Crank case pressure regulating valve

[Mahmoud]

Hello there..

lets say we the maximum evaporating temp. of a compressor is 3*C, and the room temp. is 20*C, with this delta T of 17 is it mandatory to use a crankcase pressure regulating valve there to avoid HP cut out...

actually the case that the consultant wanted to avoid the usage of the AC units, and wanted to use refrigeration condensing units with commercial coolers, so the above limits is for the available R134 copeland unit.

[frank]

Hi Mahmoud

I'm not certain, but I don't think it is mandatory to fit a CPR in this situation. It may be good engineering practice though.

From my limited knowledge, I think it is only mandatory to install safety devices such as pressure relief valves to comply with local regulations.

Which part of Dubai are you in? - nice city, had a lovely time there last Xmas

[Mahmoud]

Dear frank,,

next time to be in Dubai,give me a call , yes it is a lovely city to work and live in, am in Dubai-Deira, and i work there, u'll never know the city man , now they started building the longest tower in the world. -they- coz am not from Emirates.. am from Jordan, but i work here as millions do here from all over the world .. u know that ..

thx man ..

[US Iceman]

Hi Mahmoud,

A CPR is normally used on refrigeration applications that have periodic high loads applied to the compressors. This helps to limit the compressor crankcase pressure to prevent overloading the compressor motor and condenser.

High suction pressure contributes to higher mass flow rates and increases the heat rejection to the condensers, so you see higher discharge pressures as a result.

If the refrigeration system is subjected to high evaporating temperatures during numerous hot start ups, it may be worthwhile to use this valve. However, if the system is supposed to run all of the time at one temperature I'm not sure what you will gain by using a CPR.

If the room is operating at 20C, why do you have such a high TD? The refrigeration evaporators I am familiar with are not properly circuited for a 17C TD.

Air conditioning evaporators are designed for this. Refrigeration evaporators are not normally used with this high of a TD.

In the refrigeration evaporator catalogs their rating is based on heat transfer per degree of TD. This does not mean that any TD is acceptable to my knowledge.

Regards,
US Iceman

[frank]

If the room is operating at 20C, why do you have such a high TD? The refrigeration evaporators I am familiar with are not properly circuited for a 17C TD.
Air conditioning evaporators are designed for this.

Hi Iceman
AC coils are not normally sized for 17C delta T, maybe 17F. If the room temp is , say, 25C with a 17C drop you would be getting air off temps around 8C which would lead to cold draughts.
Do you have any data on this?

Cheers

[US Iceman]

HI Frank,

I was afraid I would open a can of worms with that statement, but I'll try to explain my logic.

Refrigeration evaporators are normally based on heat transfer per deg. TD. Sorry for the generic language, but I'm not sure of the exact units used in the UK for this. Is it kW or kCal?

For these units, the individual refrigerant circuits (in the coil) are designed for some maximum pressure loss per circuit. For a given coil size a specific number of refrigerant circuits are usually available.

The manufacturers have predetermined all of this to meet a certain operating range to reduce engineering for custom coils.

If the kW/degree TD is too high, the mass flow through each circuit will be too high, resulting in larger than expected pressure losses in the refrigerant circuits.

This affects the coil capacity.

For refrigeration coils the normally used TD is in the range of say 10F to 18F (5.5C to 10C). The TD is based on entering air temperature (essentially room temperature) minus the refrigerant evaporating temperature.

We would have the following:

kW/deg.C TD times 5 deg.C TD= XX kW

if we use a 10C TD the capacity doubles and so does the mass flow of refrigerant required.

With a 17C (30.6F) TD for refrigeration, the resulting coil circuit pressure drop can negatively affect the coil performance. This is particularly important on low temperature applications and TXV's.

The air temperature drop is very small since we use large volumes of air. We might see air TD's as low as 1C on refrigeration evaporators.

On AC coils, using your example and comparing it to the refrigeration evaporator selection method: if the room is 25C the normal expected refrigerant evaporating temperature in the coil would be about 7.2C (45F DX feed), or, 25C minus 7C is 18C TD.

The actual air temperature exiting the DX coil (depending on sensible heat ratio, bypass factor, etc.) would be somewhere around 12-13C (54F) without reheat.

The actual air TD across the coil then becomes approximately 25C minus say 13C = 12C (about 21F) for discussion purposes. Please bear with me as I'm using numbers to explain the logic, not the exact answer.

It is my understanding this is not how AC coils are selected, but using the refrigeration evaporator selection method as an analogy, this is how the numbers come out.

This is what I tried to say about the AC coils being sized for high TD's. The temperature difference between the entering air temperature and the coil evaporating temperature is much higher for DX AC coils than the those for DX refrigeration applications.

If the selection method for refrigeration evaporators in the EU is similar to what we use on our side of the pond, 17C for a TD is too high for a refrigeration application. Especially for low temperature applications...

This would require a much lower suction pressure and larger compressor to provide the required capacity.

I have seen some chilled air systems using about 8C air discharge temperatures (off of the coil), but they were using ice water on thermal storage and reheat to reduce the cold drafts (sorry, our spelling version).

Does this help? What are your thoughts on this?

US Iceman

[frank]

Hi Iceman

Most of the selection data over here (from my limited knowledge) bases the evaporater size (refrigeration) on a temp diffence of either 6 or 8K. http://www.searle.co.uk/client/choos...tType.asp?id=1

A/C equipment over here does not give the same sort of selection process but a system capable of matching a total heat load.

The thoughts about low air off temps was based on experience. With the new inverter driven systems, evaporator capacity varies with room load and coil/air temps vary accordingly.

I've had many occasions where complaints were forthcoming due to low air off temps but these were nearly always based on low load conditions with on/off compressers

[Gary]

I suspect some posters are confusing delta-T with TD. These are two entirely different things.

Air on minus air off is delta-T.

Air on minus saturated suction temperature (SST) is TD.

In other words the delta-T is the drop in air temperature as it goes through the coil, while the TD is the difference between (entering) air temperature and (saturated) refrigerant temperature.

[frank]

Quite aware of that Gary thanks - no offence though

[US Iceman]

As long as we are discussing differences let's also cover:

Air on minus saturated suction temperature (SST) is TD.

This should be the "saturated evaporating temperature", which should not be confused with saturated suction temperature. Two different things.

These occur at two different locations in the system. Same is true for condensing and discharge. Both are different.

I must say this is the first time where I've seen any definition given to the distinction between dT and TD based on an implied context though.

I must have missed this in school.