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View Full Version : Advice for lab scale low temperature chiller: 15 kW at -50F







Adam Libert
04-12-2015, 07:44 PM
Hello!

I'm designing a chiller system for a lab scale test stand and comparing quotes from chiller vendors. I was hoping that the combined wisdom of this community will help me to ensure I'm making the smartest choices about system architecture, etc.

Design Criteria:
We have a ~100 gallon bath of kerosene that I want to keep at -35F +/- 5F. There is a process adding between 2 kW and 12 kW of heat to this bath, so I'm designing a chiller to reject all this heat.

The chiller has to be outdoor-rated and air-cooled, capable of rejecting 15 kW of heat at -50F or colder fluid output in an ambient environment up to 100F. I could flow my kerosene directly through the chiller's evaporator (in which case the chiller would have to be Class 1 Div 2 rated because kerosene is flammable) or I could use an intermediate loop of a heat transfer fluid and a brazed plate heat exchanger between this and my process fluid.

Proposed Vendor Solution:
It seems that most vendors converge on the same solution. A simple single-refrigeration cycle design using a Bitzer 2-stage semi-hermetic reciprocating compressor (model S6J-16.2 (21.9 kW) or S6H-20.2 (26 kW)) with R404a refrigerant. They all include some kind of hot gas bypass, but no other frills that I can tell.

Questions:

How do I know whether they've sized the compressor appropriately and aren't selling me an unrealistically high COP/EER that won't actually work? When I contacted Bitzer to size out a compressor, they recommended an S6G-25.2 (30.1 kW)
One vendor chose a Bitzer screw, and one vendor chose 2x Copeland scroll compressors. I'm going to pass on both of these options and stick with the majority vote for the 2-stage reciprocating, unless you guys recommend otherwise
Thoughts on flowing my kerosene directly through the evaporator versus using an intermediate heat transfer fluid? The former seems more efficient, but the latter gives more options for temperature control (turning on and off coolant flow)
Some say that a mechanical/capillary TXV is totally fine, while others are using an electronically controllable one. What's the tradeoff?
Is R404a the right choice, or should I press for R507a? Am I cutting it a bit close with these refrigerants, or am I fine?
Will the suction side of this thing be operating at a slight vacuum? Is that an issue?
Will hot gas bypass this work at such low temperatures? What should I be looking for in terms of false loading? is it okay to turn the compressor on and off to maintain our temperature? What duty cycle is acceptable?
If I use an intermediate heat transfer fluid, what would you recommend? Is EGW 60/40 good enough? PGW? Some sort of silicone based fluid?
Prices range from $45k to $80k with leadtimes from 7 to 14 weeks. It's difficult to discern what I'm getting from each vendor for more or less money, though. Hopefully this sounds reasonable?
What other concerns should I have or what other questions should I be asking? Especially for such low temperatures so close to low end capability of the refrigerant


Thanks for any and all advice!

Magoo
04-12-2015, 11:37 PM
Hi Adam.
an interesting project, my initial thoughts would be an immersion coil in tank with bubbling air for fluid agitation. similar to an ice bank but without ice formation on coils.
Do you have a specific heat value for kerosene, would help. Is tank insulated, closed or open top.

regards magoo

Adam Libert
04-12-2015, 11:45 PM
interesting. why would you prefer this architecture over a standard process chiller? i think i'd have a hard time pitching an immersion coil with aeration, but if you have good reason, i'm all ears.

the kerosene tank is insulated with a closed top, though it is not pressure capable. specific heat of kerosene is 1.71 kJ/kg-K at -35 F

Magoo
05-12-2015, 12:30 AM
Simplicity of a system, and control. Add extra agitation for variable volume temperature options no need for hot gas.
Trying to work with you not against you. 100 gallons is not exactly huge.
your quote on specific heat value is SI metric and then ' F delta T but no a huge variation in that range obviously

Adam Libert
05-12-2015, 12:45 AM
hahaha yeah i'm aware of my inconsistency with units. for basic temperatures, i use imperial, but for anything involving energy or power i use SI. it's a bit silly, but at the same time, in my opinion, it's better than dealing with BTU (really: BTU/hr)

varying agitation to vary temperature, so no need for hot gas bypass to vary chiller output. it's a cool idea, i like it. but i don't think i'll be able to sell the team on it. mainly because we are trying to keep our kerosene degassed, and agitating with bubbles will do the opposite. as well, i think it'll be tough to reliably design a system that can meet our requirements while not wildly over-designing, as calculating the heat transfer of a bubbling immersion coil seems difficult compared to well-understood brazed plate or shell and tube heat exchangers.

any chance you're willing to offer advice on the system i have had quoted already?

monkey spanners
05-12-2015, 12:59 AM
What temperature does kerosene start going waxy, if indeed it does? Would be concerned about it blocking up a plate heat ex, less of a problem with an immersion coil or plates. A pump would do the same job of moving the fluid about as airation.

Adam Libert
05-12-2015, 01:22 AM
our specific type of kerosene (similar to Jet A) should be good to -40. colder than that, it starts to get a bit waxy but doesn't "freeze" or solidify until much colder (like -80F).

it's certainly a concern, but i figured that if we run the evaporator at -45F, we could have the RP-1 entering at -15F (coming from our process) and leaving back to the reservoir at -35F. we'd spec an evaporator with oversized channels to ensure we don't have any blocking issues.

thoughts?

RANGER1
05-12-2015, 11:37 PM
If using a PHE a conventional TX valve could be a pain to set up
If you use electronic, ask what type.
With this you can observe how it is performing.

Assume it has subcooler on interstage of Bitzer compressor.

Some sort of protection for compressor for liquid flood back, as want to protect compressor.
The other point is that it will probably operate in a vacuum so nice to have all pressures etc monitored with pressure & temperature transducers throughout the system.

If you use a secondary refrigerant system would run lower suction pressures again.

If there is any possibility of a freeze up for any reason all bets might be off with PHE or shell & tube , if it does not have good protection, electronic flow protection.

Magoo
06-12-2015, 12:17 AM
Adam
have you considered a cascade system, probably better suited to low temp application with hot gas defrost.

hookster
06-12-2015, 10:07 AM
First reply you received about a coil in tank was the best suggestion you received. Except I would use an EX rated agitation pump over the air agitation to prevent contamination.
Coil in the tank is standard application for process cooling as it is robust and has a far smaller failure rate, simplification of design / control.

The usual influencing factor for coil in tank is the size of the coil as you need a large surface area, hence tank volume. Personally I would get a few more local quotes to your initial system design, try find more process based companies.

I advise you consider the multi stage compressors (scroll over screw) as your process is going to be variable as you stated varying compressor steps. Although screw can be controlled, the screw will be designed at your peak loads and at the low end of your load requirements the volumetric efficiency of the screw is very poor.

I would also consider the electronic expansion valve as this will allow far more accurate control of the refrigerant and I advise all customers on electronic expansion valves at the moment as the refrigerants are changing so quick, an electronic expansion valve can be more readily adapted to a change in refrigerant / blend if required.

R404a has a high GWP and in Europe I would always advise against it as it is being phased out by 2020.

As for the rest it is all budget based, you can look at head pressure control, waste heat recovery etc. etc.