a then also a better operation control for low evaporating pressure and whether you can program a time delay for restart or maybe i should throw a time delay relay separately or simply set a differential so the compressor doesn't restart until a certain degree of equalization has occured, although the EXV wouldn't have sensors on both sides. Do their controllers take an ambient sensor or high side sensor for adding to programming regimens.

This would certainly become the brains of the system at that point and maybe chillers are or should begoing this direction although all the commercial ones i've seen in this modest size range (admittedly older units) use a standard TXV with no hot gas bypass. they use some form of condensor fan modulation, a flow switch and a freeze stat as their principal protections. (There are typical low and high side pressure limits but these are in the more extreme range and don't really serve as operating monitors with freeze protection or liquid return prevention as their regular operational function.)

thanks for any experience with and recommendations for EXVs

brian