Heat Pump Defrost Cycle Design 1

[w2jo]

My job is to interface various HVAC and other equipment to Building Automation Systems. I have just (about) completed the interface of a Carrier Infinity Heat Pump to a standard Building Automation Controller. This involved complete replacement of the existing Infinity HP controller so as to allow detailed control and monitoring of the HP by the building automation system.

Every thing is finished and working fine, but I am thinking that the DEFROST CYCLE for the HP is more complex than it needs to be. Maybe someone can educate me.

What I implemented (in accord with Carrier's Manual) is:
1) Set minimum defrost interval to 30 minutes at power up.
2) If minimum defrost interval timer has expired AND OCT (outside expansion coil exit temp) is less than 32F for 5 minutes, then initiate defrost cycle.
3) If the prior defrost cycle took less than 3 minutes (for OCT to reach 65F which signifies "defrost complete", set minimum defrost interval to 90 minutes. If it took 3>6 minutes set next interval to 60 minutes. If more than 6 minutes set next interval to 30 minutes. (This 30/60/90 selection is the continuous running time UNTIL the next defrost is ALLOWED to occur. The defrost cycle is not run unless #2 above is satisfied.

I have noticed that the typical OCT coil exit temp to Outside Air temp when running in HIGH HEAT MODE is about 8 degrees F after stabilizing for 5 minutes. I also have noticed that the OCT temp difference to outside air temp increases as the outside coil ices up as one would expect. When the coil to Outside Air temp is about 12F, the coil is getting "fuzzy" with ice.

It seems to me that if I keep a long term trend of the delta-T (temp difference, Outside Coil to Outside Air in HEAT MODE) and use that as a baseline, I should enter the defrost cycle about when the delta-T gets to 12F or so.

Is there anything I am missing with the above protocol?

Also, Is there a good rule of thumb criteria for a temperature below which a heat pump should not be run? Some have told me 40F, others 37F and others 32F. It seems to me that as long as the compressor AMPS are within specs and the defrost cycle is working the machine should be capable of operating independent of any set minimum temperature.

My criteria for terminating heat pump operation might be if the defrost cycle took more than (say) 20% of the "between defrost cycle" HP running time then stop using the heat pump until the temperature rises. Anyone have any thoughts here?

Many thanks for your helpul comment.

Joe

 

[w2jo]

Hi Gepman,
The charts you linked to are very interesting. The "Operating Map" in particular is useful.

The operating map appears to be for a heat pump in heating mode whereas the table appears to be in air conditioning mode. Do you also have an operating map for a heat pump in cooling mode and a table for the heat pump in heating mode.

Is there more to the document? Perhaps you can give me a link to the entire document??

Thanks
Joe

 

[gepman]

w2jo
The way that Copeland achieves two steps of capacity is with a "valve" or "port". It does not affect the suction and discharge pressure which basically are controlled by the evaporator and the condenser but affects the "effective" swept volume of the scrolls. The less swept volume the less mass flow of refrigerant and therefore the less capacity of the compressor. It is very similar to unloading a cylinder on a reciprocating compressor. You can see an animation of this at

http://www.copelandscroll.com/index2.html

the click on "products", then click on "ultra tech", the click on "view ultra tech animation".

The operating data that I sent you was generated by the Copeland Compressor Selection Software. The operating envelope is for the compressor whether it runs as a heat pump or an air-conditioner. Remember the compressor doesn't know if it is rejecting the heat inside the house (heat pump) or outside the house (air conditioner). It just cares about the suction and discharge pressures. The performance table was generated around the particular set of operating conditions that I gave it (where the star is on the operating envelope). I can generate a performance table around any set of operating conditions that you want so if you have a particular set of conditions, let me know. I can also generate the information for the compressor at 67% capacity. I can put in any amount of superheat and subcooling that you want. If you are referring to the 95 deg. F ambient "air over" in the upper left of the performance table I think this is a fixed value. This does not refer to the temperature of the air over the condenser (remember this table is for the compressor only and does not care what the condenser is), it is for the temperature of the ambient air around the compressor which affects whether the compressor will overheat. Obviously in heat pump mode the compressor will be cooled more efficiently by 10 deg. F air than by 95 deg. F air. That is why you could put some temperature sensors in the motor winding and in the discharge gas to see if you are overheating instead of relying solely on the operating envelope. Copeland has some information on this for OEM's that design around their compressors.

 

[w2jo]

Hi Gepman,
I did look at the Copeland site as you suggested. I would like to see a operating map for a heat pump in cooling mode and a table for the heat pump in heating mode for the Copeland ULTRA model, nominal capacity in colling. My 25HNA9 unit claims a nominal cooling capacity of 48,000btuh at outlet air 72 and Outside Air of 100F. Superheat is specified at 12F for this system.

I tried to sign up for access to the site, but I never got a confirmation and my chosen PW does not work. Maybe I have to phone them to gain access.

Thanks for your help. The information you have furnished so far have helped my understanding a LOT!
Joe

 

[gepman]

w2jo
Now we get into the problem that I have every day when I try to back calculate efficiency or capacity of condensing units. I do this because many times there is no published information on the EER of a condensing unit at various conditions, only the capacity but not the EER. Manufacturers almost NEVER give the degree of approach, temperature difference, or heat transfer rating in BTU/deg. F of either their indoor or outdoor coils (in the case of you A/C Heatpump you can't say what the function is of each coil since it changes).

What I have done is assumed a 22 deg. TD on the indoor coil (72-22=50) and a 30 deg. TD on the outdoor coil (100+30=130). Using 12 deg. F of "useful" superheat (superheat which occurs inside the evaporator) and 3 deg. F of "non-useful" superheat (superheat which occurs in the suction piping after the evaporator). Useful superheat contributes to the cooling effect of the refrigeration system, non-useful superheat does not. This gave me very close (within 100 BTU/H) of the capacity which you stated. I am not much of an expert on these small commercial units but 10-20 degrees on the typical evaporator and 20-30 degrees on the typical condenser is what I have seen. Industrially on large ammonia systems I would normally design 8-10 degrees on the evaporator and 10-15 degrees on the condenser (usually an evaporative condenser). Other people may go higher to minimize capital cost.

I have attached the actual calculation at that those conditions along with the performance table. As I said previously the performance table and the operating envelope will be the same whether it is a heat pump or air conditioner since the compressor doesn't care. When calculating the performance tables the software does NOT allow me to enter the ambient air temperature so it always defaults to 95 deg. F. The ambient air temperature, as I said previously, does NOT affect the compressor capacity but would slightly affect the operating envelope, since I believe that this only affects the cooling of the compressor. Lowering the ambient air temperature would probably raise slightly the allowable condensing pressure in the operating envelope since more heat could be removed. You would have to call Carrier or Copeland to find out how much.

 

[EmeraldCoastHVACR]

gepman,

The evap coil to inlet air temp varies 10 to 20 TD depending on the system, but, in HVACR engineering, the AHU's are sized by the TD of the inlet air before the evap coil and supply air after the coil, which rarely exceeds 20 deg (Q = 1.08 x cfm x TD).

The condensor TD is an actual temp of condensor coil from compressor discharge to end of condensor coil -- 30 deg is a standard efficiency condensor with higher efficient ones condensing closer to 20 deg above ambient.

To actually measure the amount of subcooling at the condensor & superheat from the evap outlet to compressor suction, manifold guages must be installed to correlate refrigerant temp to piping temp -- only a licensed & skilled technician like myself is qualified to do this -- if this procedure isn't done correctly, oxygen could mix with the refrigerant and cause a violent explosion.

superheat is never measured in the evaporator, only from the evap outlet to suction of compressor -- along with subcooling at the end of the compressor, the superheat adds to the total "refrigerant effect" of the system.

 

[w2jo]

Hello EmeraldCostHVACR.

If you are warning me not to "mess around" with refrigeration equipment, I am modestly qualified. I have a HVAC Contractor's License, and a Electrical Contractor's License. And I have installed and engineered a myriad of smaller Air Conditioning systems. BUT.. My experience with the engineering details and operation of heat pumps is indeed minimal as you can likely surmise.

Therefore, I <really DO> appreciate any tutorials that you and Gepman may choose to pass my way. I am soaking it all up!

I have added some interesting graphs of my 4 ton Carrier Infinity Heat Pump (modified with new defrost algorithm) for your possible interest. They are at the bottom of the article at

http://gpsinformation.org/joe/HeatPump.html

The LEGEND is shown in the first graph. From the graphs you can see that the new algorithm a) cuts the expected defrost cycles by a large percentage. I still have not had weather much below 30F and I expect the improvement to be a lot less in below freezing operation.

So far, I have operated the system down to about 28F ambient and the COP has stayed above 2 even at 28F. Not bad.

 

[gepman]

w2jo
I salute you for the great amount of good work and intelligent thinking that you accomplished. You might want to see if you can sell your information to Carrier. You did hit on a difficult problem which is determining when to defrost. This is a problem in a lot of cold storages and as you noted it wastes a lot of energy. Heatcraft has a new method of defrost initiation that I need to check out.

My wife would kill me if I put that much time into our HVAC system!

 

[w2jo]

Gepman:
My motto is:
You spend 40 years of your life working for others so that in your retirement you can run all the experiments you want of your own selection - guilt free! And sometimes make a $ in the process..

 

[w2jo]

I have placed the latest article on my work at

http://www.gpsinformation.org/joe/HeatPump2.html

This system has now allowed my heat pump to operate normally and with FAR FEWER DEFROST CYCLES all the way down to 16F. I presently have it set to switch over to natural gas at 20F as this particular 48,000btu/h heat pump is down to 30,000btu/h at 20F and that plus the second furnace output of 48,000btu/h is not adequate to go to lower temps. I then go to the backup 90K btu/h + the 48K btu/h gas furances.

But above 20F the heat pump runs fine and as you can see from the graphs in the article, it averages less than 1 defrost cycle per day.

 

[sterl]

The rate at which you frost up will be closely related to the Ws of the air and the dew point depression....as your suction pressure drops and refrigerant mass flow decreases, the heat absorbed at the evaporator and consequentially the mean temp difference at the evaporator decreases. If the Tube Outside Temperature never reduced below the ambient dewpoint you would not make any frost...

As to Defrost due to (air) pressure difference, there has been LOTS of eperimenation on that topic with lots of published data. much of it for industrial uses but not that much different when you get past the intensity of the vernacular...

Also Sight Eye type devices compacted so they wrapped around and looked at the coating on a single tube....

The neatest one I have seen used emissivity in a similar way to a motion sensor, and the sensor element (tranmitter and receiver combined) moved around on one side of a wall of 6-coils in a very large industrial food freezer. Don't know if the electronics could be rationalized. but such a "seeing eye", fixed at a short range, wcould have a fkeld of view of enough coil that all the local effects would be averaged out...

Just a thought.

 

[w2jo]

Thanks Sterl for your comments. I think an optical scheme such as you suggest would work, but my experience with such devices outside in the weather has not been good. Dirt, water, and films are constant environmental factors which cause optical devices in the "dirty outdoors" to be problematical.

As you can see from the latest graphs at

http://www.gpsinformation.org/joe/HeatPump2.html

the differential pressure sensor is working out quite well. We just went through a couple of days of snow and sleet and the unit defrosted every 2 hours or a bit less during the worst of it. So.. During such times this differential pressure scheme defrosts at about the same rate at the T&T defrost designs. However, I like to point out the weeks it has gone without ANY defrosts when the temp was in the 30 to 40F range and the humidity was below 85%. As you noted, Humidity and dewpoint are what determines an icing situation. Most defrost controllers do not have a clue about that but the differential pressure scheme takes it into account automatically.

 

[gepman]

sterl
Do you have a brand name or a link to the IR defrost initiation device? It has been a while but the last time I tried any of these devices they didn't last long in the -40 to -70 deg. F. blast freezers that I used in the frozen foods industry.

w2jo
Can you post a sketch of the orientation of your dp sensor to airflow and the coil?