0-50,000 BTU/hr system

Hello,
We are trying to build a system that we could use to vary the AC from 3,000 BTU/hr all the way up to 50,000 BTU/hr. This is for a equipment test setup. We were thinking of using a vfd to drive the motor/compressor, but we have many concerns. We want to avoid using a reciprocating compressor (just preference.) It is also must run on R134a.

We have been talking to Bitzer regarding their screw compressor (VSK-3161-15Y) but they said not to run it below 20 Hz and the capacity of the unit is linear with frequency. They were concerned with oil pressure loss. This isn't going to work since we need to be able to go 3,000 BTU/hr.

Then we thought about running the refrigerant through two sets of condensers and evaporators to control our capacity of one unit and place the other evaporator outside of where we need it. This adds too many variables and makes it difficult to control the capacity of the half we are using.

We were thinking of using a scroll compressor due to their durability. Has anyone ever ran a scroll compressor at very low rpm for extend periods? Are there any issues? Is there any possible system set-ups we haven't thought of yet.

We also must run R134a, most scroll compressor's literature is done with R22 or R404a. Is there a correction factor to approximate the capacity of a compressor rated with R22 or R404a if we would use R134a? Is it even safe to run a compressor with a different refrigerant then designed?

Thank you for taking time to read and respond to this. This project has only lead to more questions than answers.

how close must the steps be. Old Fujitsu VRF had 3 compressors of different sizes which went on/off in stages to give different capacities from low to high rather than variable speeds. Cheaper and easier to control too

Lok at using two Bitzer mni screws with step unloaders and VSD speed control for min/max capacity, the minimum speed would be around 25 hertz to maintain oil differential flow to screw, at low speed capacity you will need some form of liquid injection into suction to maintain saturated suction temps for motor cooling and oil cooling, controlled by discharge gas temperature.

Is it the application that requires the varying load?, or are you trying push the limits of a refrigeration system?
How close control do want!
The application determines the design.
You know your min and max loads.
Do you know your load profile (time to change)
Is the load at a certain temp or does the required temp vary.

If it is a high temp application you could consider using a chiller with 3 port valve arrangement, possibly on multiple evaporators.

Thanks for all the responses. This is for a testing cell and will power varying equipment as we test it in a calorimeter. The idea is if we have a small unit we can dial down the compressor to very low load to provide the flows the evaporator would see to evaluate the performance. On the other hand, we want to be able to hook up large units and use the same compressor.

Control capacity should be fairly precise. I would need to check how precise.

I was interested in using multiple compressors, but my boss wants one. I think I need to persuade him otherwise. I would think 1 5,000 1 10,000, and one 35,000 BTU/hr units would give you the most accurate capacity. Have each unit be vfd controlled so you could range in 50-100% capacity and give him the capacity range he wants.

The time it takes to change the capacity isn't a huge issue as long as it is simple.

Has anyone used a copeland digital scroll? It appears to be able to go to as long as 10% capacity but essentially turning on and off quickly. Anyone have any experience?

I'm not sure of the temperature range he is after, I need to find a table of what is considered high temperature. I was hoping to find that today but we just got a new environmental chamber in that is suppose to be my new baby.

Thanks for all the help. I appreciate the suggestions and I wish I knew more on the details, I'm learning as I go here.

An evaporator varying in load from 3.000 - 50.000 BTU will also create it's own issues, so

Completely out of the box here...

What about a wet system?
A chiller cooling a tank of water/brine which in turn feeds cooling coils via variable speed pumps or proportional diverting valves...

Providing the tank is large enough and insulated enough, the chiller will have a easy life with long running cycles regardless of load, cooling will literary be "on tap" (OK, poor pun I know) and the losses will be minimal.

Just a thought.
:cool:

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Quote:

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Originally Posted by The Viking

An evaporator varying in load from 3.000 - 50.000 BTU will also create it's own issues, so

Completely out of the box here...

What about a wet system?
A chiller cooling a tank of water/brine which in turn feeds cooling coils via variable speed pumps or proportional diverting valves...

Providing the tank is large enough and insulated enough, the chiller will have a easy life with long running cycles regardless of load, cooling will literary be "on tap" (OK, poor pun I know) and the losses will be minimal.

Just a thought.
:cool:

.

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See post #5 :D

I guess I should explain what we are doing in a little more detail. The evaporator changes with each test set up. One day we may be working with a 5,000 BTU/hr evaporator and the next a 50,000 BTU/hr evaporator. The condenser has enough capacity for our purposes, we just change the fan speed.

The chiller idea is great Frank/Viking. Took a little more explain for me to understand :(

Ooops, sorry Frank, I should have read all the replies more carefully.:)

Now, Craig,
You say you will change the evaporators daily???
No way you want to do that with a DX (straight refrigerant) set up. You definitely want a wet system of some sort.

Or is it evaporators you are testing???

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Evaporators are what we are testing.

I would also agree that a fluid system was better, I would look at 2 "3 port valve" a large one and small one, for very fine control and low loads.
There are many ways of of achieving the results that you are after. You must first totally understand all of your process. Min and max loads, min and max temps, and how they correlate with each other.
50,000 BTU at -20C is a completely different beast to 50,000BTU at +20C, but a system 3000Btu at -20C, is some what closer to a system of 50000 BTU at +20C.

Quote:

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Originally Posted by CraigFr

Evaporators are what we are testing.

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Aha!
As far as I can tell most of the replies above relates to test chambers for other stuff... Where the evaporator is just part of the "tool kit".

Sorry.


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You are checking refrigeration evaporators?
Well that is a different kettle of fish.
What SST ranges are you looking at.

If your SST are similar for all duties, then the solution is relatively easy.
If your SST is wide across all working duties, the basic solution is the same, but with greater complexity.
You need to focus on steady state conditions at the evap, and do not worry so much about what is happening at the compressor end.

How about setting up a couple of additional evaporators that can be turned ON or OFF as necessary to give the compressor pack an easier life, like a supermarket system?

If you are testing the evaporators for design purposes, there are a lot of variables you want to fix not just compressor speed...

How about a false heat load when running compressor at 20hz?

If you are testing Evaps and all your measurements are targetting the evap INSIDE a test chamber: you should be able to run a Hot Gas bypass arrangement and meter/measure the flows to the evaporator only, meanwhile the hot gas would be cooled prior to the compressor by a liquid injection arrangement to a mixing vessel. You won't be able to use the compressor as any form of measurement device but if measuring mass flow to the evaporator and temperatures as parameters is enough to indicate performance, this will permit you to manage that down to a few percent flow and steady state with fairly conventional pieces and parts.