[intellitech]

Hello everyone,

Let's assume an example system: Walk-in freezer with outdoor condensing unit, R-404a, 0 to -10 degree box temp, and the method of head pressure control is a mechanical fan cycle control.

What head pressure should be maintained using R-404a? What would be the "recommended" fan cut-in and cut-out settings? I ask this because my fellow techs vary with their answers to this question and I want to be accurate when I am setting a fan cycle control. Can this info be found in any of Sporlan's literature?

I look forward to your answers.

 

[Dowadudda]

Head Pressure setpoint if you do it correctly is found by determining minnimum delta P which the TXV can perform at it's rated BTUH capacity.

I can also tell you that nobody believes that. But it is the correct way. I see a lot of grocery stores try to low low lower head pressure based on energy management programs, but don't get the very fact that BTUH capacity will diminish rather quickly underneath a certain setpoint. It's all about net refrigeration effect.

I do this on simple single units. On a simple fan cycle only. I pick 95 degree saturated temp. And go 25 PSI above and below. Most TXV's rate their BTUH sweet spot on 90 or 95 degree full column liquid.

If you have a headmaster and fan cycle, the fan cycle setpoint is above the bottom of the headmaster setpoint.

 

[cold in alberta]

Head Pressure setpoint if you do it correctly is found by determining minnimum delta P which the TXV can perform at it's rated BTUH capacity.

I can also tell you that nobody believes that. But it is the correct way. I see a lot of grocery stores try to low low lower head pressure based on energy management programs, but don't get the very fact that BTUH capacity will diminish rather quickly underneath a certain setpoint. It's all about net refrigeration effect.

I do this on simple single units. On a simple fan cycle only. I pick 95 degree saturated temp. And go 25 PSI above and below. Most TXV's rate their BTUH sweet spot on 90 or 95 degree full column liquid.

If you have a headmaster and fan cycle, the fan cycle setpoint is above the bottom of the headmaster setpoint.

dow,that is a good response.

 

[Burnerfixer]

Go to www.heatcraft.com They have an excellent download that includes just about everything.

 

[rayr]

If you have a headmaster and fan cycle, the fan cycle setpoint is above the bottom of the headmaster setpoint.

Many D's LOL Tell him if you have a headmaster and and a fan cycle control------------lose the fan cycle control.

 

[t527ed]

Many D's LOL Tell him if you have a headmaster and and a fan cycle control------------lose the fan cycle control.

i was always taught you never have the 2 together.........


maybe up in his area both are needed on some units??

 

[ndfrozen]

I've only put the fan cycle on when the headmaster sticks in the non-bypass condition, then I install it just like d4 says.
cheaper fix for cheap customers.

 

[man from trane]

I would think that the fan cycling control would be more energy efficient. Around here we pay 53 cents per Kw so I try to run as few fans as possible on the condenser to keep a 100 degree saturated temp. Of course it never gets below 73 degrees outside, either so the headmaster is non-existent.

From my work on Trane chillers I know that there is a point at which the fan energy exceeds the reduction in energy caused by lower head pressure, but that is manufacturer specific.

 

[wannafreeze]

i was always taught you never have the 2 together.........


maybe up in his area both are needed on some units??

Manitowoc Quiet Cube uses headmaster and FFC together.

 

[Biblepoet]

I agree with all the above answers and thought I would throw another one forward. This is a "rule of thumb" way of setting it and should be set correctly as soon as you are able.

It is as follows:Grab the line after the liquid drier and set the fan to come on just after it starts to feel uncomfortable and set the differential to minimum.
This was taught to me by my father and it is an "old techs secret".

But, remember also return to set it correctly - that tip is more for the late night, in a pinch, want to get home quick type scenario.

 

[t527ed]

Manitowoc Quiet Cube uses headmaster and FFC together.

but manitowoc has never done anything right.........

 

[wannafreeze]

but manitowoc has never done anything right.........

On the remote CU they don't let the hot gas go trough the condenser coil during harvest by using the head master.Maybe someone can explain it better.

 

[t527ed]

are they cycling fan during freeze cycle to maintain head pressure or just shutting it off during harvest?

 

[hvacpope]

I would think that the fan cycling control would be more energy efficient. Around here we pay 53 cents per Kw so I try to run as few fans as possible on the condenser to keep a 100 degree saturated temp. Of course it never gets below 73 degrees outside, either so the headmaster is non-existent.

From my work on Trane chillers I know that there is a point at which the fan energy exceeds the reduction in energy caused by lower head pressure, but that is manufacturer specific.

Running your fans on and off frequently is not a good thing for the motors and more energy gets consumed that way, every time a motor takes off a large amount of energy is used to break inertia once the motor reach it rated RPM the energy consumption decreases, as rule motors should not alternate more than 6 times per hour.

 

[man from trane]

Running your fans on and off frequently is not a good thing for the motors and more energy gets consumed that way, every time a motor takes off a large amount of energy is used to break inertia once the motor reach it rated RPM the energy consumption decreases, as rule motors should not alternate more than 6 times per hour.

I strongly agree. It's good to cycle them once in a while to scare off the ants, though. They love to build nests on the contacts for some reason. Mine hardly ever cycle since it never gets below 73° outside. I have the sat temp setpoint at 100°F. Right now I have 1 out of 10 fans inop due to no one stocking fuses on this silly island. I get 3 or 4 cycles per day, usually at night during the freezer defrost. Also the condenser coils are fried because of the salt air so airflow is low.

 

[wannafreeze]

are they cycling fan during freeze cycle to maintain head pressure or just shutting it off during harvest?

If I remember correctly fans go off during harvest,and head master makes the hot gas by pass the condenser coil.