Commercial Refrigeration Ch 1 & 2

What is considered the "normal" box temperature of a walk-in refrigerator?

35�F-37�F

What is considered the "normal" box temperature of a reach-in refrigerator?

38�F-40�F

What is considered the "normal" box temperature of a walk-in freezer?

?10�F

What is considered the "normal" box temperature of a reach-in freezer?

0�F

Why is the temperature of walk-ins usually lower than the temperature of reach-ins?

Walk-ins are designed for long-term storage and lower temperatures.

How can the latent heat in a cold suction line be transferred to higher outdoor air temperatures?

Compress the suction vapor until its condensing temperature is higher than the ambient.

What is the primary difference between the AC system of Figure 1-1 and the refrigeration system of Figure 1-2?

The refrigeration unit has to lower the evaporator temperature.

Which refrigerant is used most in recently installed walk-in refrigerators?

R404A

Which refrigerant is used most in recently installed reach-in refrigerators?

R134a and R404a

Which refrigerant is used most in recently installed walk-in freezers and reach-in freezers?

R404A

Why do zeotropic blends have temperature glide?

The component refrigerants boil off at different rates.

Under what circumstances should a technician use TROT?

When the factory information is not available.

Discharge Vapor Contains what 4 sources of Heat?

1. Evap Superheat 2. SuctionLine Superheat 3. Compressor motor heat 4. Heat of compression

T.R.O.T. FOR EVAPORATOR TD AND HUMIDITY for AC

35�F TD at 50 percent RH

T.R.O.T. FOR EVAPORATOR TD AND HUMIDITY for Reach-In

25�F TD at 65 percent RH

T.R.O.T. FOR EVAPORATOR TD AND HUMIDITY for Walk-In

10�F TD at 85 percent RH

T.R.O.T. FOR EVAPORATOR TD AND Humidity for High-Humidity Boxes

8�F TD at 90+ percent RH

T.R.O.T. When to defrost Box Temp 37� and above?

Off Cycle (No Clock Needed)

T.R.O.T. When to defrost Box Temp 35�?

Planned (Clock Only)

T.R.O.T. When to defrost Box temperature 33�F (and below)

Heat and clock

Defrost defrosts, what?

Frost not Ice

Freezer defrost Sequence:

1. Defrost initiated 2.Comp & Evap Fans Stop 3.StripHeat or Hot Gas 4. Heat till temp at ice melted 5.Compressor Starts 6.Evap Fan on at 24�F

DFTD = ?

Defrost Termination / Fan Delay

During Freezer Defrost, what is powered OFF?

Evaporator Fan, thermostat, and solenoid coil

During Freezer Defrost, what is powered ON?

Defrost Heater & 1 side of Defrost Termination Solenoid

One indication of a bad fan delay is...

icicles on the box ceiling and ice on the fan blades because closed contacts would allow the fan to start as soon as the unit is back in the freeze cycle.

Adjustable Range to end defrost termination on an Adjustable Defrost Termination Control is between...

60�F-75�F

The adjustment screw on an Adjust able Defrost Termination Control, a CW turn increases fan delay by

3�F

The Primary Function of an Evaporator coil is...

to absorb heat from the refigerated space

TD =

Air Temp Entering the Evaporator - Refrigerant Temp in Evap Coil

The Greater the TD the _____ moisture is removed from the air by the evaporator.

More.

Latent Heat is...

heat absorbed in the evap. by changing state of refrigerant w/o change in temp

Sensible Heat is...

heat absorbed in the evaporator after all ref. has been evap'ed w/ change in temp.

Superheat is...

measurement of sensible heat after all refrigerant boiled off, and indicatore of flooded or starving evap.

What is considered the "evaporator temperature"?

The temperature of the refrigerant inside the evaporator tubing

How do you calculate evaporator temperature from the suction pressure?

Use a P/T chart

How is evaporator TD calculated for a walk-in cooler?

Subtract the SST (Saturated Suction Temperature) from the box temperature

What is "hot pull-down"?

When the evaporator is subject to higher temperatures and loads than under normal operating conditions

What is the approximate humidity in a walk-in refrigerator with a 10�F TD evaporator?

85 percent

As TD increases, how does it affect box humidity?

Decreases humidity

If airfl ow across an evaporator is decreased, what effect does it have on evaporator temperatures and suction pressure?

Evaporator temperature and suction pressure both decrease

Why are multiple circuits used on larger evaporator coils?

They provide less pressure drop than a long single circuit evaporator

If the suction pressure of an R22 unit is 55 psig, what is the approximate evaporator temperature?

30�F

If the suction pressure of an R404A unit is 21 psig, what is the approximate evaporator temperature?

-14�F

How do you determine the evaporator superheat of a refrigeration system?

Subtract evaporator temperature from the temperature of the suction line at the expansion valve bulb

A -10�F walk-in freezer using R404A has a suction pressure of 15 psig. The suction line temperature at the TEV bulb is -20�F.

What is the evaporator superheat? 2�F

With 2�F Superheat, the evaporator is _______.

Flooding.

Why is it necessary for the fin spacing in a freezer evaporator to be wide?

Frost buildup will not occur as fast if the fin spacing is wide

What is the basic sequence of operation at the beginning of a freezer defrost?

Evaporator fans shut off, compressor shuts off, and heaters come on

Which freezer defrost system is more efficient, hot gas or electric? Why?

Hot gas bc it heats the coil from the inside

If a walk-in is operating at a box temperature of 35�F, what type of defrost should it have?

Planned defrost: a defrost clock only

What is an effective way to check a DTFD control?

Freeze the control in another freezer then measure the continuity of the contacts with an ohmmeter