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kengineering
22-04-2006, 01:34 PM
How do you properly adjust the head presure on a water cooled condensing unit? I was told to adjust water flow to produce a head presure equivalent to 105 degrees F (after convertion of presure to temperature) regardless of refrig type. Also, how much does the entering water temperature influence this procedure.

I have a need to set up a display case for shipping. I will only have tap water, which is approximately 55 degrees F, for the set up. If the location has a cooling tower I'm fairly certain it will be a different temp than my tap.
Ken

US Iceman
22-04-2006, 05:02 PM
Ken,

Are you using a water regulating valve controlled by discharge pressure?

The valve setting will be different for cold water, since the required flow rate will be lower.

If a cooling tower is used, the valve setting will require more water during the summer because of the higher wet bulb temperatures and the effect of this on the outlet water temperature from the tower.

105F is a nominal design temperature that a lot of people use for some reason. If the system can operate with lower condensing temperature (and most of them can), the water regulating valve can be adjusted further open.

The final adjustments will have to be done during start-up.

Was the system designed for 105F condensing, or is this just a number someone is throwing around?

Peter_1
23-04-2006, 10:23 AM
when we installed WC condensers in a ship, we adjusted the valve to the saturated pressure of the warmest expected seawater + 10 K for the refrigerant used.
So the water can become max 18C during summer, so it was set to condens a 28C.

If it's a capillary fed system, then you will need to design your system at the highest possible condensing temperature and maintain that pressure very stable.
So you can use in your case 65F to 70 F.
The lower you condens, the better for the power consumption.

But if it is fresh water, then you have to make compromises.
This fresh water costs a lot of money so it flows all to the sewer. The gain you have in power consumption is lost in water consumption.

In such a case, I should let in condens on the same pressure a normal installation should condens when it was aircooled.

Install also a SV in case the pressure regulating valve should leak.
if it leaks for weeks, the then you have spoiled a lot of fresh water.

Ever wondered that 1 l of fresh water in bottles costs almost the same of 1 l. fuel?? And think once at the almost uncountable handling's, transports, refining, ... there has to be done to make 1 l. of fuel!
You can plot this in a graph: water cost versus power savings and look where lines are crossing.

You have the same with fans running on a condenser to lower HP and to increase the COP.
On a certain moment, too many fans will run for the savings you gain on the compressor side.


PS: IN general for all readers, try to place SI units between brackets. I tend not to read posts with to many IP units.
And SI posters better place the IP units between brackets.

NoNickName
23-04-2006, 01:36 PM
Danfoss and Jonhson control have modulating pressure valves to adjust the water flow based on condensing pressure.
Be careful that cold water will cause unexepectedly high subcooling at low flow rates.

kengineering
25-04-2006, 12:48 AM
In such a case, I should let in condens on the same pressure a normal installation should condens when it was aircooled.

Here in the States Copeland data sheets rate condensing units on a chart starting at 90 degrees f (32 c ) and as teperature goes up from there the capacities diminish. So it would seem that would be the place to start. Thanks, Ken

Chillin'
30-05-2007, 10:37 PM
Not that simple.

As a general rule of thumb, we use a condensing temp of 90 to 100F. The lower your cond temp the greater your subcooling and therefore the higher the efficiency and capacity. To correctly charge the system however becomes more tricky. I will assume AC conditions. Ensure you have a full load - no unloading. First charge the system with the minimum charge (or add refrigerant while the unit is running). Adjust the head pressure control until you get 210 to 250 psi (R22). Continue to add gas until your discharge superheat reaches 75F or less. Do not go lower that 60F (overcharge). If you lower your head pressure too much, you will not have good oil return.

Hope this helps