Dunham Bush

[frank]

Hi Guys

The old vertical screw continues to pop up with nuisance trips.

Refrigerant = R407c
Ambient = 27C
Discharge Temp = 69C
machine dt = 6C
Discharge Pr = 296psig
Entering temp = 14C
leaving temp = 8 C

What would you expect the discharge pressure to be at this ambient and what sort of superheat?

The machine was running at full load.

[Lc_shi]

what you mean "machine dt".
according to the discharge pressure ,the 296psig,the tc should be at 54C ,it's air cooled condenser? the tc usually is 50C. The dischage temp is lower than average which may be due to the discharge cooling or heat loss large. The te is 2C for the water chiller design ,the lp should be at 52psig(is it correct). As you mentioned above the chiller's COP around
2.7.
Maybe the condenser is not working well, pls confirm.

Only for your reference.
Hope u offer more data ,so we can think it deeper.

rgds
LC

[frank]

what you mean "machine dt".

The difference between the return temp and the leaving temp

it's air cooled condenser?

Yes it is

The dischage temp is lower than average

The saturation temp @ 296psig is around 52C so at this point the superheat is (69 - 52 = 17C). I don't understand your comment

[CuGe]

What is your suction superheat and sub cooling? plus check the liquid injection, should be set to come on @ around 76 deg C with an 8 deg diff to cut out. Also check if you have vapour injection, the ssh for this should be 10 k to 15 K.

[Lc_shi]

Hi frank,
I understand what you talked about is air cooled water chiller with vertical Dunham screw compressor.
I calculate in the coolpack by your input data and see the dischage temp is lower than average(usually suppose the comprssor isentropic efficiency at 0.75 as average).
"The saturation temp @ 296psig is around 52C so at this point the superheat is (69 - 52 = 17C). " ??
I'm not sure if it could be called "superheat".Usually we use "subcool" for condenser output liqiud temp diff and "superheat" for evaporator output gas temp diff comparing with the saturation temp at the corresponding pressure. For the discharge temp,it depends on the suction gas state and the compressing process which is between different pressure. I'm puzzled what you really mean for "superheat" here.
Maybe I don't take your point-.

rgds
LC

[US Iceman]

LC,

There are at least two areas where superheat occurs in a refrigeration system (or air conditioning also, if we make a distinction that they are different).

The discharge superheat is the one Frank is discussing I believe. The actual compressor discharge temperature minus the saturated condensing temperature is the discharge superheat.

Liquid subcooling is of course different.

Most screw compressors need a minimum discharge superheat of at least 10-15C higher than the saturated condensing temperature to control oil dilution and oil quality (warm enough to vaporize any liquid refrigerant).

The condensing temperature plus the discharge superheat is the actual discharge temperature.

Depending on the features included in the package there may be vapor injection and/or liquid injection. Either of these can have a impact on the actual discharge temperature. And of course, the suction superheat can too.

Does this package have a flooded chiller with the oil recovery venturi's? One area to check for low suction superheat is the venturi's returning too much liquid. It has been a long time since I have worked on one of these machines.

Frank, if the ambient air temperature is 27C and the condensing temperature is 52C this seems to have a high temperature difference for an air-cooled condenser. (25C)-- (52C condensing temp. minus 27C ambient entering air). I would have expected about half of that value.

If I understand all of the information correctly, your original question was directed to the discharge pressure.

If the system was originally designed properly, I would expect the condenser TD to be around 11-13C as a estimate range.

Therefore, on a hot summer day with an ambient temp. around 35C the condensing temperature would be around 46-48C. However, you said the condensing temp. is 52C with an entering air temp. of 27C.

If the condenser was sized for the range (11-13C)-- (I am using this TD as similar values we use here in the US), the condensing temperature with 27C entering air would be approximately 38-40C.

Again, if I have understood all of this correctly, the discharge pressure does appear to be too high.

Causes: air/non-condensable gas in condenser, dirty condenser, recirculation of condenser exhaust air, too much refrigerant, etc.

Some of the old Dunham-Bush air-cooled condensers have dampers to control the discharge pressure in lower ambients. Some also had a small tank on each condenser circuit to provide a seal on the subcooling coil. Several different things to check.

Hope this helps you.

[Lc_shi]

HI iceman
Thanks for your splendid explanation. I think i learn much from it.-

rgds
LC

[US Iceman]

Frank,

I have some Dunham Bush service manuals that were current in the early 1980's. Depending on the age of your system I may have some information that might provide additional insight.

If you can provide some equipment model numbers, I'll do a quick search to see if I have anything that could be helpful to you.

[frank]

Thanks Iceman

I won't be back on site until the latter part of next week so I'll see if there is a name plate somewhere, although I haven't seen one to date.

[US Iceman]

Hi Frank,

If I remember correctly, the tags were on the left side of the main control panel.

At least they used to be there about 25 years ago.

If you can find the model numbers, I hope I can find the information.